How to use the Heat Index Calculator
- Enter the air temperature in Fahrenheit, measured in the shade.
- Enter the relative humidity as a percentage.
- Read the heat index in both Fahrenheit and Celsius.
- Check the risk category against your planned activity.
- Note how much hotter it feels than the raw temperature reading.
How the calculation works
The heat index estimates apparent temperature by accounting for the way humidity suppresses evaporative cooling. Sweat cools the body only when it evaporates, and evaporation slows as the air approaches saturation. At 90°F and 40% humidity the body copes reasonably; at 90°F and 85% humidity it feels close to 110°F because the primary cooling mechanism has largely stopped working.
The calculation uses the Rothfusz regression, a polynomial fit that the US National Weather Service adopted to approximate Steadman's much more complex 1979 biometeorological model. The regression is defined for temperatures at or above 80°F; below that, humidity has little effect and the raw temperature is reported instead, which is exactly what this calculator does.
The model assumes shade and light wind. In direct sunlight the effective heat index can be up to 15°F higher, which is why outdoor work guidance is stricter than the shaded figure suggests. It also assumes a typical adult; children, older people, those on certain medications and anyone unacclimatised face elevated risk at lower readings.
HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T² − 0.05481717R² + 0.00122874T²R + 0.00085282TR² − 0.00000199T²R², with T in °F and R in %Source: Rothfusz, L.P. (1990), The Heat Index Equation, NWS Southern Region Technical Attachment SR/SSD 90-23; based on Steadman (1979).
Worked example
An outdoor crew is scheduled at 95°F with 65% relative humidity.
- Enter T = 95°F, RH = 65%.
- The regression returns a heat index of about 121°F.
- That falls in the Danger band, where heat cramps and exhaustion are likely.
- In direct sun, add up to 15°F, pushing it towards Extreme Danger.
About 121°F apparent temperature — 26°F above the thermometer reading, and firmly in the range where work-rest cycles and mandatory hydration breaks are required.
Frequently asked questions
Why does the calculator ignore the formula below 80°F?+
The regression is only fitted above that point and returns nonsense below it. Under 80°F humidity has minimal effect on apparent temperature.
Is heat index the same as wet bulb globe temperature?+
No. WBGT also accounts for direct solar radiation and wind, and is the measure occupational safety standards actually specify for outdoor work.
What heat index is dangerous?+
Above 103°F is Danger and above 125°F is Extreme Danger under NWS categories, but individual tolerance varies widely with acclimatisation, age and health.
Does wind reduce the heat index?+
Slightly, by aiding evaporation, but only until air temperature exceeds skin temperature — beyond that, wind adds heat rather than removing it.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.