Heat Index Calculator
Calculate the heat index from temperature and humidity, with heat risk level.
About Heat Index Calculator
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
Worked example
With the default values below, Heat Index Calculator returns Heat Index (°F) of 113.1. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Unit Systemone of Imperial (°F), Metric (°C)
- Imperial (°F)
- Air Temperature (°F)accepts 80–135
- 95
- Air Temperature (°C)accepts 27–57
- 35
- Relative Humidity (%)accepts 0–100 %
- 60 %
Results
- Heat Index (°F)How hot it feels accounting for humidity.
- 113.1
- 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
- 1Inputs= T = 95°F, RH = 60%
- 2Rothfusz regression result= 113.1°FBased on 9-term polynomial regression against Steadman lookup tables.
- 3Heat index= 113.1°F (45.1°C) — +18.1°F above air temperature
- 4Heat risk level= Danger — heat cramps and exhaustion likely; heat stroke possible. Avoid prolonged outdoor activity.
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
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.