Heat Index Calculator
Turn air temperature and humidity into the "feels-like" heat index and a heat-stress risk level (Caution → Extreme Danger), using the US NWS formula. °C/°F toggle, fully offline.
Open Heat Index Calculator →What is the Heat Index Calculator?
A heat-index calculator that turns air temperature and relative humidity into the "feels-like" temperature plus a heat-stress risk level, from No heat stress through Caution up to Extreme Danger. It uses the US National Weather Service formula, works in either °C or °F, updates as you type, and runs entirely in your browser offline.
How to use Heat Index Calculator
- Choose your unit — Use the °C / °F toggle next to the temperature field to match how you read the weather.
- Enter the air temperature — Type the current air temperature into the temperature box.
- Enter the relative humidity — Type the relative humidity as a percentage (0 to 100).
- Read the feels-like result — The heat index appears with the equivalent in the other unit, a colour-coded risk band, and a short guide to what that level means.
Frequently asked questions
What does the heat index actually tell me?
It estimates how hot it feels to the human body when humidity is combined with the air temperature, which is often higher than the thermometer reading.
Which formula is used?
It uses the US National Weather Service (Rothfusz) heat-index regression, the same method behind official heat advisories.
Does it account for direct sunlight?
No. The heat index assumes shade and light wind. Standing in full sun can feel up to around 8 °C / 15 °F hotter than the calculated value.
What do the risk bands mean?
They range from No heat stress to Caution, Extreme Caution, Danger and Extreme Danger, each with a note on the heat-related effects that become likely at that level.
Is anything sent to a server?
No. The calculation runs entirely in your browser, so it keeps working offline and nothing you enter is uploaded.
Tips
- Relative humidity must be between 0 and 100 percent; values are clamped to that range.
- Switch the unit toggle to see the same result in both °C and °F.
- Add a margin if you'll be working in direct sun, where it feels hotter than the index.
- Use the risk band to decide on rest breaks and hydration for outdoor work.