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Heat Index Calculator

Calculate how hot it actually feels using the National Weather Service's heat index formula, with the NWS Caution-to-Extreme-Danger risk category.

Heat index
99.7°F (37.6°C)
Risk category
Extreme caution
Air temperature
32.2°C

Where this falls

99.7 falls in the "Extreme caution" range.

How it works

This uses the U.S. National Weather Service's Rothfusz regression, the standard 'feels like' formula behind heat advisories: HI = -42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T² − 0.05481717R² + 0.00122874T²R + 0.00085282TR² − 0.00000199T²R², where T is air temperature in °F and R is relative humidity in percent — verified via web search against multiple independent sources and cross-checked numerically against the well-known ~106°F result at 90°F/70% humidity.

Two correction terms apply at the extremes the base polynomial handles poorly: in low humidity (under 13%, between 80-112°F) an amount is subtracted because the polynomial otherwise overstates the heat effect; in high humidity (over 85%, between 80-87°F) an amount is added because it otherwise understates it. Below conditions that would produce roughly an 80°F result, the NWS instead uses a simpler formula rather than the full regression, which isn't fit for milder conditions.

The result is also classified into the NWS's four heat-risk categories — Caution (80-90°F), Extreme Caution (90-103°F), Danger (103-125°F), and Extreme Danger (125°F+) — shown on a gauge alongside the precise number, the same fixed-band classification pattern this site's BMI calculator uses.

FAQ

Why is there no metric/imperial toggle for temperature?

The NWS formula's coefficients were fit specifically to Fahrenheit and percent relative humidity, so converting temperature to Celsius first and running the same formula would give a wrong answer — the same reason this site's Wind Chill calculator has no toggle either. The result is also shown in Celsius for convenience, but the calculation itself always runs in °F.

Why does the calculator reject temperatures below 80°F?

Below about 80°F, humidity doesn't meaningfully change how the air feels, and the Rothfusz regression wasn't fit to those conditions — rather than extrapolate a formula outside its validated range, the calculator treats anything below 80°F as out of scope.

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