Heat Index Calculator
At 90 degrees Fahrenheit with 70 percent humidity, it feels like about 106 degrees. This heat index calculator combines air temperature and relative humidity to estimate the feels-like temperature, the value your body actually senses on a humid day. It uses the National Weather Service heat index formula and returns the result in Fahrenheit and Celsius along with a heat risk level. Enter the temperature and humidity to see how hot it really feels.
Quick answer
The heat index is how hot it feels when humidity is factored in with the air temperature.
What this tells you
- •The heat index is how hot it feels when humidity is factored in with the air temperature.
- •High humidity slows sweat evaporation, so your body cools less and the air feels hotter.
- •The calculator uses the National Weather Service formula and is most accurate at 80 degrees Fahrenheit and above.
- •The risk level moves from Caution to Extreme Danger as the heat index climbs.
How to Use
- 1Enter the air temperature in degrees Fahrenheit.
- 2Enter the relative humidity as a percentage from 0 to 100.
- 3Click Calculate to get the feels-like temperature in Fahrenheit and Celsius plus the heat risk level.
How It Works
Formula
HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T^2 - 0.05481717R^2 + ... (NWS regression)The heat index uses the National Weather Service Rothfusz regression, where T is the temperature in Fahrenheit and R is the relative humidity. A simpler average formula is used first, and when that value reaches 80 degrees the full regression is applied, with small adjustments for very dry or very humid air. At 90 degrees and 70 percent humidity the result is about 106 degrees Fahrenheit.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
How hot does 90 degrees at 70% humidity feel?
At 90 degrees with 70 percent humidity, the heat index is about 106 degrees Fahrenheit. That falls in the Danger range, where heat cramps and heat exhaustion are likely with prolonged exposure.
A hot, drier day at 100 degrees and 40% humidity
Even at 40 percent humidity, 100 degrees feels like about 109 degrees because the air still limits how fast sweat evaporates.
Heat Index Risk Levels
How the National Weather Service classifies heat index ranges.
| Heat Index (Fahrenheit) | Risk Level | What It Means |
|---|---|---|
| 80 to 90 | Caution | Fatigue possible with prolonged activity |
| 91 to 103 | Extreme Caution | Heat cramps and heat exhaustion possible |
| 104 to 124 | Danger | Heat exhaustion likely, heat stroke possible |
| 125 and above | Extreme Danger | Heat stroke highly likely |
Values are for shade. Full sun can raise the heat index by up to 15 degrees Fahrenheit.
Common mistakes
- Entering humidity as a decimal instead of a percentage. Use 70 for 70 percent, not 0.7.
- Expecting accurate results in cool weather. The heat index formula is built for warm conditions at about 80 degrees Fahrenheit and above.
- Assuming the reading applies in direct sun. The heat index is a shade value, and standing in full sunlight can feel up to 15 degrees hotter.