Heat Index Calculator: Use Temperature and Humidity or Dew Point
Judge heat stress, not comfort alone: Work out the heat index from air temperature and relative humidity or dew point, in °F or °C, with the NWS danger category for the result.
Heat index
105°F (41°C)
NWS category
Danger
!
How much humidity adds to the heat
Above the air temperature10°F
Recommended responseReduce prolonged outdoor activity
This result assumes shade and a light breeze. Direct sun can make conditions feel up to 15°F (8°C) hotter.
NWS heat index categories
| Heat index | Category | What can happen with prolonged exposure |
|---|---|---|
| 80-90°F (27-32°C) | Caution | Fatigue is possible with prolonged exposure or activity |
| 90-103°F (32-39°C) | Extreme caution | Heat cramps and heat exhaustion are possible |
| 103-124°F (39-51°C) | Danger | Heat cramps and heat exhaustion are likely; heat stroke is possible |
| 125°F (52°C) and above | Extreme danger | Heat stroke is highly likely |
How humidity changes the body's ability to cool
The heat index is how hot humid air feels to the body, because sweat evaporates more slowly when the air is already moist. This calculator uses the same regression the US National Weather Service uses for its heat advisories, including its corrections for very dry and very humid air.
For the evidence, limits and practical use behind this result, read Heat Index Explained. The weather tools guide explains when each calculator is the right one.
NWS Rothfusz regression (T ≥ 80°F)Steadman simple formula below 80°FDew point converted to RH with the Magnus formula
Limits and safe use of the heat index
- It assumes shade and a light breeze. In full sun the felt heat can run up to 15°F (8°C) higher than the heat index.
- Below 80°F (27°C) it barely matters. Humidity only starts to add real heat stress once the air is warm.
- Dew point is the steadier input. Relative humidity swings through the day as the temperature changes; dew point does not.
Heat-stress and formula questions
Is a heat index of 105°F dangerous?
Yes. 105°F (41°C) falls in the NWS "Danger" band, where heat exhaustion is likely with prolonged activity and heat stroke is possible. Many NWS offices issue a heat advisory from a heat index of about 100-105°F.
Why does the calculator ask for dew point as an option?
Forecasts and weather stations often quote dew point, and it is a more stable measure of moisture than relative humidity. The calculator converts it to relative humidity first, then applies the heat index formula.
Does the heat index work in cool weather?
Not meaningfully. Below about 80°F (27°C) the formula is outside the range it was fitted for, so the calculator falls back to the simpler Steadman approximation, which stays close to the air temperature.