Enter the temperature and relative humidity to see how hot it feels and the NWS heat danger level.
How the NWS calculates the heat index
The heat index is how hot it feels when humidity slows the cooling effect of sweat. The National Weather Service computes it in steps, with T in °F and RH in percent:
1. Simple formula first.
HI = 0.5 × (T + 61.0 + (T − 68.0) × 1.2 + RH × 0.094)
Average that with T. If the average is below 80 °F, that average is the heat index.
2. Otherwise, the Rothfusz regression:
HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T·RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²·RH + 0.00085282T·RH² − 0.00000199T²·RH²
3. Adjustments. If RH is under 13% and T is 80 to 112 °F, subtract ((13 − RH) ÷ 4) × √((17 − |T − 95|) ÷ 17). If RH is over 85% and T is 80 to 87 °F, add ((RH − 85) ÷ 10) × ((87 − T) ÷ 5).
Worked example
At 90 °F and 50% humidity, the simple formula gives 0.5 × (90 + 61 + 22 × 1.2 + 50 × 0.094) = 91.05, which averaged with 90 is 90.5: above 80, so use the regression. It gives 94.6 °F, which rounds to 95 °F, the same as the NWS chart. That's in the Extreme Caution range.
NWS heat danger levels
| Heat index | Level | What can happen with long exposure or activity |
|---|
| 80–90 °F | Caution | Fatigue |
| 90–103 °F | Extreme caution | Heat cramps or heat exhaustion possible |
| 103–124 °F | Danger | Heat cramps or heat exhaustion likely; heat stroke possible |
| 125 °F or higher | Extreme danger | Heat stroke highly likely |
Limitations
- The heat index is for shade. The NWS says full sunshine can raise it by up to 15 °F.
- It assumes a light breeze. Strong hot wind or heavy work makes things worse.
- The regression isn't valid for extreme combinations outside the range of Steadman's original data, roughly above the NWS chart's edges. Past the chart's top (about 137 °F), Calcist says "off the chart" instead of giving a number. Below 80 °F the heat index is essentially the air temperature.
- Children, older adults and people with some health conditions are at risk at lower values.
Sources
How we checked this
Before every update, this calculator is run in a real browser with these known answers, and it must match all of them:
- 90 °F and 50% humidity: heat index 95 °F (NWS chart: 95) ✓
- 96 °F and 65%: 121 °F, Danger (NWS chart: 121) ✓
- 85 °F and 90%: 101.8 °F with the high-humidity adjustment ✓
- 100 °F and 10%: 94.1 °F with the low-humidity adjustment ✓
- 75 °F and 50%: 74.8 °F from the simple formula ✓
- 90 °F and 50%: simple-formula average 90.525 ≈ 90.5, so the regression is used ✓
- 120 °F and 80%: regression gives 295 °F, shown as off the chart ✓
- 80 °F and 40%: 79.79 shows as 80 °F, so the level is Caution ✓
- 30 °F: the heat index is essentially the air temperature ✓
Last checked October 26, 2026. Spotted something wrong? The math is done in your browser with the formula shown on this page, so you can check any result by hand.
Questions people ask
What is the heat index at 90 °F and 50% humidity?
About 95 °F (94.6 °F unrounded), which is in the NWS Extreme Caution range.
At what heat index is it dangerous?
The NWS calls 103 to 124 °F "Danger" and 125 °F or higher "Extreme Danger". Heat illness is possible from 90 °F.
Is the heat index the same as the humidex?
No. Canada's humidex uses the dew point and a different formula, so the numbers differ. This calculator uses the US heat index.
Why is the heat index lower than the temperature in dry air?
In very dry air, sweat evaporates quickly and cools you well, so it can feel slightly cooler than the thermometer says.