Heat Index Calculator

Heat Index Calculator: Your Complete Guide to Heat Index, WBGT, and Heat Stroke Risk

A heat index calculator combines air temperature and relative humidity into one number that shows how hot the air actually feels to the human body. This number is called the apparent temperature, and it exists because humidity slows the evaporation of sweat, which is the body’s main cooling method. Many forecast apps market this feature as a “heat index app,” but the underlying formula is the same regardless of which tool displays it.

Safety managers, coaches, outdoor workers, military trainers, and parents use a heat index calculator to decide when heat becomes dangerous. A more advanced version of this same idea, Wet-Bulb Globe Temperature (WBGT), adds wind speed and sunlight to the calculation, which is why occupational safety agencies rely on WBGT instead of the basic heat index for legal work-rest limits.

This guide explains the full heat index calculation, the Rothfusz regression formula behind it, WBGT, dew point, heat stroke risk scoring, sweat loss, work-rest planning, cooling system sizing, and heat acclimatization. Every section connects to the next, so the sweat-loss numbers you calculate later depend on the heat index and WBGT values you calculate first.

Four components make up this heat safety system: an ambient heat index engine, a physiological risk engine (core temperature, sweat rate, dehydration), an intervention engine (electrolytes, work-rest cycles, cooling), and a final decision engine that turns all three into one Go or No-Go safety call.

Which Section Do I Need?

Use this table to jump to the right part of the guide based on your role.

How the Heat Index Calculator Works

The heat index calculator takes two required inputs: air temperature and relative humidity (RH). Some versions add wind speed and direct sun exposure as optional inputs, since wind speeds evaporation and direct sun can add another 15°F (8°C) to the perceived heat.

The calculator returns three outputs. The first is the heat index value in Fahrenheit and Celsius. The second is a risk category (Caution, Extreme Caution, Danger, or Extreme Danger). The third is a short list of matching symptoms and recommended actions for that category.

The National Weather Service (NWS) built the heat index for shaded conditions with light wind. Direct sunlight raises the real apparent temperature above the calculator’s base output, so the calculator applies a sun exposure adjustment when that input is selected.

If you exercise or train regularly, pair your heat index reading with a calories burned calculator to see how exertion intensity adds to your metabolic heat load on top of the ambient conditions.

The Heat Index Formula: Rothfusz Regression

The heat index is 90°F (32°C) when the air temperature is 90°F and the relative humidity is 50%.

The NWS uses a formula called the Rothfusz regression for most everyday conditions. Lans P. Rothfusz developed this regression in 1990 by fitting a multiple regression analysis to Steadman’s 1979 physiological heat-stress model, which is why you will also see it called the Steadman model in some sources.

The full nine-term Rothfusz regression is:

HI = -42.379 + 2.04901523T + 10.14333127R – 0.22475541TR – 0.00683783T² – 0.05481717R² + 0.00122874T²R + 0.00085282TR² – 0.00000199T²R²

In this formula, T stands for air temperature in Fahrenheit and R stands for relative humidity as a whole number (50, not 0.50). This nine-term version applies once the heat index result climbs above roughly 80°F (27°C).

Below that threshold, the NWS uses a simpler linear approximation instead, since the full regression becomes unreliable at lower temperatures. The calculator switches between these two formulas automatically based on your inputs, so you never need to pick the right formula yourself.

Two boundary corrections apply at the extremes. When relative humidity drops below 13% and temperature sits between 80°F and 112°F (27–44°C), the formula subtracts a correction factor because dry air evaporates sweat faster than the base regression assumes. When relative humidity exceeds 85% and temperature sits between 80°F and 87°F (27–31°C), the formula adds a correction factor because near-saturated air blocks evaporation almost completely.

Heat Index Chart

Use this chart to read your approximate heat index without running the formula by hand.

A heat index of 90°F to 103°F (32–39°C) falls in the Extreme Caution range, where heat cramps and heat exhaustion become possible with prolonged exposure or physical activity. A heat index of 103°F to 124°F (39–51°C) falls in the Danger range, where heat cramps and heat exhaustion become likely and heat stroke becomes possible. A heat index above 125°F (52°C) falls in the Extreme Danger range, where heat stroke is highly likely.

