Heat Index Calculator

How this is calculated

THIS IS THE NWS EQUATION, NOT AN APPROXIMATION OF IT. The heat index is the Rothfusz regression, a curve fitted to Steadman's 1979 human-comfort model: HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783TT - 0.05481717RR + 0.00122874TTR + 0.00085282TRR - 0.00000199TTRR where T is temperature in F and R is relative humidity in percent. ⚠️ THE HEAT INDEX IS A SHADE MEASUREMENT. The NWS is explicit that the values 'were devised for shady, light wind conditions, and exposure to full sunshine can increase heat index values by up to 15 degrees F.' That is not a rounding error - it is the difference between Extreme Caution and Danger. The full-sun figure is shown here as a separate output precisely so it cannot be mistaken for the headline number. THERE ARE THREE FORMULAS, NOT ONE, AND THE OTHER TWO ARE USUALLY LEFT OUT. The regression misbehaves at the edges, so the NWS procedure wraps it: 1. A simple form is computed first - 0.5 x (T + 61 + (T-68) x 1.2 + R x 0.094) - and averaged with the air temperature. If that average is below 80 F, the simple form IS the answer and the regression is never used. 2. LOW HUMIDITY: if R is below 13% and T is between 80 and 112 F, subtract ((13-R)/4) x sqrt((17-|T-95|)/17). Dry desert air genuinely feels cooler than the raw regression says. 3. HIGH HUMIDITY: if R is above 85% and T is between 80 and 87 F, add ((R-85)/10) x ((87-T)/5). Both adjustments are implemented here. A calculator that omits them is wrong exactly where the weather is most unusual, which is when people look it up. THE NWS BANDS: Caution 80-90 F (fatigue possible with prolonged exposure or activity), Extreme Caution 90-103 F (heat cramps, heat exhaustion or heat stroke possible), Danger 103-124 F (cramps or exhaustion likely, heat stroke possible), Extreme Danger 125 F and above (heat stroke highly likely). Every one of those descriptions carries the phrase 'prolonged exposure and/or physical activity' - the band is about what you are DOING in it, not just standing in it. ⚠️ WHY HUMIDITY MATTERS SO MUCH IS THAT SWEATING IS THE WHOLE MECHANISM. Your body sheds heat by evaporating sweat, and evaporation slows as the air fills with water. At high humidity the cooling system simply stops working well, which is why 90 F at 70% humidity is more dangerous than 100 F in dry air. The temperature reading alone genuinely does not tell you how hard your body is working. THIS DESCRIBES AIR, NOT A PERSON. The equation assumes a notional adult in the shade. Age, medication, heart and lung conditions, acclimatisation and how hard you are working all move real risk substantially, and none of them is an input here.

Common questions

Why does the heat index assume shade?

Because the model it comes from does. The National Weather Service states plainly that heat index values 'were devised for shady, light wind conditions, and exposure to full sunshine can increase heat index values by up to 15 degrees F'. That is why this calculator shows the full-sun figure as its own output rather than folding it in - it is an upper bound rather than an average, so averaging it into the headline number would misrepresent both. If you are working or exercising in direct sun, the sun figure is the one that describes your afternoon.

Why is there more than one formula?

Because the regression is a curve fitted to a model, and fitted curves misbehave at their edges. The NWS procedure therefore wraps it in three parts: a simple formula used when the result would be below about 80 F, a subtraction when humidity is under 13% at temperatures between 80 and 112 F, and an addition when humidity is over 85% between 80 and 87 F. All three are implemented here. Most calculators implement only the regression, which makes them wrong precisely in unusual weather - which is exactly when somebody bothers to look the number up.

Why can the heat index be LOWER than the air temperature?

Because dry air helps you. Sweating cools you by evaporation, and evaporation is fast when the air is dry, so in low humidity your body sheds heat more easily than the raw temperature suggests. The 'hotter than the air temperature' output goes negative in those conditions and that is correct, not a bug. It is also why 100 F in Arizona and 100 F in Florida are genuinely different experiences rather than a matter of opinion.

What do the NWS bands actually mean?

Caution is 80-103 F split into two: 80-90 F where fatigue is possible with prolonged exposure or activity, and Extreme Caution 90-103 F where heat cramps, heat exhaustion or heat stroke become possible. Danger is 103-124 F, where cramps or exhaustion are LIKELY and heat stroke is possible. Extreme Danger is 125 F and above, where heat stroke is highly likely. The wording that matters is in every one of them: 'prolonged exposure and/or physical activity'. The bands describe risk while you are doing something, not risk from standing still in the shade.

Why does humidity matter so much more than it seems like it should?

Because sweating is the entire cooling mechanism, and humidity is what stops it working. Your body does not cool by making sweat, it cools by evaporating it - and evaporation slows as the surrounding air fills with water. Above roughly 75% humidity the system is badly impaired, and the heat your muscles produce has nowhere to go. That is the reason 90 F at 70% humidity is more dangerous than 100 F in dry desert air, and why the air temperature on its own genuinely does not tell you how hard your body is working.

Does this account for wind?

No, and neither does the NWS heat index. The equation assumes light wind. Moving air normally helps by carrying evaporated sweat away, which is why a fan feels good - but the NWS also warns that strong wind with very hot, dry air can be hazardous rather than helpful, because it can heat you faster than the evaporation cools you. Above roughly body temperature, wind stops being reliable relief. There is no wind input here because adding one would require a different model, not a correction to this one.

Is this a medical assessment?

No. It describes AIR, not a person. The equation assumes a notional adult in the shade, and the things that actually decide who gets into trouble on a hot day are not in it: age, heart and lung conditions, many common medications, whether you are acclimatised, whether you are drinking, and how hard you are working. Older adults, young children and anyone on medication that affects sweating or fluid balance are at meaningfully higher risk at the same heat index. If someone stops sweating, becomes confused or faints in the heat, that is an emergency rather than a number to look up.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Heat Index Calculator at https://www.bfcbrilliance.com/tools/heat-index-calculator.

What I entered:
- Air temperature (°F): ___
- Relative humidity (%): ___

What it calculated:
- Heat index - what it feels like: ___
- Hotter than the air temperature: ___
- In full sun, up to: ___
- Above the NWS Caution line (80°F): ___
- Above the NWS Danger line (103°F): ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

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