Free tool · Training
Heat index calculator for exercising in the heat
Short answer
The heat index is how hot it feels once humidity is counted, because humid air slows sweat from cooling you. At 90 °F (32 °C) and 70% humidity it is about 106 °F (41 °C), which the National Weather Service rates "Danger". The values are for shade; direct sunlight can add up to 15 °F.
Last reviewed 28 October 20262 published sourcesNot medical advice
Reps keeps track of this for you on iPhone, iPad and Apple Watch.
Get Reps freeHeat index calculator
Use the air temperature and relative humidity from a weather app or station, measured in the shade.
Enter the temperature and humidity to see the heat index.
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| Air temperature | 20% RH | 40% RH | 60% RH | 80% RH |
|---|---|---|---|---|
| 80 °F (27 °C) | 79 (Below the NWS caution level) | 80 (Caution) | 82 (Caution) | 84 (Caution) |
| 85 °F (29 °C) | 82 (Caution) | 84 (Caution) | 89 (Caution) | 97 (Extreme caution) |
| 90 °F (32 °C) | 86 (Caution) | 91 (Extreme caution) | 100 (Extreme caution) | 113 (Danger) |
| 95 °F (35 °C) | 91 (Extreme caution) | 99 (Extreme caution) | 113 (Danger) | 134 (Extreme danger) |
| 100 °F (38 °C) | 97 (Extreme caution) | 109 (Danger) | 129 (Extreme danger) | Over 137 (Extreme danger) |
| 105 °F (41 °C) | 104 (Danger) | 121 (Danger) | Over 137 (Extreme danger) | Over 137 (Extreme danger) |
How the National Weather Service calculates it
The simple formula is 0.5 × (T + 61 + (T − 68) × 1.2 + RH × 0.094), averaged with the temperature. If that reaches 80 °F, the full Rothfusz regression applies, with a downward adjustment when humidity is below 13% and an upward one when it is above 85% at moderate temperatures (NWS Weather Prediction Center).
The Weather Service's categories describe what to expect with prolonged exposure or physical activity, from fatigue at Caution to heat stroke being highly likely at Extreme Danger (National Weather Service).
Training in the heat
On hot days, pacing by heart rate rather than by speed keeps your effort in check. Work out your ranges with the heart rate zone calculator, and check your sweat losses with the water intake calculator.
Know the weather before you go
Reps looks at the weather before your run, so you know what you are heading into before you lace up.
Free to download · iPhone, iPad and Apple Watch
Common questions
What heat index is dangerous?
The National Weather Service calls 80–90 °F Caution, 90–103 °F Extreme Caution, 103–124 °F Danger and 125 °F or higher Extreme Danger. In the Danger band, heat cramps or heat exhaustion are likely and heat stroke is possible with prolonged exposure or physical activity.
Is the heat index the same in the sun?
No. The heat index chart is for shady locations. The National Weather Service says direct sunlight can raise the heat index by up to 15 °F.
How is the heat index calculated?
The National Weather Service first uses a simple formula. If the result is 80 °F or more, it switches to the Rothfusz regression, a nine-term equation in temperature and humidity, with small corrections for very dry or very humid air.
Sources
Figures on this page were checked against these publications on 28 October 2026.
- National Weather Service, Weather Prediction Center. The heat index equation. NOAA. www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml
- National Weather Service. What is the heat index? NOAA. www.weather.gov/ama/heatindex
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