Heat Index and Dew Point

Dew point measures the exact temperature at which air becomes saturated with water vapor, and it drives the humidity side of every heat index calculation.

A dew point below 60°F (16°C) feels comfortable. A dew point between 60°F and 69°F (16–21°C) feels sticky. A dew point above 70°F (21°C) feels oppressive and signals a high heat index even at moderate air temperatures.

The Magnus-Tetens approximation calculates dew point from air temperature and relative humidity using a logarithmic function of vapor pressure. If you already have a dew point reading from a local forecast, you can skip re-deriving it and plug the number directly into a dedicated dew point calculator to check vapor pressure deficit for evaporative cooling planning.

Heat Index vs. WBGT vs. Humidex

Three different indexes measure heat stress, and each one serves a different audience.

Wet-Bulb Globe Temperature (WBGT) is the most complete heat-stress index because it accounts for solar radiation and wind speed on top of temperature and humidity. OSHA and ACGIH both use WBGT, not the standard heat index, to set legal and recommended work-rest limits for outdoor labor.

Humidex

Humidex is Canada’s equivalent of the heat index, developed by Environment Canada and calculated from air temperature and dew point rather than relative humidity. A Humidex reading above 40 signals dangerous conditions, roughly matching a heat index in the Danger range. A Humidex above 45 signals conditions where most outdoor activity should stop, comparable to the Extreme Danger heat index tier. Humidex uses a simpler two-variable formula than the Rothfusz regression, which makes it faster to calculate but slightly less precise across extreme humidity ranges. Humidex is not used by OSHA or ACGIH for occupational safety limits in the United States, so treat it as a public-comfort index rather than a workplace safety standard.

Heat Stroke Risk Scoring

Heat stroke risk is calculated from four weighted factors: environmental heat load, physical exertion level, age and health status, and current core body temperature trend.

The scoring model assigns points on a 0–100 scale. Environmental heat load (heat index or WBGT) contributes up to 40 points. Metabolic Equivalent of Task (MET), which measures exertion intensity, contributes up to 25 points. Age and pre-existing conditions contribute up to 20 points. Core temperature trend contributes up to 15 points.

Check your MET value against a VO2 max calculator if you train for endurance sports, since higher aerobic capacity shifts your effective exertion score downward at the same workload.

A total score below 30 signals Low Risk. A score of 30–54 signals Elevated Risk, where heat cramps and heat syncope (brief fainting from heat-related blood pressure drops) become possible. A score of 55–79 signals Severe Strain, where heat exhaustion becomes likely. A score of 80 or above signals Critical risk, where heat stroke becomes a genuine medical emergency requiring immediate cooling and evacuation.

Heat exhaustion and heat stroke are not the same condition. Heat exhaustion causes heavy sweating, weakness, and nausea, and the body can still sweat and regulate temperature. Heat stroke causes core temperature above 104°F (40°C), confusion, and often dry skin, because the body’s cooling system has failed entirely. Heat stroke requires emergency medical treatment within minutes, since organ damage begins quickly once core temperature stays elevated.

Pre-Existing Conditions: Weighted by Category

Not all pre-existing conditions carry equal heat stroke risk. Treat a single “condition present” flag as a starting point, then apply this weighted breakdown for a more accurate score.

Cardiovascular and renal conditions deserve the heaviest score adjustment, since both directly limit the body’s ability to move blood to the skin and manage fluid balance during heat stress.

Individual Risk Factors Beyond This Calculator

This calculator estimates group-level risk from measurable inputs, but three individual factors change real risk without appearing in the formula.

Fitness level changes heat tolerance directly, since a person with higher cardiovascular fitness sweats earlier and more efficiently at a given workload. Check your baseline with a BMR calculator, since a higher resting metabolic rate also raises baseline heat production. Hydration status at the start of exposure matters as much as fluid intake during exposure, because a person who starts a shift already dehydrated reaches dangerous core temperatures faster. Prior heat-illness history raises future risk, since a documented heat stroke or heat exhaustion episode increases susceptibility to repeat episodes for months afterward.

Treat the calculator’s score as a baseline, then adjust it upward for anyone with these individual risk factors present.

Heat Acclimatization and Adjusted Risk Thresholds

Heat acclimatization is the physiological adaptation that lowers heat stroke risk after 7 to 14 days of repeated heat exposure.

