Heat Index Calculator

Find how hot it feels when humidity is combined with the air temperature. The heat index calculator uses the US National Weather Service method and shows its warning level.

Updated
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Units

In the same unit as the air temperature

Heat index
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How to use the Heat Index Calculator

  1. Choose °F or °C.
  2. Enter the Air temperature and either the Relative humidity or the Dew point.
  3. Press Calculate. The heat index appears with the NWS warning level and both units.

How it works

The NWS method, in °F:

simple = 0.5 × (T + 61 + (T − 68) × 1.2 + RH × 0.094)
if (simple + T) ÷ 2 ≥ 80:
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

Small adjustments apply when humidity is below 13% or above 85%. A dew point is turned into relative humidity with the Magnus formula.

Examples

  • 90 °F at 70% humidity feels like 105.9 °F: the Danger level.
  • 32 °C at 60% humidity feels like 37.1 °C.
  • 85 °F at 40% humidity feels like 84.3 °F.

NWS heat index levels

  • Caution, 80-90 °F (27-32 °C): fatigue is possible with long exposure.
  • Extreme caution, 90-103 °F (32-39 °C): heat cramps and heat exhaustion are possible.
  • Danger, 103-124 °F (39-51 °C): heat exhaustion is likely.
  • Extreme danger, 125 °F (52 °C) and above: heat stroke is highly likely.

Limitations

  • Assumes shade and a light breeze. Direct sun can add up to 15 °F (8 °C).
  • Below 80 °F (27 °C) the index is close to the air temperature.

Frequently asked questions

What is the heat index?

A measure of how hot it feels when humidity slows sweat from evaporating.

What heat index is dangerous?

The NWS calls 103 °F (39 °C) and above Danger.

Why does humidity make it feel hotter?

Sweat cools the body by evaporating. Humid air slows that, so the body keeps more heat.

Often used together with the Heat Index Calculator.