How to use the heat index calculator
- Choose °F or °C.
- Enter the air temperature, measured in the shade, as in a weather report.
- Enter the relative humidity in percent.
- Read the heat index, its NWS caution level, the working, and a table showing how the heat index changes with humidity at your temperature.
How the heat index is calculated
The heat index estimates how hot it feels when humidity slows the cooling effect of sweat. The National Weather Service computes it in three steps. First it tries a simple formula. If the average of that result and the temperature is below 80 °F, the simple result is the heat index. Otherwise it uses the Rothfusz regression, a fit to R. G. Steadman's 1979 tables, and then applies one of two corrections:
Celsius inputs are converted to °F first and the answer is converted back. The method is described on the NWS Weather Prediction Center's heat index equation page. Results are rounded to whole degrees.
Worked example
The defaults are 90 °F with 60% relative humidity:
- Simple formula: 0.5 × (90 + 61 + 22 × 1.2 + 60 × 0.094) = 0.5 × 183.04 = 91.52 °F
- Average with the temperature: (91.52 + 90) ÷ 2 = 90.76 °F, which is 80 or more, so the full regression is used
- Rothfusz regression: 99.68 °F
- Neither adjustment applies (humidity is between 13% and 85%)
- Heat index: 100 °F (38 °C), in the extreme caution range
In Celsius, 32 °C at 60% humidity gives a heat index of 37 °C (99 °F). On a mild day of 75 °F and 40% humidity, the simple formula applies and gives 74 °F. In dry desert heat, 100 °F at 10% humidity, the low-humidity correction takes 0.63 °F off the regression, and the heat index is 94 °F, cooler than the air because sweat evaporates quickly.
NWS heat index caution levels
| Heat index | Level | What can happen with long exposure or activity |
|---|---|---|
| 80–90 °F (27–32 °C) | Caution | Fatigue |
| 90–103 °F (32–39 °C) | Extreme caution | Heat cramps and heat exhaustion possible |
| 103–124 °F (39–51 °C) | Danger | Heat cramps and exhaustion likely; heat stroke possible |
| 125 °F (52 °C) or more | Extreme danger | Heat stroke highly likely |
The heat index assumes shade and a light breeze. Full sunshine can raise it by up to 15 °F (8 °C), and hard work, heavy clothing or protective gear add more. Children, older adults, people with heart or lung conditions and anyone not used to the heat are at higher risk. Drink water regularly, rest in the shade, and call for medical help if someone becomes confused, stops sweating, faints or has a very high body temperature.
Limits of the heat index
The regression was fitted to Steadman's tables and works best in the range covered by the NWS chart, roughly 80 to 110 °F. At extreme combinations of heat and humidity it is an extrapolation. It also describes how a typical adult feels, not the temperature of anything: a car parked in the sun gets far hotter than the heat index. Humidity is often easier to judge from the dew point; use the dew point calculator to convert between the two. For cold, windy weather, use the wind chill calculator.
Frequently asked questions
What is a dangerous heat index?
The NWS treats 103 °F (39 °C) and higher as dangerous, with heat cramps and heat exhaustion likely and heat stroke possible. Many local offices issue heat advisories somewhere between 100 °F and 110 °F, depending on what the area is used to.
Why can the heat index be lower than the temperature?
In very dry air, sweat evaporates fast and cools you well, so the air can feel slightly cooler than its temperature. The NWS low-humidity correction applies when humidity is under 13%.
Is the heat index measured in the shade?
Yes. It is calculated from shade temperatures and assumes you are in the shade with a light wind. In direct sun it can feel up to 15 °F hotter than the heat index.
Can I use the heat index below 80 °F?
You can, but it matters little. Below about 80 °F the NWS uses its simple formula, and the result stays close to the air temperature.
Related calculators
- Dew Point Calculator — Dew point from temperature and relative humidity, or humidity from temperature and dew point, in °F or °C.
- Wind Chill Calculator — Wind chill (feels-like) temperature with the NWS formula, in °F or °C, with the frostbite risk.
- Speed Calculator — Speed from distance and time, distance from speed and time, or time from distance and speed, in any unit.
Results are calculated in your browser. Double-check anything important before you rely on it. See our disclaimer.