Calculate the feels-like apparent temperature using the official NWS Rothfusz regression with solar exposure adjustments, OSHA heat stress guidelines, and reference matrix.
Climate Presets:
Conditions
Actual air temperature in °F. The NWS regression applies at 80°F and above.
Relative humidity percentage, from 0 to 100%.
Result
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Enter conditions above to compute.
Dew Point:—
Steadman Apparent Temp:—
Heat-Stress Screening:—
Heat Index Risk Categories
Current risk—
Enter conditions on the Heat Index tab to see your risk category.
OSHA Outdoor Work & Exertion Guidance
Heat index is an environmental screening estimate, not an OSHA work/rest determination. Occupational decisions also require workload, acclimatization, clothing/PPE, recovery conditions, and preferably WBGT.
Work/Rest Recommendation
Enter weather conditions to generate OSHA work/rest schedules.
Hydration Advisory
Standard recommendation: 8 oz water every 20 minutes during exertion.
Workplace Precautions
Enforce buddy system and monitor outdoor workers for heat exhaustion symptoms.
NWS Heat Index Chart
Official NWS heat index by air temperature and relative humidity. Your current conditions are outlined.
Method, Sources & Safety Limits
Last verified: July 2026
Next review: July 2027 or sooner if official guidance changes
This calculator uses the National Weather Service heat-index methodology for a shade-based screening estimate. Direct sun is shown as a clearly labeled approximation; it is not a WBGT calculation.
This tool is educational and cannot diagnose heat illness or determine a workplace work/rest schedule. Stop activity and seek urgent medical help for confusion, fainting, seizures, loss of consciousness, or suspected heat stroke.
The heat index tells you how hot the air actually feels once humidity is factored in, because high humidity slows the evaporation of sweat that your body relies on to cool itself.
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Walk-through
How to Use This Calculator
3 steps▸
1
Enter the air temperature
Type the actual air temperature in the shade — not a temperature already adjusted for humidity. Toggle between °F and °C with the unit buttons above the input.
2
Enter the relative humidity
Enter the relative humidity as a percentage, from 0 to 100. Most local weather reports list this alongside temperature.
3
Read the feels-like result and risk level
The result card shows the heat index — how hot it actually feels — along with the NWS risk category. Switch to the Risk Level tab for guidance, or the Chart Reference tab to see the full NWS lookup table.
T is air temperature in °F and R is relative humidity as a percentage (0–100). This multiple-regression equation, developed by Lans Rothfusz for the National Weather Service, approximates Steadman's 1979 apparent-temperature model. It is only used once the simpler average-based estimate ((T + simple estimate) / 2) reaches 80°F or higher; below that, the simpler estimate is used directly. Two adjustments are applied at the edges of the valid range: heat index is reduced when humidity is below 13% (temperature between 80–12°F), and increased when humidity is above 85% (temperature between 80–87°F), matching the official NWS implementation.
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Glossary
Key Terms Explained
7 terms▸
Heat indexA single number that combines air temperature and relative humidity into how hot the air actually feels to the human body. Also called the apparent temperature.
Feels-like temperatureThe everyday name for heat index (in hot conditions) or wind chill (in cold conditions) — the temperature your body perceives rather than the thermometer reading.
Apparent temperatureThe technical term for feels-like temperature. Heat index is the National Weather Service's apparent-temperature model for warm, humid conditions.
Relative humidityThe amount of water vapor in the air compared to the maximum the air could hold at that temperature, shown as a percentage. Higher humidity slows sweat evaporation, which is why it makes heat feel more intense.
NWSThe National Weather Service, the U.S. government agency that develops and publishes the official heat index formula and risk-category thresholds used by this calculator.
Heatstroke riskThe likelihood of heat-related illness — ranging from heat cramps and heat exhaustion to life-threatening heat stroke — which rises sharply as heat index climbs through the NWS risk categories.
Shade assumptionThe published heat index value assumes the person is in shade with light wind. Direct sunlight can add another 10–15°F to how hot conditions actually feel.