Three changes drive this adaptation. Sweat onset starts earlier at a lower core temperature, which begins cooling the body sooner. Sweat sodium concentration drops, which reduces electrolyte loss per liter of sweat. Blood plasma volume expands by 3–5%, which improves heart efficiency and lowers heart rate at a given workload.

The typical acclimatization timeline breaks into three phases. During Days 1–4, risk stays highest because none of the three adaptations have occurred yet, and this window accounts for a disproportionate share of workplace heat stroke cases in new hires. During Days 5–10, sweat onset and heart rate improve rapidly, cutting effective risk by roughly a third compared to Day 1. By Days 10–14, most healthy adults reach near-full acclimatization, and safe exposure duration at a given WBGT can increase by 50% or more compared to an unacclimatized worker.

Acclimatization fades within one to two weeks of reduced heat exposure, so a worker returning from a week of vacation, illness, or indoor duty needs a partial re-ramp before resuming full exposure limits. Seasonal transitions matter too, since the first hot week of spring poses higher risk than an equally hot week in August, when most outdoor workers have already adapted.

Apply a risk-score reduction of roughly 15–20 points for a fully acclimatized individual under identical environmental conditions, and add that reduction back for anyone in their first week of heat exposure.

Medications and Heat Tolerance

Certain medications and substances measurably impair the body’s ability to regulate heat, independent of fitness or acclimatization status.

Diuretics increase fluid loss through urination, which compounds sweat-related dehydration and raises heat stroke risk. Antihistamines and anticholinergic medications suppress sweat gland activity, which removes the body’s primary cooling mechanism. Certain psychiatric medications, including some antidepressants and antipsychotics, interfere with the hypothalamus’s temperature-regulation signals. Alcohol compounds all three effects by increasing fluid loss while impairing judgment about exposure limits.

Flag any of these medications during a pre-shift or pre-training health screening, and treat a positive flag as an automatic upward adjustment to the individual’s calculated heat stroke risk score.

Sweat Rate and Electrolyte Replacement

Sweat rate varies by body size, fitness, heat acclimatization, and exertion intensity, typically ranging from 0.8 to 1.4 liters (27–47 oz) per hour during moderate outdoor labor in hot conditions. The National Academies of Sciences, Engineering, and Medicine sets general daily fluid intake benchmarks at 3.7 liters (about 16 cups) for men and 2.7 liters (about 11 cups) for women, and outdoor heat exposure raises those baselines significantly.

Sweat sodium concentration falls into three tiers, based on sweat-testing research published by the Gatorade Sports Science Institute. Low-sodium sweat contains under 20 mmol/L and mainly needs plain water replacement. Average sweat contains 20–60 mmol/L and benefits from a standard electrolyte drink. High-sodium “salty sweater” sweat exceeds 60 mmol/L, identifiable by visible white salt residue on skin or clothing after exercise, and requires higher-sodium replacement fluids.

Replace fluids at a rate matching measured sweat loss, since both underhydration and overhydration carry risk. Drinking far more plain water than sweat lost can cause hyponatremia, a dangerous drop in blood sodium that produces symptoms similar to dehydration. Compare your daily energy and fluid needs with a TDEE calculator to build a hydration plan around your actual activity level, not a generic average.

Work-Rest Cycle Planning

OSHA and ACGIH publish work-rest cycle guidelines based on WBGT and workload intensity, expressed as the percentage of each hour spent working versus resting in shade or air conditioning. This guidance appears in the OSHA Technical Manual, Section III, Chapter 4, and in the ACGIH Threshold Limit Values (TLV) documentation for heat stress.

Apply the heat-acclimatization adjustment from the earlier section by shifting an unacclimatized worker one full row down this table, since their effective safe threshold sits roughly one WBGT tier lower than a fully acclimatized worker.

Cooling System Sizing

Cooling tents, break trailers, and vehicle cabins need enough cooling capacity to offset both the outdoor heat load and the metabolic heat generated by the people inside.

The cooling load formula multiplies the required air-changes-per-hour (ACH) by air density, specific heat, and the temperature difference between outdoor and target indoor conditions, then applies a 1.3 latent-heat multiplier to account for moisture removal alongside temperature reduction. This calculation is a direct application of the thermodynamic work formula used in a general-purpose work calculator, adapted for airflow instead of mechanical displacement.

