Heat Index Calculator
Enter the air temperature and the relative humidity and get the National Weather Service heat index — how hot it feels in the shade — with its risk category, the exact procedure the NWS publishes and a full heat index chart. Works in °F or °C. Runs entirely in your browser — free, no sign-up.
0–100. The NWS chart covers 40–100 %.
Results
100 °F
Heat index (feels like)
37.6 °C
In °C
Extreme caution
NWS risk level
Effect on the body: Heat stroke, heat cramps or heat exhaustion are possible with prolonged exposure and/or physical activity.
Formula used: Rothfusz regression. In full sunshine the heat index can feel up to 15 °F (8 °C) higher, and strong hot, dry winds add to the danger.
The working (temperatures in °F)
- Simple formula: 0.5 × (T + 61.0 + 1.2 × (T − 68.0) + 0.094 × RH) with T = 90 and RH = 60 gives 91.52.
- Averaged with the air temperature: (91.52 + 90) / 2 = 90.76.
- At or above 80 °F the Rothfusz regression applies: HI = −42.379 + 2.04901523 T + 10.14333127 RH − 0.22475541 T RH − 0.00683783 T² − 0.05481717 RH² + 0.00122874 T² RH + 0.00085282 T RH² − 0.00000199 T² RH² = 99.68.
- Heat index = 99.7 °F.
Heat index chart
| Relative humidity | 80 °F | 85 °F | 90 °F | 95 °F | 100 °F | 105 °F | 110 °F |
|---|---|---|---|---|---|---|---|
| 40 % | 80 | 84 | 91 | 99 | 109 | 121 | 136 |
| 50 % | 81 | 86 | 95 | 105 | 118 | 134 | above 140 °F |
| 60 % | 82 | 89 | 100 | 113 | 129 | above 140 °F | above 140 °F |
| 70 % | 83 | 93 | 106 | 123 | above 140 °F | above 140 °F | above 140 °F |
| 80 % | 84 | 97 | 113 | 134 | above 140 °F | above 140 °F | above 140 °F |
| 90 % | 86 | 102 | 122 | above 140 °F | above 140 °F | above 140 °F | above 140 °F |
| 100 % | 89 | 108 | 132 | above 140 °F | above 140 °F | above 140 °F | above 140 °F |
Each cell is the heat index for that air temperature and humidity, computed with the same procedure. Values assume shade and light wind. Cells marked above 140 °F (60 °C) are beyond anything the regression was fitted to.
How to Use the Heat Index Calculator
- 1. Enter the air temperature in the shade and choose °F or °C. The NWS formula works in °F internally; the calculator converts for you.
- 2. Enter the relative humidity as a percentage. If you only have the dew point, the Relative Humidity Calculator converts it first.
- 3. Read the heat index and the risk level. The highlighted card is the feels-like temperature, with the NWS category and what it means for the body.
- 4. Follow the working. Each step of the NWS procedure is listed, including whether a low- or high-humidity adjustment applied, and the chart shows neighbouring values.
What the Heat Index Is
Your body cools by evaporating sweat. When the air is humid, sweat evaporates more slowly, so the same air temperature feels hotter. The heat index combines temperature and relative humidity into a single feels-like value, based on Robert Steadman's model of human heat balance. The National Weather Service computes it with a regression developed by Lans Rothfusz (NWS Technical Attachment SR 90-23). It is defined for shade and light wind; in full sunshine the NWS says the value can feel up to 15 °F (8 °C) higher.
The NWS Procedure
- Simple formula first. Compute HI = 0.5 × {T + 61.0 + [(T − 68.0) × 1.2] + (RH × 0.094)} with T in °F and RH in percent, then average the result with T. If that average is under 80 °F, it is the heat index.
- Otherwise use the Rothfusz regression. HI = −42.379 + 2.04901523 T + 10.14333127 RH − 0.22475541 T RH − 0.00683783 T² − 0.05481717 RH² + 0.00122874 T² RH + 0.00085282 T RH² − 0.00000199 T² RH².
- Low-humidity adjustment. If RH is under 13 % and T is between 80 and 112 °F, subtract [(13 − RH) / 4] × √{[17 − |T − 95|] / 17}.
- High-humidity adjustment. If RH is over 85 % and T is between 80 and 87 °F, add [(RH − 85) / 10] × [(87 − T) / 5].
The NWS warns that the regression is not valid for extreme temperature and humidity beyond the range of data Steadman considered, which is why the calculator flags readings outside the standard chart of 80–110 °F and 40–100 % humidity.
Worked example: 90 °F at 60 % humidity
Simple formula: 0.5 × (90 + 61 + 1.2 × 22 + 0.094 × 60) = 91.52; averaged with 90 gives 90.76, which is over 80, so use the regression.
Rothfusz regression: 99.68 °F. Neither adjustment applies (RH is neither under 13 % nor over 85 %).
Heat index ≈ 100 °F (37.6 °C), in the extreme caution band.
| Heat index | NWS category | Effect with prolonged exposure or activity |
|---|---|---|
| 80–90 °F | Caution | Fatigue possible |
| 90–103 °F | Extreme caution | Heat stroke, cramps or exhaustion possible |
| 103–124 °F | Danger | Cramps or exhaustion likely; heat stroke possible |
| 125 °F or higher | Extreme danger | Heat stroke highly likely |
Boundary values are placed in the higher category. Source: National Weather Service heat index classification.
Using the Number Sensibly
- Sun and wind change it. The index assumes shade. Direct sun can add up to 15 °F, and hot, dry wind can be dangerous even when the index looks modest.
- It is an average person. Age, fitness, medication and acclimatization all shift how a given heat index feels. Children, older adults and people working outdoors are at higher risk.
- Below 80 °F it does nothing. The heat index is not defined for cooler weather; for cold, use the Wind Chill Calculator.
- Other indexes exist. Canada reports the humidex, and sports medicine and occupational safety often use the wet-bulb globe temperature, which also counts sun and wind. They are different formulas and give different numbers.
Frequently Asked Questions
- How is the heat index calculated?
- The National Weather Service computes a simple formula first and averages it with the air temperature. At 80 °F or above it switches to the Rothfusz multiple regression in temperature and relative humidity, adding or subtracting a correction at very low or very high humidity. This calculator lists each step.
- What heat index is dangerous?
- The NWS calls 103–124 °F danger and 125 °F or higher extreme danger, but 90–103 °F is already extreme caution because heat stroke, cramps or exhaustion are possible with prolonged exposure or activity. Take precautions earlier if you are working or exercising outdoors.
- Why does humidity make it feel hotter?
- Sweat cools you only as it evaporates. In humid air evaporation slows, so your body sheds heat less effectively and the temperature feels higher than the thermometer reads.
- Is the heat index the same as the temperature in the sun?
- No. It is calculated for shade and light wind. Standing in direct sunshine can make it feel as much as 15 °F hotter than the heat index says.
- Why does the calculator give a value close to the temperature when it is cool?
- Below 80 °F the NWS does not use the regression, and the simple formula averaged with the temperature stays near the air temperature. The heat index is meant for hot weather only.
- What is the difference between heat index and wind chill?
- The heat index describes how hot warm, humid weather feels; wind chill describes how cold windy weather feels on exposed skin. The two never apply at the same time: heat index starts at 80 °F, while wind chill applies at or below 50 °F.