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Scenarios
Real-World Examples
3 worked examples▸
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Summer afternoon
90°F at 70% relative humidity
Air temperature 90°FRelative humidity 70%
The heat index comes out to about 106°F — solidly in the NWS “Danger” category. Heat cramps or heat exhaustion are likely, and heat stroke is possible with prolonged outdoor activity, so strenuous exercise should be postponed until conditions cool.
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Extreme heat wave
100°F at 60% relative humidity
Air temperature 100°FRelative humidity 60%
The heat index climbs to roughly 129°F, well into “Extreme Danger.” Heat stroke is highly likely with any outdoor exertion — the NWS guidance at this level is to avoid outdoor activity entirely and get to air conditioning.
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Mild spring day
72°F at 65% relative humidity
Air temperature 72°FRelative humidity 65%
Below the 80°F threshold, the NWS heat index formula does not apply — there is no meaningful added heat risk from humidity at this temperature, so the calculator shows the actual temperature with no risk category.
The heat index tells you how hot the air actually feels once humidity is factored in, because high humidity slows the evaporation of sweat that your body relies on to cool itself. This calculator uses the same Rothfusz regression the National Weather Service uses to issue heat advisories, so the feels-like number and risk category match what you would see on an official forecast.
How the Heat Index Calculator works
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The calculator applies the NWS Rothfusz regression, a multiple-regression equation fit to Steadman's 1979 apparent-temperature model. Below 80°F the formula isn't used at all — heat index only becomes meaningful once temperature and humidity combine to create measurable heat stress. Two correction terms are applied at the edges of the formula's valid range: a cooling adjustment in very dry air (relative humidity under 13%, temperature between 80–12°F) and a warming adjustment in very humid air (relative humidity over 85%, temperature between 80–87°F). These corrections are part of the official NWS implementation and keep the result accurate at the boundaries where the base regression is less reliable.
Inputs and what they mean
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Air temperature should be the actual, unadjusted reading in the shade — not an already-adjusted feels-like value. Relative humidity is the percentage reading from a local weather station or forecast, typically highest in early morning and lowest in mid-afternoon. Small changes in humidity can shift the heat index by several degrees, especially at high temperatures, so it's worth checking a current reading rather than guessing.
Limits and edge cases
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The published heat index assumes shade and light wind; direct sunlight can add another 10–15°F to how conditions actually feel, and this calculator does not account for wind speed, individual clothing, body size, or activity level the way a full thermal-comfort model would. Below 80°F the formula is not defined and the calculator will not compute a heat index. For very hot, dry climates the analogous "wet-bulb globe temperature" used by some athletic and military guidelines can differ from heat index — treat heat index as a general public-safety guide, not a substitute for local heat advisories from the National Weather Service.
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Questions
Frequently Asked Questions
6 questions▸
What formula does the Heat Index Calculator use?+
It uses the National Weather Service's Rothfusz regression, the same equation used to issue official heat advisories in the United States. Below 80°F the calculator does not apply the regression, since heat index is not meaningfully defined at cooler temperatures.
What does “feels like” mean?+
It's the temperature your body perceives once humidity is factored in, rather than the raw thermometer reading. High humidity slows sweat evaporation, which is the body's main cooling mechanism, so hot and humid air feels considerably hotter than the actual air temperature.
What's the valid range for this calculator?+
The NWS heat index formula is valid for air temperatures of 80°F (about 26.7°C) and above. Below that threshold, humidity does not add meaningful heat stress, so the calculator shows the actual temperature with no risk category instead of a computed heat index.
What do the risk categories mean?+
The NWS defines four heat index risk categories — Caution, Extreme Caution, Danger, and Extreme Danger — each with its own guidance on activity level, hydration, and heat-illness symptoms to watch for. Switch to the Risk Level tab to see the full breakdown and where your current conditions fall.
Does the heat index assume I'm in the shade?+
Yes. The published heat index value assumes shade and light wind. If you're standing in direct sunlight, the actual feels-like temperature can be 10–15°F higher than the calculated heat index.
How is heat index different from the humidex?+
Both combine temperature and humidity into a single feels-like number, but they use different formulas and scales. Humidex is used in Canada and is derived from dew point rather than relative humidity; heat index is the U.S. National Weather Service standard used by this calculator. The two produce similar but not identical results for the same conditions.
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