For a detailed ACH-to-BTU breakdown for a specific tent or trailer size, use a dedicated CFM calculator to convert your required airflow into the cooling unit capacity you need to purchase. For a broader look at how thermal energy moves through a system, a kinetic energy calculator covers the related physics of energy transfer that underpins cooling-load math.

Post-Exposure Recovery Time

Core temperature returns to baseline following Newton’s Law of Cooling, which states that the rate of temperature loss is proportional to the difference between current body temperature and ambient temperature.

Recovery time depends heavily on the cooling environment. A person moved to shade with a breeze recovers baseline core temperature in 20–40 minutes after moderate heat strain. The same person moved to air conditioning recovers in 10–20 minutes. Active cooling methods like ice water immersion cut recovery time to under 10 minutes for severe cases, which is why ice immersion remains the gold-standard treatment for suspected heat stroke.

Understanding the Safety Decision Engine

The final safety decision pulls three inputs together into one Go or No-Go call: the ambient heat index or WBGT score, the WBGT-based work-rest tier, and the individual heat stroke risk score.

The decision engine flags “Go With Caution” when the combined risk score sits between 30% and 45%, meaning work can continue with mandatory rest breaks, hydration checks, and buddy monitoring. The engine flags “No-Go / Suspended” once the combined score crosses 45%, meaning outdoor activity should stop until conditions improve or additional cooling and acclimatization measures are in place.

If a user has not completed every upstream input, the engine defaults to the more conservative “Go With Caution” recommendation rather than a full Go, since missing data should never produce a false sense of safety.

Practical Example 1: Commercial Paving Crew

A five-person asphalt paving crew works an 8-hour shift starting at 7 a.m. in direct sun.

By 1 p.m., air temperature reaches 96°F (36°C) with 55% relative humidity, producing a heat index of 116°F (47°C), which falls in the Danger category. The crew’s heavy labor rating produces an estimated MET of 6.5, and their duty uniforms carry a Clo (clothing insulation) value of 0.6, both of which raise their WBGT-equivalent exposure above the plain heat index reading.

Combining a MET of 6.5, a WBGT near 89°F (32°C), and two crew members in their first week on the job (unacclimatized), the calculator returns individual risk scores of 58–72, placing most of the crew in the Severe Strain category. The work-rest table calls for 25% work and 75% rest at this WBGT and workload tier. The decision engine returns “No-Go / Suspended” for the two unacclimatized workers and “Go With Caution” for the three acclimatized crew members, recommending a rotating schedule where the newer workers take extended shade breaks while acclimatized crew members continue at the reduced work-rest ratio.

Practical Example 2: Military Physical Training in High Heat

A group of 20 trainees completes a 6-mile (9.7 km) ruck march carrying 35 lb (16 kg) packs starting at 6 a.m.

Conditions reach 88°F (31°C) with 70% relative humidity by 9 a.m., producing a heat index of 103°F (39°C) and a WBGT of 85°F (29°C) once solar load and wind are factored in. Ruck marching at this pace and load produces a MET of 8, one of the highest exertion levels covered in this guide, and standard duty uniforms carry a Clo value of 0.5.

At a MET of 8 and WBGT of 85°F, the risk scoring model returns scores of 62–78 for trainees in weeks 1–2 of training and 40–52 for trainees past their third week of acclimatized conditioning. The work-rest table places this scenario at 25% work / 75% rest for heavy work at this WBGT tier. Military heat category flag systems typically match this WBGT range to “Category 4,” which caps continuous work cycles at 30 minutes with mandatory water intake of at least 1 quart (0.95 L) per hour. The decision engine returns “Go With Caution” for acclimatized trainees under close monitoring and recommends postponing the march for unacclimatized trainees until early-morning conditions extend the safe window.

Heat-Related Illness Stages

Heat illness progresses through distinct clinical stages, and recognizing early stages prevents progression to the most dangerous ones.

Heat stroke is a medical emergency. Call emergency medical services immediately and begin active cooling, such as ice water immersion or wet towels with fanning, while waiting for help to arrive.

