Heat Index Calculator (NWS Equation)
Enter the air temperature and relative humidity from your forecast or weather station to see the heat index, the "feels like" temperature the National Weather Service uses in its heat alerts. The math is the equation published by the NWS Weather Prediction Center.
This is an estimate, not a forecast or medical advice. The NWS says heat index values were devised for shade and light wind; full sunshine can raise them by up to 15 °F, and strong winds with very hot, dry air can be extremely hazardous. Follow your local NWS heat alerts, and take extra care with children, older adults, outdoor workers and anyone in poor health.
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Results
| Air temperature used | — |
|---|---|
| Relative humidity used | — |
| Heat index (°F, rounded) | — |
| Heat index (°C) | — |
| Method | — |
Formula used
- Simple formula first: HI = 0.5 × {T + 61.0 + [(T − 68.0) × 1.2] + (RH × 0.094)}, then averaged with T. If that average is below 80 °F, it's the answer.
- Otherwise the full NWS regression: HI = −42.379 + 2.04901523 T + 10.14333127 RH − 0.22475541 T·RH − 0.00683783 T² − 0.05481717 RH² + 0.00122874 T²·RH + 0.00085282 T·RH² − 0.00000199 T²·RH², with T in °F and RH in percent.
- Low humidity: if RH is below 13% and T is 80–112 °F, subtract [(13 − RH) ÷ 4] × √{[17 − |T − 95|] ÷ 17}.
- High humidity: if RH is above 85% and T is 80–87 °F, add [(RH − 85) ÷ 10] × [(87 − T) ÷ 5].
- Celsius inputs are converted first (°F = °C × 9/5 + 32). The NWS notes the regression isn't valid for extreme combinations beyond the range of its source data, so very high results are rough.
Worked example (from the NWS)
- An air temperature of 96 °F with 65% relative humidity gives a heat index of 121 °F.
For heat illness warning signs and what to do, read our heat index and heat safety guide.
Sources
- NWS Weather Prediction Center: The Heat Index Equation
- National Weather Service: Heat Forecast Tools (heat index)
Sources checked October 29, 2026.