Heat index calculator
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Heat index
— °FAdd your readings for an estimate.
No location lookup or live weather.
Not medical advice. No result is a safety clearance. Follow official local and workplace guidance.
Read the band, not just the number
No band until a supported calculation is made.
- Caution
80–<90°F - Extreme caution
90–<103°F - Danger
103–<125°F - Extreme danger
125°F+
Ordered position, patterns and a “Current band” label identify the result. Below these bands is not safe.
Your calculation record
Not medical advice.
Not medical advice
Heat index combines air temperature and relative humidity to estimate how hot conditions feel in the shade with light wind. It is not your body temperature and it cannot tell you whether an activity is safe for you.
Enter a measured air temperature and relative humidity from the same place and time. Choose °F or °C for temperature; humidity is a percentage, not a decimal fraction. This calculator uses manual inputs, not live local weather.
What is heat index?
Heat index combines air temperature and humidity for shade and light wind. It is not a personal safety clearance.
Read the short heat-index guide for the reference example, assumptions and comparison with wet bulb and WBGT.
Understand the risk label
| Heat index, °F | Display band | What the band means |
|---|---|---|
| 80 to below 90 | Caution | Fatigue is possible during prolonged exposure or activity |
| 90 to below 103 | Extreme caution | Heat-related illness is possible with prolonged exposure or activity |
| 103 to below 125 | Danger | Heat illness is a serious concern; heat stroke is possible |
| 125 or higher | Extreme danger | NWS identifies a very high heat-stroke risk |
These general bands are adapted from the NWS heat-index table. Shared boundary values are placed in the higher band here. There is no “safe” category: illness can occur below these bands, and risk also depends on exertion, clothing, health and acclimatization.
The NWS formula and its adjustments
The NOAA Weather Prediction Center method uses T in °F and R as relative humidity in percent. Convert Celsius first using T = 1.8 × C + 32.
First compute S = 0.5 × [T + 61 + 1.2 × (T − 68) + 0.094 × R], then average it with air temperature: A = (S + T) / 2. If A < 80°F, use A as the simple-method result, not the full hot-weather regression. This is not a wind-chill calculation.
If A ≥ 80°F, calculate the Rothfusz regression:
HI = -42.379 + 2.04901523*T + 10.14333127*R
- 0.22475541*T*R - 0.00683783*T*T
- 0.05481717*R*R + 0.00122874*T*T*R
+ 0.00085282*T*R*R - 0.00000199*T*T*R*R
If R < 13 and 80 ≤ T ≤ 112, subtract [(13 − R)/4] × sqrt[(17 − abs(T − 95))/17].
If R > 85 and 80 ≤ T ≤ 87, add [(R − 85)/10] × [(87 − T)/5].
Keep full precision through the calculation and convert the final value to Celsius with (HI − 32) / 1.8 when needed. The regression is not valid for extreme temperature/humidity combinations outside the underlying data. A numeric answer alone does not establish validity.
Plan beyond the number
Check official local forecasts, warnings and the plan used by your school, event or workplace. OSHA recommends on-site WBGT measurement for workplace environmental heat assessment; heat index alone does not account for workload, radiant heat or clothing.
Choose a heat-index chart for a quick comparison, dew point to understand moisture, or WBGT to learn why worksite measurement needs more information.
Method and transparency
Last updated: 6 November 2026. Not medical advice.