Heat index combines air temperature and relative humidity to estimate how hot conditions feel in shade with light wind. It is not body temperature, a live forecast or a personal safety clearance.
Open the heat-index calculator
Not medical advice.
Temperature + humidity → heat index
The National Weather Service reference example uses these inputs. Humidity makes evaporation of sweat more difficult; a heat-index value can therefore exceed the air temperature.
Compare the three measures
Heat index
Air temperature + relative humidity. A feels-like estimate for shade and light wind.
Wet bulb
Evaporative cooling under specified conditions. It is a different temperature, not a feels-like value.
Read the wet-bulb guideWBGT
Wet-bulb globe temperature uses additional environmental measurements. Heat index cannot replace it.
Explore WBGT methodsHow to read an estimate
- Use air temperature and relative humidity measured at the same place and time. Choose °F or °C; enter humidity as a percentage.
- Check the method and supported range before reading the number. Unsupported combinations do not receive a valid answer.
- Read the named heat-index band and its limitations, not just the temperature. A low value is not a guarantee of safety.
- Check official local forecasts, warnings and the plan used by your event or workplace. The calculator does not evaluate personal circumstances.
Check the assumptions
- Shade and light wind: direct sunlight is not modeled. NWS says full sunshine can increase apparent heat by up to 15°F; do not automatically add 15°F to every answer.
- Manual readings: there is no location lookup or live weather feed.
- Site range: 50–112°F air temperature, 0–100% RH; regression outputs above 137°F are not evaluated here.
- Limited scope: exertion, radiant heat, clothing, health and acclimatization are not assessed.
Frequently asked questions
Is heat index the same as air temperature?
No. Air temperature is an input; heat index combines it with relative humidity to estimate feels-like conditions under the stated assumptions.
Is every weather app's feels-like value a heat index?
No. Apps can use different methods, especially in cold or windy conditions. Check the provider's definition.
Does a low heat index mean an activity is safe?
No. There is no safe category. Exposure and personal circumstances are outside this tool's assessment.
Method and next steps
This site uses the NWS/WPC equation and adjustments, including the simple-method branch and humidity adjustments. The full method and named bands are on the calculator; use the heat-index chart to compare inputs.
Reviewed 6 November 2026. Not medical advice.