How to use the heat index calculator
- Enter the air temperature and choose Fahrenheit or Celsius.
- Enter the relative humidity as a percent from 0 to 100.
- Read the heat index, the unrounded value and the table showing the same temperature at other humidity levels.
- Remember that the index assumes shade and light wind. Direct sun adds to what you feel.
Formula
HI = -42.379 + 2.04901523 T + 10.14333127 RH - 0.22475541 T RH - 0.00683783 T^2
- 0.05481717 RH^2 + 0.00122874 T^2 RH + 0.00085282 T RH^2 - 0.00000199 T^2 RH^2
Low humidity: if RH < 13 and T is 80 to 112 F, subtract ((13 - RH) / 4) x sqrt((17 - |T - 95|) / 17)
High humidity: if RH > 85 and T is 80 to 87 F, add ((RH - 85) / 10) x ((87 - T) / 5)
Simple formula: HI = 0.5 x (T + 61 + 1.2 (T - 68) + 0.094 RH), used when the average of it and T is below 80 F
T is the air temperature in degrees F and RH is relative humidity in percent. This is the Rothfusz regression and the adjustments published by the National Weather Service. Inputs in Celsius are converted to Fahrenheit first.
Worked example
At 90 degrees F and 70% humidity, the regression gives 105.92 degrees F, reported as 106. No adjustment applies, because humidity is between 13 and 85 percent.
At 96 F and 40% the value is 100.89 F. At 100 F and 10% the low humidity adjustment lowers it to 94.12 F. At 85 F and 90% the high humidity adjustment gives 101.78 F. The tool refuses air temperatures below 80 F, where the index is not defined.
What the heat index measures
The heat index combines temperature and humidity to estimate how hot it feels to the body. When the air is humid, sweat evaporates more slowly, so the body cools less effectively and the same temperature feels hotter. It is an apparent temperature for a person in the shade, not an actual air temperature.
Valid range and caution
The National Weather Service says the regression is not valid for extreme combinations beyond the data it was fitted to, and that below about 80 F the simple formula is used. The reference chart covers 80 to 110 F and humidity of 40 percent and up. This tool warns above 112 F and below 40 percent humidity. Full sunshine can raise the heat index by up to 15 F, and strong wind in very hot, dry air can make conditions worse.
Staying safe in the heat
Drink water, take breaks in shade or cool places and check on people who are older, young or ill. This page is a guide and not medical advice. For other weather indexes see the dew point calculator and the wind chill calculator, and track hydration with the water intake calculator.
Assumptions and limits
- The formula, adjustments and rule for choosing between them follow the National Weather Service heat index equation page.
- Values assume shade and light wind. Sun, wind, clothing and activity change how hot it feels.
- Temperatures below 80 degrees F are refused. Values above 112 F or under 40 percent humidity are flagged as outside the chart.
- The result is rounded to a whole degree, as weather services report it.
Frequently asked questions
How do you calculate the heat index?
Use the Rothfusz regression with temperature in degrees F and relative humidity in percent, then apply the low or high humidity adjustment when its conditions are met.
What is the heat index at 90 F and 70 percent humidity?
About 106 F. The unrounded value is 105.92 F.
Why can I not enter a temperature below 80 F?
The heat index is only defined for warm conditions. Below about 80 F the effect of humidity on how you feel is small, so air temperature is the better guide.
Does the heat index include direct sunlight?
No. It assumes shade. According to the National Weather Service, full sunshine can add up to 15 degrees F to the value.