Long-Term Effects of Repeated Heat Exposure

Repeated heat exposure over months or years carries health risks beyond any single shift’s heat stroke risk score.

Chronic kidney disease of unknown origin (CKDu) has been documented in outdoor agricultural and construction workers who face repeated severe dehydration over years of employment. Research led by Dr. Catharina Wesseling and colleagues, studying Nicaraguan sugarcane workers, linked repeated heat-stress dehydration cycles to elevated rates of kidney damage independent of traditional risk factors like diabetes or hypertension. This condition is sometimes called Mesoamerican Nephropathy in occupational health literature, reflecting the population where it was first documented at scale.

Repeated dehydration cycles also correlate with higher rates of kidney stone formation, since concentrated urine over years of marginal hydration raises mineral crystallization risk. Treat consistent full hydration, not just single-shift compliance, as a long-term health requirement for anyone working outdoor heat exposure jobs across multiple seasons.

UV Index and Sunburn Risk

Outdoor heat exposure typically coincides with high UV exposure, and sunburned skin loses some of its ability to regulate temperature through blood vessel dilation. Check the UV Index alongside the heat index for any outdoor shift longer than one hour, and apply SPF 30 or higher sunscreen as a standard part of heat safety planning, not a separate concern.

Climate Trends and Extreme Heat

Heat waves have grown longer and more frequent across most regions over the past several decades, driven by broader climate change trends. Urban heat island effects add another 1–7°F (0.5–4°C) on top of surrounding rural temperatures in dense city centers, since concrete and asphalt absorb and re-radiate heat that vegetation would otherwise dissipate. Check your local Heat Advisory status through NOAA, Weather.gov, or a trusted forecast source like AccuWeather or The Weather Channel before planning extended outdoor exposure, since advisory thresholds account for local acclimatization norms that a single national heat index chart cannot capture. Many forecast providers now build a heat index app directly into their mobile platforms, pulling live NOAA data automatically rather than requiring a manual formula lookup.

Common Mistakes to Avoid

Avoid these frequent errors when using a heat index calculator for safety planning.

• Reading heat index in the shade and applying it to direct-sun conditions without adjustment
• Ignoring wind speed, which can lower effective heat stress by 5–10°F (3–6°C) at moderate speeds
• Treating one day’s acclimatization as permanent, when it fades within one to two weeks of reduced exposure
• Replacing sweat loss with plain water only, risking hyponatremia during multi-hour high-sweat-rate exposure
• Skipping a medication screening for diuretics, antihistamines, or psychiatric medications before high-heat shifts

Pro Tips for Accurate Results

Measure relative humidity with a local sensor rather than a regional forecast, since humidity can vary by 10–15 percentage points across a few miles depending on nearby water, vegetation, or pavement. Recalculate heat index and WBGT every two hours during a shift, since afternoon peaks often arrive later than air temperature alone would suggest. Log individual risk scores per worker rather than relying on one crew-wide average, since age, medication use, and acclimatization status can shift an individual’s real risk by 20 points or more from the group average.

Glossary of Heat Safety Terms

Heat Index (HI): The apparent temperature calculated from air temperature and relative humidity, also called the “feels like” temperature.

Apparent Temperature: A general term for any calculated “feels like” reading, including heat index, WBGT, and Humidex.

Wet-Bulb Globe Temperature (WBGT): A heat-stress index that adds wind speed and solar radiation to temperature and humidity, used by OSHA and ACGIH for occupational limits.

Dew Point: The temperature at which air becomes fully saturated with water vapor, used to calculate relative humidity and vapor pressure deficit.

Metabolic Equivalent of Task (MET): A unit measuring exertion intensity relative to resting metabolic rate; higher MET values mean more metabolic heat generated per hour.

Clo: A unit measuring clothing insulation; higher Clo values trap more body heat and raise effective heat stress.

Heat Acclimatization: The physiological adaptation to repeated heat exposure over 7–14 days, including earlier sweat onset and expanded blood plasma volume.

Vapor Pressure Deficit (VPD): The difference between the amount of moisture air can hold and the amount it currently holds, which drives evaporative cooling efficiency.

Humidex: Canada’s heat-stress index, calculated from air temperature and dew point.

