Heat Index Calculator

Given

Dew Point

°C ▾

Relative Humidity

% ▾

Related

Heat Index Calculator

The heat index calculator estimates the apparent temperature felt by the human body based on environmental temperature and relative humidity. It automatically converts values between Celsius, Fahrenheit, and Kelvin for convenience.

What is the Heat Index?

“The heat index is the temperature, or humidex, as felt by the human body.”

The heat index combines air temperature and relative humidity to indicate how hot it feels. High humidity reduces the effectiveness of sweating, increasing perceived temperature above the actual air temperature.

How to Calculate Heat Index

The heat index can be calculated using the air temperature and relative humidity. Sunlight, shade, and wind also affect the actual perceived temperature.

Heat Index Formula

HI = c₁ + c₂T + c₃φ + c₄Tφ + c₅T² + c₆φ² + c₇T²φ + c₈Tφ² + c₉T²φ²

Where:

T = Air temperature (°C or °F)
φ = Relative humidity (%)
c₁, c₂, …, c₉ = Constants depending on the temperature scale

Constants for Heat Index Calculation

The constants depend on the temperature scale. Use a chart for accurate results:

Parameter	°C	°F
c₁	-8785	-42,379
c₂	1611	2049
c₃	2339	10,143
c₄	-0.146	-0.225
c₅	-1.231 × 10⁻²	-6.838 × 10⁻³
c₆	-1.642 × 10⁻²	-5.482 × 10⁻²
c₇	2.212 × 10⁻³	1.229 × 10⁻³
c₈	7.255 × 10⁻⁴	8.528 × 10⁻⁴
c₉	-3.582 × 10⁻⁶	-1.990 × 10⁻⁶

Effects of the Heat Index

High heat index conditions reduce the efficiency of sweating, increasing the risk of:

Heat cramps
Heat exhaustion
Heat stroke

Always check humidity levels before outdoor activities in hot conditions.

Heat Index Chart

Precautionary measures based on heat index values:

Celsius	Fahrenheit	Precautions
27–32°C	80–90°F	Fatigue possible with prolonged exposure. Risk of heat cramps.
32–41°C	90–105°F	Fatigue likely. Heat cramps may occur.
41–54°C	105–130°F	*Danger: heat cramps, heat exhaustion likely; heat stroke probable with continued activity.*
Over 54°C	Over 130°F	*Extreme danger: heat stroke imminent. Avoid all physical activity.*