How this calculator works
- Step 1Convert°C → °FThe formula works in Fahrenheit.
- Step 2Simple estimateSteadmanUsed when it feels below 80 °F.
- Step 3Full regressionRothfuszUsed above 80 °F, with humidity adjustments.
Humid air slows the cooling of sweat, so it feels hotter than the thermometer shows. The heat index puts a number on that feeling.
The calculator follows the US National Weather Service algorithm step by step, including its corrections for very dry and very humid air.
Worked example: 35 °C at 60% humidity
- Rothfusz (1990)
HI = −42.379 + 2.049T + 10.143RH − … (°F)
- TTemperature 35 °C = 95 °F
- RHRelative humidity 60%
- Simple estimateaverage with Tabove 80 °F
- RegressionRothfusz, no adjustment113 °F
- Heat indexback to °C45 °C (113 °F)
That is in the NWS “Danger” band. Heat cramps or exhaustion become likely with long exposure or hard work.
Humidity changes everything
- 96 °F, 65%121 °FNWS: feels like 121 °F.
- 100 °F, 55%124 °FNWS: feels like 124 °F.
- 100 °F, 15%96 °FNWS: dry air feels cooler, 96 °F.
Read the result
The heat index is a feel-like temperature in the shade. In direct sun it can be up to 15 °F (about 8 °C) higher.
Dry heat is still dangerous: 40 °C at 30% humidity feels like 43 °C (110 °F). Drink water and rest in the shade whatever the reading.
NWS risk levels
- Caution: 80–90 °FFatigue possible with long exposure or activity.
- Extreme caution: 90–103 °FHeat stroke, cramps or exhaustion possible.
- Danger: 103–124 °FCramps or exhaustion likely; heat stroke possible.
- Extreme danger: 125 °F+Heat stroke highly likely.
Related weather tools
Heat index combines temperature and humidity. Dew point and wind chill are related measures.
For how much moisture the air holds, use the dew point calculator. In cold, windy weather, the wind chill calculator shows how cold it feels.
Common mistakes
Mistake: Using a sunny thermometer reading.
Instead: Use the shade temperature from a weather report.
Mistake: Treating the result as exact.
Instead: The NWS regression has an error of about ±1.3 °F.
Mistake: Ignoring dry heat.
Instead: High temperatures are risky even at low humidity.
Checking the method
Every golden case is re-worked in exact fractions from the NWS text, and three NWS worked examples are reproduced.
An independent script implements the NWS steps without floating point and matches the published 121 °F, 124 °F and 96 °F examples.
Read how we test our calculators, or browse more health calculators.