Methodology and Sources

This calculator’s heat index formula follows the National Weather Service’s official Rothfusz regression, developed by Lans P. Rothfusz in 1990 from Steadman’s 1979 physiological heat-stress model. Work-rest cycle guidance follows the OSHA Technical Manual (Section III, Chapter 4) and the ACGIH Threshold Limit Values for heat stress. Recovery time modeling applies Newton’s Law of Cooling, a standard thermodynamic principle. Sweat sodium tier guidance reflects sweat-testing research from the Gatorade Sports Science Institute. Long-term kidney-health guidance references research on Mesoamerican Nephropathy led by Dr. Catharina Wesseling. This guide summarizes established public formulas and guidelines; it does not replace medical advice or an employer’s official heat-safety program.

Frequently Asked Questions

What is the difference between heat index and WBGT?

Heat index uses only air temperature and relative humidity, while WBGT adds wind speed and solar radiation. WBGT gives a more accurate outdoor heat-stress reading, which is why OSHA and ACGIH use WBGT rather than heat index for occupational work-rest limits.

How does clothing affect heat stress?

Clothing insulation, measured in Clo units, traps body heat and blocks evaporative cooling, raising effective heat stress above the ambient heat index reading. A standard duty uniform with a Clo value of 0.5–0.6 can raise perceived WBGT by 3–5°F (2–3°C) compared to light athletic clothing.

How much sodium do I lose sweating?

Sweat sodium concentration ranges from under 20 mmol/L for low-sodium sweaters to over 60 mmol/L for high-sodium “salty sweaters,” with most people falling in the 20–60 mmol/L average range. A visible white salt residue on skin or clothing after exercise signals high-sodium sweat loss.

What heat index is dangerous?

A heat index above 103°F (39°C) falls in the Danger category, where heat exhaustion becomes likely with continued exposure. A heat index above 125°F (52°C) falls in the Extreme Danger category, where heat stroke becomes highly likely.

Can you get heat stroke in the shade?

Yes, heat stroke can occur in the shade if humidity is high enough to block sweat evaporation and physical exertion generates enough metabolic heat. Shade lowers direct solar heat load but does not eliminate humidity-driven heat stress.

How long does heat acclimatization take?

Full heat acclimatization takes 10 to 14 days of repeated heat exposure for most healthy adults. The largest risk reduction happens in the first 5 to 10 days, as sweat onset speeds up and blood plasma volume expands.

Can certain medications increase heat stroke risk?

Yes, diuretics, antihistamines, anticholinergic medications, and certain psychiatric medications all measurably impair heat tolerance. Alcohol compounds these effects by increasing fluid loss and impairing judgment about safe exposure limits.

What’s the difference between heat exhaustion and heat stroke?

Heat exhaustion causes heavy sweating, weakness, and nausea while the body can still regulate temperature. Heat stroke causes core temperature above 104°F (40°C) with confusion and often dry skin, signaling the body’s cooling system has failed entirely and requiring emergency medical treatment.

How do I size a cooling tent for an outdoor event?

Calculate required air-changes-per-hour based on tent volume and expected occupancy, then convert that airflow requirement into cooling unit capacity using a CFM calculator. Apply a 1.3 latent-heat multiplier on top of the base temperature-difference calculation to account for humidity removal.

Does repeated heat exposure cause long-term health problems?

Yes, repeated severe dehydration from years of outdoor heat exposure is linked to chronic kidney disease and higher kidney stone rates. Research on outdoor agricultural workers documented this pattern, sometimes referred to as Mesoamerican Nephropathy in occupational health studies.

Conclusion

A heat index calculator turns two simple readings, temperature and humidity, into an apparent temperature that predicts how heat will actually feel and how dangerous it can become. The Rothfusz regression behind this calculator matches the exact formula the National Weather Service uses, not a simplified approximation.

Pair the heat index with WBGT for outdoor labor and athletic settings, since WBGT accounts for wind and sun that the base heat index ignores. Layer in heat acclimatization status, medication use, weighted pre-existing conditions, and individual fitness before trusting any single risk score, since these factors shift real risk well beyond what temperature and humidity alone can predict.

Use the work-rest tables, cooling-load formulas, and related tools in this guide to turn a raw heat index or WBGT reading into an actual safety decision, whether you manage a paving crew, a military training cycle, or a weekend athletic event.