Find how hot it really feels by combining air temperature and relative humidity.
The heat index is the “feels like” temperature when humidity is added to air temperature. At 90 °F (32 °C) with 70% humidity it feels like about 106 °F (41 °C) — in the “danger” range for prolonged exposure.
Humidity makes hot weather feel hotter because sweat evaporates more slowly, so your body cools less effectively. The heat index combines air temperature and relative humidity into a single “feels like” figure. This calculator uses the US National Weather Service formula and shows the result in °F and °C together with the NWS risk level.
Use it to judge outdoor work, sport and travel in hot, humid weather. The figure applies to shade and light wind; direct sunshine can make it feel up to 15 °F (8 °C) hotter. For cold weather, see the Wind Chill Calculator; for humidity itself see the Dew Point Calculator.
The NWS method uses a simple formula for mild weather and a regression equation for hot weather.
| Air temperature | Humidity | Heat index | Risk |
|---|---|---|---|
| 90 °F | 70% | 105.9 °F · 41.1 °C | Danger |
| 90 °F | 50% | 94.6 °F · 34.8 °C | Extreme caution |
| 100 °F | 50% | 118.3 °F · 48 °C | Danger |
| 35 °C | 60% | 113.1 °F · 45.1 °C | Danger |
| 32 °C | 80% | 111.9 °F · 44.4 °C | Danger |
| 30 °C | 40% | 85.4 °F · 29.7 °C | Caution |
| Heat index | Level | What can happen |
|---|---|---|
| 80–90 °F (27–32 °C) | Caution | Fatigue possible with prolonged exposure and activity |
| 90–103 °F (32–39 °C) | Extreme caution | Heat cramps and heat exhaustion possible |
| 103–124 °F (39–51 °C) | Danger | Heat cramps and exhaustion likely; heat stroke possible |
| 125 °F (52 °C) or higher | Extreme danger | Heat stroke highly likely |
A “feels like” temperature that combines air temperature and relative humidity to show how hot the weather feels to the body.
With the US National Weather Service regression equation using temperature in °F and relative humidity in percent.
From about 80 °F (27 °C) upwards. Below that the formula gives values close to the air temperature.
103 °F (39 °C) and above is classed as “danger”, and 125 °F (52 °C) or more as “extreme danger”.
High humidity slows the evaporation of sweat, the body's main way of cooling down.
No. It is what the heat feels like. The actual air temperature is unchanged.
Yes. The index assumes shade; full sun can add up to 15 °F (8 °C).
No, but they are related. The dew point measures moisture in the air; the heat index combines humidity and temperature into a comfort figure.
Yes — choose °C and the result is shown in both °C and °F.
No. Everything runs in your browser.
