Heat Index Calculator
Calculate the feels-like temperature from air temperature and humidity, or reverse-solve for temperature or humidity. Free, fast, and easy to use.
Calculate
Choose a calculation type, enter your values, and click Calculate Now to get results with risk categories and step-by-step solutions.
Result
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Step-by-Step Solution
Complete User Guide
What is Heat Index?
The Heat Index, also called "feels-like temperature" or "apparent temperature", measures how hot it actually feels when humidity is combined with the air temperature. High humidity reduces the body's ability to cool through evaporation, making it feel hotter. The calculator uses the NWS Rothfusz regression equation, the standard used by the U.S. National Weather Service.
Rothfusz Equation
HI = −42.379 + 2.049×T + 10.143×RH − 0.2248×T×RH
− 6.838×10⁻³×T² − 5.482×10⁻²×RH²
+ 1.229×10⁻³×T²×RH + 8.528×10⁻⁴×T×RH²
− 1.99×10⁻⁶×T²×RH²
Where T = temperature (°F), RH = relative humidity (%)
4 Calculation Types
Heat Index
Enter temperature ≥ 80 °F and humidity ≥ 40 %. Get feels-like temperature + risk category.
Temperature from HI
Enter heat index and humidity. Finds the actual air temperature using iterative solving.
Humidity from HI
Enter heat index and temperature. Finds the relative humidity using iterative solving.
Convert °F ↔ °C
Simple temperature unit conversion. °C = (°F−32)×5/9.
Heat Index Risk Categories
| Heat Index (°F) | Category | Health Effects |
|---|---|---|
| 80 – 89 | Caution | Fatigue possible with prolonged exposure. |
| 90 – 102 | Extreme Caution | Heat cramps and heat exhaustion possible. |
| 103 – 124 | Danger | Heat cramps, exhaustion likely; heat stroke possible. |
| 125 + | Extreme Danger | Heat stroke highly likely. |
Important Notes
- Heat index is only valid for T ≥ 80 °F (27 °C) and RH ≥ 40 %.
- Direct sunlight can increase the heat index by up to 15 °F (8 °C).
- Elderly, children, and those with health conditions are more susceptible.
- Stay hydrated and take breaks in shaded areas when heat index is high.