Calculate the "feels like" temperature combining air temperature and relative humidity using the NWS Rothfusz equation.
| Temp \ RH | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|
| 80°F | 80 | 81 | 82 | 83 | 84 | 86 |
| 85°F | 84 | 86 | 89 | 93 | 97 | 102 |
| 90°F | 91 | 95 | 100 | 106 | 113 | 122 |
| 95°F | 99 | 105 | 113 | 123 | 134 | 147 |
| 100°F | 109 | 118 | 129 | 143 | 158 | 176 |
| 105°F | 121 | 134 | 149 | 166 | 187 | 209 |
| 110°F | 136 | 152 | 171 | 194 | 219 | 247 |
| < 80°F | Safe |
| 80–90°F | Caution |
| 91–103°F | Extreme Caution |
| 103–124°F | Danger |
| ≥ 125°F | Extreme Danger |
At 2 p.m. a youth soccer coach checks the forecast: 95°F and 60% humidity. The air temperature sounds like an ordinary summer day, but the heat index is 113°F, which sits in the NWS Danger band (103 to 124°F) and not in the Caution band the thermometer suggests. This calculator applies the National Weather Service's Rothfusz regression to a temperature and relative humidity and returns the heat index in both scales with its category.
In °F with relative humidity RH in percent, the page computes HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541·T·RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²·RH + 0.00085282T·RH² − 0.00000199T²·RH². At 95°F and 60%, that evaluates to 113.09, shown as 113.1°F or 45.1°C. It falls in the Danger band (103 to under 125).
Edge cases. Below 80°F the page declines to compute and says so: 79°F gives no figure at any humidity, and 26.5°C (79.7°F) does not either, while 27°C (80.6°F) at 70% gives 83.9°F. Humidity is clamped to 0 to 100. At the other extreme, the NWS chart covers 80 to 110°F, but the page does not stop you from going beyond it: 110°F at 90% returns 247.0°F, which is the regression running out of data, not a real condition. The page also skips the two small adjustments the NWS applies for very dry air (below 13% humidity) and very humid air (above 85%), worth at most about 3 and 2 degrees. Treat anything outside the chart's range with suspicion.
The Rothfusz regression was fit for hot conditions and misbehaves in cooler air, so the page returns nothing under 80°F instead of an unreliable figure. That does not mean the weather is risk-free for people exerting themselves.
Yes. NWS values are for shade and light wind. Full sunshine can raise the effective heat index by up to 15°F, and the page makes no adjustment for it.
No. It only means the index is under 80°F and no advisory band applies. Children, older adults, outdoor workers and pets can be affected at lower numbers, and anyone with symptoms of heat exhaustion or heat stroke should seek medical help. This page is not medical advice; follow NWS and local heat guidance.
Forecast offices use the NWS procedure with its low- and high-humidity adjustments and their own rounding, and the humidity at your location may differ from the station's. Official NWS products should be the reference for warnings.
A great deal. At 90°F the index is 90.7°F at 40% humidity, 99.7°F at 60% and 113.3°F at 80%. Same air temperature, a swing of about 23 degrees.