Heat Index Calculator
Work out how hot it really feels once humidity is factored in (heat index).
How to use the heat index calculator
- Enter the air temperature in °C.
- Enter the relative humidity in percent.
- The feels-like temperature (heat index) updates automatically.
Why does it feel hotter at high humidity?
Your body cools itself by evaporating sweat from your skin. When the air is already saturated with moisture, sweat evaporates more slowly — your body becomes less able to shed excess heat, so the temperature feels higher than what the thermometer actually shows.
Frequently asked questions
Why doesn't the calculator work at cold temperatures?
Because heat index as a phenomenon is only defined for warm temperatures — at cold temperatures, it's wind that affects how cold it feels instead. See our wind chill calculator for that.
Is a high heat index dangerous?
Yes — a high heat index significantly increases the risk of heat stroke and dehydration, since your body cools less effectively than normal. Health authorities recommend drinking extra fluids, avoiding exertion during the hottest part of the day, and seeking shade when the feels-like temperature is substantially higher than the actual air temperature.
What's the difference between heat index and wind chill?
Heat index shows how hot it feels due to humidity at high temperatures, while wind chill shows how cold it feels due to wind at low temperatures — two separate, opposite phenomena that each apply within their own temperature range.