What is the heat index?
The heat index is a "feels like" temperature that combines air temperature with relative humidity. When the air is humid, sweat evaporates more slowly, so your body sheds heat less effectively and feels hotter than the thermometer shows. The U.S. National Weather Service (NWS) uses it to issue heat advisories and warnings.
This calculator takes the air temperature in °F and relative humidity from 0 to 100%, and returns the heat index in °F and °C plus the NWS risk category.
How the heat index is calculated
The calculator follows the procedure published by the NWS Weather Prediction Center, which is based on a regression equation by Lans Rothfusz (1990) fitted to Robert Steadman's original apparent-temperature tables.
-
Simple formula first. HI = 0.5 × (T + 61.0 + (T − 68.0) × 1.2 + RH × 0.094), where T is the temperature in °F and RH is relative humidity in percent. This result is averaged with the temperature.
-
If that average is 80°F or more, the full Rothfusz regression replaces it:
HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T·RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²·RH + 0.00085282T·RH² − 0.00000199T²·RH²
-
Low-humidity adjustment. If RH is below 13% and T is between 80 and 112°F, subtract ((13 − RH) ÷ 4) × √((17 − |T − 95|) ÷ 17).
-
High-humidity adjustment. If RH is above 85% and T is between 80 and 87°F, add ((RH − 85) ÷ 10) × ((87 − T) ÷ 5).
The result is rounded to one decimal place, and the Celsius value is (HI − 32) × 5/9.
Worked example
90°F air temperature with 60% relative humidity:
- Simple formula: 0.5 × (90 + 61 + 26.4 + 5.64) = 91.52
- Average with the temperature: (91.52 + 90) ÷ 2 = 90.76, which is above 80, so use the regression.
- The Rothfusz regression gives 99.7°F (37.6°C).
- Neither adjustment applies (humidity is between 13% and 85%).
- 99.7°F falls in the Extreme Caution band.
At 75°F and 50% humidity the simple formula stays below 80, so the calculator reports 74.8°F with no significant risk.
Heat index risk levels
These are the NWS heat index categories, which this calculator uses for its risk label:
| Heat index | NWS level | What can happen with prolonged exposure or activity |
|---|---|---|
| 80 to 90°F (27 to 32°C) | Caution | Fatigue is possible |
| 90 to 103°F (32 to 39°C) | Extreme Caution | Heat stroke, heat cramps or heat exhaustion possible |
| 103 to 124°F (39 to 51°C) | Danger | Heat cramps or heat exhaustion likely, heat stroke possible |
| 125°F (52°C) or higher | Extreme Danger | Heat stroke highly likely |
Below 80°F the calculator shows "No significant risk".
Things the heat index doesn't include
- Sunshine. The values assume shade and light wind. The NWS says full sun can add up to 15°F.
- Wind. Strong hot wind can make conditions worse, and it isn't in the formula.
- You. Age, health, medication, hydration, clothing and how hard you're working all change your risk. Older adults, young children, outdoor workers and people with chronic illness are more vulnerable at any given heat index.
- Cold weather. The heat index is meant for warm conditions. In cold weather, wind chill is the relevant measure.
How to use it
- Find the current temperature and relative humidity from a local forecast or weather station.
- If the temperature is in Celsius, convert it with the temperature converter.
- Enter both values and read the risk level, then add a margin if you'll be in direct sun.
Related calculators
- Temperature converter: switch between °C, °F and K.
- Calorie calculator: daily energy needs, which rise with hard physical work.
- BMI calculator: a quick health screen from height and weight.
This calculator is for general information. Follow official heat advisories from your local weather service, and seek medical help for signs of heat illness.
Last updated