Heat Index Calculator

Enter the air temperature and relative humidity to see how hot it feels. Humid air makes the same temperature feel hotter because sweat evaporates more slowly.

Source: NOAA National Weather Service – What is the heat index? ↗ · NWS – The Heat Index "Equation" (SR 90-23) ↗

This result is an estimate only. Check with your bank or the relevant authority before making decisions.

How it is calculated

The NWS method works in °F, so the temperature is converted first.

A simple formula is tried first: HI = 0.5 × (T + 61 + (T − 68) × 1.2 + RH × 0.094).
If the average of that result and T is 80 °F or more, the full Rothfusz regression is used instead, with the NWS adjustments for very dry air (RH below 13%) and for very humid air (RH above 85% at 80–87 °F).
The result is converted back to °C.

The risk levels are the NWS ones: 80–90 °F Caution, 91–102 °F Extreme caution, 103–124 °F Danger and 125 °F or more Extreme danger. See also the haze API checker when haze and heat come together.

Worked example

33 °C (91.4 °F) at 75% humidity:

The regression gives about 114.3 °F, which is 45.7 °C
That falls in the Danger range, so heat cramps or exhaustion are likely with long exposure or activity

Why your number may differ

The heat index is for shade and light wind. Direct sun can add up to about 8 °C, and heavy work, clothing and your own health change how hot you feel. The formula was fitted mainly to temperatures from 80 °F up, and the NWS calls it an approximation. This is general information, not medical advice.

Frequently asked questions

What is the heat index?

The heat index is the temperature the body feels when air temperature and humidity are combined. It was developed by the US National Weather Service for shade and light wind.

Is this the same as MetMalaysia's heat alerts?

No. MetMalaysia's heat alerts use the maximum air temperature, not the heat index. Level 1 is 35–37 °C, Level 2 is above 37 up to 40 °C and Level 3 is above 40 °C, each for three days in a row.

How do I stay safe in hot weather?

Drink water regularly, rest in the shade or air-conditioning, wear light clothing, and avoid hard activity at midday. Take extra care with children, older people and people with health conditions.