Heat Index Calculator
Find the heat index, the “feels-like” temperature in hot, humid weather, from the air temperature and the relative humidity or dew point. Uses the US National Weather Service formula and risk levels.
How to Use
- Choose whether you know the relative humidity or the dew point.
- Enter the air temperature in °F or °C (measured in the shade), then the humidity in percent or the dew point.
- Read the heat index in your unit and the other one, the NWS risk level and how much hotter it feels than the air.
- The scale shows the heat index against the NWS caution, extreme caution, danger and extreme danger levels.
- Show Work gives the simple formula, which formula applies, each term of the Rothfusz regression and any adjustment.
Worked Example
90 °F and 50% humidity. The simple formula gives 0.5 × (90 + 61 + 22 × 1.2 + 0.094 × 50) = 91.05 °F. Its average with the temperature is 90.5 °F, which is 80 °F or more, so the Rothfusz regression is used: 94.6 °F, extreme caution. The NWS heat index chart shows 95 °F for the same conditions.
85 °F and 90% humidity. The regression gives 101.58 °F. Because the humidity is above 85% and the temperature is between 80 and 87 °F, the NWS adds ((90 − 85) ÷ 10) × ((87 − 85) ÷ 5) = 0.2 °F, for a heat index of 101.8 °F.
The common mistake: using the simple formula in real heat. At 95 °F and 65% the simple formula gives 97.3 °F, which looks only a little warmer than the air. It is meant for heat index values below about 80 °F. The full regression gives 117.6 °F, in the danger band, 20 °F higher.
Show Work
Formulas
Steadman’s Sultriness and Rothfusz’s Fit
The heat index comes from the work of Robert G. Steadman, who in 1979 published “The Assessment of Sultriness”. He modelled a clothed adult walking in the shade, with skin, clothing and breathing all exchanging heat with the air, and worked out the “apparent temperature” that would feel the same in dry air. His results were tables, not a formula.
In 1990 Lans P. Rothfusz of the US National Weather Service fitted a multiple regression to Steadman’s tables, published as NWS Technical Attachment SR 90-23. That nine-term polynomial, with the later adjustments for very dry and very humid air and the simple formula for mild conditions, is what the NWS uses for its heat advisories and what this calculator follows.
About This Calculator
This calculator follows the National Weather Service procedure step by step: the simple formula, the check against 80 °F, the Rothfusz regression and the two humidity adjustments. You can give the humidity as relative humidity or as a dew point (converted with the Magnus formula), and enter temperatures in °F or °C; the regression itself runs in °F, as published.
The result is classified with the NWS risk levels and drawn on a scale beside the air temperature. The heat index is for shade and light wind; full sun can add up to 15 °F, and the NWS chart only covers air temperatures up to 110 °F, so values beyond it are extrapolated. It is a guide, not medical advice.
All calculations run in your browser; nothing you enter is sent anywhere.
Related tools: Dew Point Calculator, Wind Chill Calculator, and Unit Converter.
Frequently Asked Questions
What is the heat index?
It is how hot the air feels to the human body when humidity is added to the air temperature, because damp air slows the cooling evaporation of sweat. It assumes shade and a light wind. At 95 °F (35 °C) and 65% relative humidity the heat index is 117.6 °F (47.6 °C).
How is the heat index calculated?
The NWS first uses a simple formula, 0.5 × (T + 61 + (T − 68) × 1.2 + 0.094 × RH). If the average of that and the temperature is 80 °F or more, it uses the Rothfusz regression, a nine-term polynomial in T and RH, and then adds a small correction for very dry (below 13%) or very humid (above 85%) air in certain temperature ranges. At 90 °F and 50% the result is 94.6 °F; the NWS chart shows 95.
What do the heat index risk levels mean?
The NWS uses four levels: caution from 80 to 90 °F (fatigue possible), extreme caution from 90 to 103 °F (heat cramps and exhaustion possible), danger from 103 to 125 °F (heat exhaustion likely, heat stroke possible), and extreme danger at 125 °F and above (heat stroke highly likely).
Can the heat index be lower than the air temperature?
Yes, in very dry air. At 100 °F and 10% humidity the regression gives 94.75 °F and the dry-air adjustment takes off another 0.63 °F, so it feels like 94.1 °F: sweat evaporates fast and cools you well.
Does sunshine change the heat index?
The heat index is worked out for shade. The NWS notes that full sunshine can raise heat index values by up to 15 °F, so in direct sun a 95 °F heat index can feel like as much as 110 °F.
How do I use the Heat Index Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
Outdoor work planning
90 °F with 70% humidity is a heat index of 105.9 °F, in the NWS danger band: schedule breaks, water and shade.
Sports practice
Coaches check the heat index before training; a muggy 88 °F at 90% humidity feels like 113.2 °F.
Travel to humid places
30 °C with 70% humidity feels like 35.0 °C, and 32 °C with 80% like 44.4 °C.
Weather and safety reports
Turn a forecast temperature and dew point into the heat index: 90 °F with a 75 °F dew point gives 100.6 °F.
Science classes
See how the regression terms add up, and why the simple formula is used only below about 80 °F.
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