Heat Index Calculator
Calculate the heat index from temperature and humidity, with heat risk level (valid ≥80°F).
Inputs
Rothfusz regression is valid at or above 80°F.
Heat Index (°F)
113.1
How hot it feels accounting for humidity.
Heat Index (°C)
45.1
Feels Hotter By (°F)
18.1
Heat Risk Level
Danger — heat cramps and exhaustion likely; heat stroke possible. Avoid prolonged outdoor activity.
Step by step
Inputs
= T = 95°F, RH = 60%
Rothfusz regression result
= 113.1°F
Based on 9-term polynomial regression against Steadman lookup tables.
Heat index
= 113.1°F (45.1°C) — +18.1°F above air temperature
Heat risk level
= Danger — heat cramps and exhaustion likely; heat stroke possible. Avoid prolonged outdoor activity.
How it works
The heat index quantifies how hot it actually feels when relative humidity is factored in alongside air temperature. High humidity impairs the body's primary cooling mechanism — sweat evaporation — so a humid 95°F day feels substantially hotter than a dry 95°F day. The NWS uses the Rothfusz multiple-regression equation, which was fitted against Steadman's 1979 empirical lookup tables. Two boundary adjustments refine the regression at low humidity (where it over-predicts) and high humidity near 80°F (where it under-predicts). The formula is only meaningful at or above 80°F (26.7°C) with at least 40% relative humidity.
Formula
Rothfusz regression (NWS)
HI = −42.379 + 2.049×T + 10.143×RH − 0.225×T×RH − 0.00684×T² − 0.0548×RH² + 0.00123×T²×RH + 0.000853×T×RH² − 0.000002×T²×RH²
- T
- Air temperature in °F
- RH
- Relative humidity as a percentage (0–100)
- HI
- Heat index in °F
Frequently Asked Questions
Why is the heat index only valid above 80°F?
Below 80°F, the difference between apparent and actual temperature is small enough to be negligible, and the Rothfusz polynomial was not fitted for that range. At lower temperatures, other factors such as sun angle, wind and clothing matter more than humidity for perceived warmth.
What is the difference between heat index and humidex?
Heat index (NWS, US) and humidex (Environment and Climate Change Canada) both express 'apparent temperature', but use different empirical formulas calibrated against different datasets. Humidex often produces higher numbers than heat index for the same conditions. This calculator implements the NWS heat index.
How does the heat index relate to heat illness risk?
The NWS risk categories (Caution, Extreme Caution, Danger, Extreme Danger) are guidelines for healthy adults at rest in the shade. Direct sun can increase the heat index by 10–15°F. Elderly people, infants, and those with certain medical conditions face higher risk at lower thresholds.
What are the low- and high-humidity adjustments?
The Rothfusz equation is a polynomial fit to the middle of its valid domain. At very low humidity (< 13%) and temperatures between 80°F and 112°F, it overestimates the apparent temperature, so NWS subtracts a correction. At very high humidity (> 85%) between 80°F and 87°F, it underestimates, so NWS adds a correction. Both bring the regression into agreement with Steadman's original tables.