Heat Index Calculator

Use this Heat Index Calculator to calculate the heat index (HI) based on air temperature (T) and relative humidity (RH).

How to use this calculator

1. Choose the temperature unit between degrees Fahrenheit, degrees Celsius, or Kelvin
2. Enter the air temperature
3. Enter the relative humidity and click the "Calculate" button to determine the heat index temperature (i.e., how hot it feels).

Heat Index Formula

This calculator uses the following basic formula to calculate the heat index:

HI (Heat Index, °F) = C₁ + C₂ T + C₃ RH + C₄ T RH + C₅ T² + C₆ RH² + C₇ T² RH + C₈ T RH² + C₉ T² RH² + CF

Where:

HI is the heat index (°F),

T is the air temperature in Fahrenheit (°F),

RH is the relative humidity (%),

CF is the correction factor,

C₁ = -42.379,

C₂ = 2.04901523,

C₃ = 10.14333127,

C₄ = -0.22475541,

C₅ = -6.83783 × 10 ^-3,

C₆ = -5.481717 × 10 ^-2,

C₇ = 1.22874 × 10 ^-3,

C₈ = 8.5282 × 10 ^-4,

C₉ = -1.99 × 10 ^-6.

If 80°F ≤ T ≤ 112°F, RH ≤ 13%; CF (Correction Factor) = - ([13 - RH] / 4) × ([17 - |T - 95|] / 17)^0.5

If 80°F ≤ T ≤ 87°F, RH > 85%; CF (Correction Factor) = 0.02 × (RH - 85) × (87 - T)

Example: If the temperature is 90°F and the relative humidity is 80%, the heat index is:

HI (Heat Index, °F) = -42.379 + 2.04901523 × 90 + 10.14333127 × 80
- 0.22475541 × 90 × 80 - 6.83783 × 10 ^-3 × 90² - 5.481717 × 10 ^-2 × 80²
+ 1.22874 × 10 ^-3 × 90² × 80 + 8.5282 × 10 ^-4 × 90 × 80² - 1.99 × 10 ^-6 × 90² × 80²

Heat Index = -42.379 + 184.4113707 + 811.4665016
- 1618.238952 - 55.386423 - 350.829888
+ 796.22352 + 491.22432 - 103.1616 = 113.3298493 degrees Fahrenheit.

You may also like to try our Wind Chill Calculator or Dew Point Calculator