Heat Index Calculator
Calculate the heat index or feels-like temperature from air temperature and humidity, free and instant in your browser.
Conditions
Result
Enter the temperature and humidity to see the result.
Estimates from standard published formulas for general use — not an official weather forecast or medical guidance. Calculations run entirely in your browser.
How to use this calculator
- Enter the air temperature and choose °C or °F.
- Enter the relative humidity as a percentage.
- Read the feels-like heat-index temperature and the heat-stress risk level.
How it works
The Heat Index Calculator estimates how hot it actually feels when humidity is combined with high temperature, using the US National Weather Service (Rothfusz) regression. High humidity slows sweat evaporation, so your body cools less effectively and the air feels hotter.
It computes the heat index in °F from the temperature and relative humidity (with the standard low-humidity and high-humidity adjustments) and converts to °C. The heat index is meant for warm, humid conditions — roughly 27 °C / 80 °F and above; below that it feels like the air temperature.
Worked example
32 °C (90 °F) at 70% humidity. At 90 °F with 70% relative humidity the air feels like about 105 °F (≈ 41 °C) — the Danger band, where heat cramps or heat exhaustion are likely with prolonged exposure.
Tips
- In Danger or Extreme Danger conditions, limit exertion, hydrate, and seek shade or air conditioning.
- Direct sun can make it feel several degrees hotter than the shaded heat index.
Frequently asked questions
What is the heat index?
The heat index, or apparent temperature, is how hot it feels to the human body when relative humidity is combined with the air temperature. Humid air makes it harder to cool down by sweating, so it feels hotter than the thermometer reads.
Why does humidity make heat feel worse?
Your body cools itself by evaporating sweat. When the air is already humid, sweat evaporates more slowly, so you lose less heat and feel hotter and more uncomfortable.
Is the heat index the same as humidex?
They are similar feels-like measures but use different formulas. This calculator uses the US National Weather Service heat index; Canada's humidex is calculated differently but gives a comparable idea of the discomfort.
Related tools
- Dew Point Calculator
- Wind Chill Calculator
- Apparent Temperature Calculator
- Absolute Humidity Calculator
- Air Density Calculator
- Cloud Base Height Calculator
Explore more in Weather & Environmental Monitoring.
Cite this tool
Zerdly. (2026). Heat Index Calculator. Retrieved from https://www.zerdly.com/tools/heat-index-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.