Heat Index Calculator

The heat index, or “feels like” temperature, combines air temperature and relative humidity into the thermal stress the body actually experiences. Sweating only cools you when the sweat evaporates, and humid air is already close to holding as much water as it can. That is why a dry 100°F is uncomfortable and a humid 90°F sends people to hospital.

Formula (Rothfusz regression equation, used by NOAA): HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T×RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²×RH + 0.00085282T×RH² − 0.00000199T²×RH²

Where T = temperature in °F and RH = relative humidity in %.

Simplified formula (accurate within ±1.3°F for most conditions): HI ≈ 0.5 × (T + 61.0 + ((T − 68.0) × 1.2) + (RH × 0.094))

When heat index applies: The heat index formula is meaningful only when temperature ≥ 80°F (27°C) and relative humidity ≥ 40%. Below these thresholds, humidity has little perceptible effect.

Worked example: Air temperature 95°F, relative humidity 65%.

The simplified equation gives HI ≈ 0.5 × (95 + 61.0 + ((95 − 68) × 1.2) + (65 × 0.094)) = 0.5 × (95 + 61 + 32.4 + 6.11) = 0.5 × 194.51 = 97.3°F

The full Rothfusz regression gives 117.6°F, and that is the figure this page reports. The 20-degree gap between them is the point: the simplified formula is a first pass that the National Weather Service uses only to decide whether the full equation is needed at all. It is close enough below 80°F and badly optimistic in exactly the conditions that hurt people. Anywhere you see a rule-of-thumb heat index quoted, check which one it is.

NOAA Heat Index categories:

Celsius conversion: Subtract 32, multiply by 5/9 to convert the result to °C.

Protective measures: Stay hydrated (drink before you’re thirsty), seek shade or air conditioning during peak hours (10 AM–4 PM), wear light-colored loose clothing, and never leave children or pets in parked vehicles.