Heat index calculator
Enter the temperature and humidity, see how hot it feels.
What is the heat index?
How hot it feels when humidity is added to the air temperature. Sweat evaporates more slowly in humid air, so the body cools less: 90°F at 50% relative humidity feels like 95°F.
How is the heat index calculated?
This calculator follows the National Weather Service method: a simple formula first, and if the result is 80°F or more, the Rothfusz regression with the NWS adjustments for very dry and very humid air.
What do the heat index levels mean?
The NWS uses four levels: Caution from 80°F, Extreme caution from 90°F, Danger from 103°F and Extreme danger from 125°F. The higher the level, the more likely heat illness is with long exposure or hard work.
Does the heat index work in the sun?
No — it is computed for shade. Direct sunshine makes it feel hotter than the number shown.
Can I enter Celsius?
Yes — switch to Metric. The formula works in Fahrenheit; the calculator converts both ways.
How does the NWS algorithm work, step by step?
The National Weather Service does not use one formula for every case. It first computes a simple value, 0.5 × (T + 61 + (T − 68) × 1.2 + RH × 0.094), where T is the air temperature in °F and RH the relative humidity in percent, and averages it with T. If that average is below 80°F, the simple value is the heat index. If it is 80°F or higher, the Rothfusz regression, a polynomial in T and RH, takes over. Two adjustments follow: when RH is below 13% and T is between 80°F and 112°F, a term is subtracted, and when RH is above 85% and T is between 80°F and 87°F, a term is added. The NWS notes that the regression is not valid for extreme temperature and humidity conditions beyond the range of data considered by Steadman, on whose work the method is built.
Worked example: 96°F and 65% humidity
The simple formula gives 0.5 × (96 + 61 + 28 × 1.2 + 65 × 0.094) = 98.4°F. Averaged with the air temperature that is 97.2°F, which is above 80°F, so the full regression applies. The result is 121°F, the same value the National Weather Service gives for these conditions on its heat safety page, and it falls in the danger level. In direct sun the NWS says the heat index can be up to 15°F higher, so the exposed value could reach 136°F.
What does each NWS level mean for the body?
The NWS classification describes what is possible with prolonged exposure or physical activity. Caution (80–90°F): fatigue is possible. Extreme caution (90–103°F): heat cramps, heat exhaustion and heat stroke are possible. Danger (103–124°F): heat cramps or heat exhaustion are likely and heat stroke is possible. Extreme danger (125°F or higher): heat stroke is very probable. These are general categories for a broad population, not a forecast for one person.
Common mistakes with the heat index
Typing humidity as a fraction: entering 0.65 instead of 65 is read as 0.65%, and 96°F then returns 88°F instead of 121°F. Putting the dew point in the humidity box: dew point is a temperature, relative humidity is a percentage, and they are different inputs. Ignoring the sun: the values are devised for shade and light wind, so a sunny playground or parking lot can be much hotter than the number. Using it below about 80°F, where the NWS says the regression is not appropriate and the value stays close to the air temperature. Treating a level as a limit: it describes likelihood, not a safe or unsafe line, and individuals respond differently.
Sources and related calculators
Sources: National Weather Service Weather Prediction Center, “The Heat Index Equation” (Rothfusz regression, adjustments and limits); National Weather Service, “Heat Index” safety page (shade assumption, up to 15°F more in full sun, the 96°F and 65% example) and the NWS heat index classification table. Related: dew point calculator, wind chill calculator and water intake calculator.