Most accurate for temperatures at or above 80°F — below that, heat index roughly equals the actual air temperature.
Free Heat Index Calculator
Heat index accounts for how humidity impairs the body's ability to cool itself through sweat evaporation — high humidity at a given temperature feels considerably hotter than the same temperature with low humidity. The formula is most accurate above about 80°F.
Worked Example
Example: 95°F air temperature, 60% humidity → heat index ≈ 114°F ("feels like").
Formula / Method
NWS Rothfusz regression: HI = -42.379 + 2.049T + 10.143H − 0.225TH − 6.838×10⁻³T² − 5.482×10⁻²H² + 1.229×10⁻³T²H + 8.528×10⁻⁴TH² − 1.99×10⁻⁶T²H², where T = temp (°F), H = relative humidity (%).
Worked Example
Example: 95°F air temperature, 60% humidity → heat index ≈ 114°F ("feels like").
Formula / Method
NWS Rothfusz regression: HI = -42.379 + 2.049T + 10.143H − 0.225TH − 6.838×10⁻³T² − 5.482×10⁻²H² + 1.229×10⁻³T²H + 8.528×10⁻⁴TH² − 1.99×10⁻⁶T²H², where T = temp (°F), H = relative humidity (%).