Find out what the temperature really feels like outside using the official NWS heat index formula.
Enter the air temperature and relative humidity, then click Calculate
This free heat index calculator uses the official U.S. National Weather Service (NWS) Rothfusz regression to convert air temperature and relative humidity into a "feels like" temperature — the heat index — along with an estimated heat-related illness risk level.
Enter the current air temperature and relative humidity, and the calculator applies the Rothfusz regression, a statistical formula fitted to a physiological heat-stress model developed by Steadman in 1979. For temperatures below 80°F, where humidity has little practical effect on comfort, the calculator instead uses a simpler estimate. The formula also applies small corrections for very low or very high humidity within the valid temperature range, matching the exact method used by the NWS.
Anyone checking how hot it will actually feel before spending time outdoors in summer, outdoor workers and coaches assessing heat-stress risk, and event planners scheduling activities during hot, humid weather. The result is most meaningful for temperatures at or above 80°F (27°C), where humidity has a real impact on perceived heat.
The official NWS Rothfusz regression this calculator uses
The formula is based on a physiological model of how efficiently sweat evaporates at different temperature and humidity combinations, which drives your body's ability to cool itself.
Rothfusz fitted this multi-term polynomial to Steadman's original heat-stress model so it could be calculated quickly without complex physiological equations.
Small corrections are applied for unusually low or high humidity within the 80-112°F range, keeping the result accurate at the extremes of the formula's typical use.
From entering conditions to reading your result
Type the current air temperature in Fahrenheit or Celsius.
Type the current relative humidity as a percentage.
See the heat index "feels like" temperature and its estimated heat danger risk level.
A hot, humid summer day and a hot, drier day
Official NWS heat index risk categories and effects
| Heat Index Range | Risk Level | Possible Effects |
|---|---|---|
| Below 80°F | Safe | Little to no heat-related risk |
| 80-90°F | Caution | Fatigue possible with prolonged exposure or activity |
| 90-103°F | Extreme Caution | Heat cramps and heat exhaustion possible |
| 103-124°F | Danger | Heat cramps and heat exhaustion likely; heat stroke possible |
| 124°F and above | Extreme Danger | Heat stroke highly likely with continued exposure |
These categories are general guidance from official NWS heat index charts. Actual risk varies by individual health, hydration, activity level, and sun exposure — always follow local heat advisories and warnings.
Where knowing the "feels like" temperature matters
Check whether conditions are safe for a run, workout, or sports practice.
Assess heat-stress risk for construction, agriculture, or landscaping work.
Schedule outdoor events with realistic expectations for summer heat conditions.
Take extra precautions for children, older adults, or pets during high heat index conditions.
Understand what hot, humid destinations will actually feel like before you arrive.
What this heat index calculator does well, and where it has boundaries
Get an accurate heat index reading
Summary: This heat index calculator uses the official NWS Rothfusz regression to turn air temperature and humidity into a "feels like" temperature and heat danger risk level — free, instant, and useful for hot-weather planning. Pair it with the Wind Chill Calculator or Dew Point Calculator for other weather comfort calculations.
Common questions about heat index
Official weather agency references to complement this calculator
Other weather and measurement tools