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How to calculate heat index from temperature and humidity

How to calculate heat index from temperature and humidity

A 90 °F afternoon at 70% humidity feels like 106 °F. The heat index puts a number on that, and the US National Weather Service uses it to decide when heat becomes dangerous.

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Why the thermometer understates the heat

Sweat cools the body by evaporating. Humid air slows that down, so the body holds on to more heat than the air temperature suggests. At 90 °F and 70% relative humidity the heat index is 105.9 °F, which the National Weather Service puts in its Danger level. At 85 °F and 40% humidity it is 84.3 °F, a little below the air temperature.

Planning a run, a shift outdoors, or a kids' game on the thermometer reading alone misses that difference. The NWS levels start at Caution from 80 °F (27 °C), and Danger begins at 103 °F (39 °C).

Calculate it, then convert it

The Heat Index Calculator takes the air temperature and either the relative humidity or the dew point, in °F or °C. It follows the NWS method: a simple formula first, then the full regression formula when the result reaches 80 °F, with small adjustments for very low or very high humidity. A dew point is turned into relative humidity with the Magnus formula. The answer comes with its NWS warning level, in both units.

If a forecast gives Celsius and you think in Fahrenheit, or the other way round, the Temperature Converter switches between °C, °F, K, and °R. 32 °C is 89.6 °F, and at 60% humidity it feels like 37.1 °C.

Calculate the heat index

  1. Read the air temperature and the relative humidity from a forecast or a weather station. A dew point works too.
  2. Convert the temperature to °F if it is in °C, because the NWS formula works in Fahrenheit.
  3. Work out the simple estimate: 0.5 × (T + 61 + (T − 68) × 1.2 + RH × 0.094).
  4. If the average of that estimate and the temperature is 80 °F or more, use the full NWS regression instead. The Heat Index Calculator does this step for you.
  5. Compare the result with the NWS levels: Caution from 80 °F, Extreme caution from 90 °F, Danger from 103 °F, and Extreme danger from 125 °F.

What the heat index does not account for

  • It assumes shade and a light breeze. Direct sun can add up to 15 °F (8 °C).
  • Below 80 °F (27 °C) the index stays close to the air temperature.
  • It is general information, not medical advice. How a person copes with heat also depends on health, age, and activity.

Frequently asked questions

What is the heat index at 90 °F and 70% humidity?

105.9 °F, in the NWS Danger range.

What heat index is dangerous?

The NWS calls 103 °F (39 °C) and above Danger, and 125 °F (52 °C) and above Extreme danger.

Can I calculate the heat index in Celsius?

Yes. Choose °C in the Heat Index Calculator. It converts to °F for the formula and shows the result in both units.

Does wind change the heat index?

Wind speed is not part of the formula. The index assumes shade and a light breeze.

Tools used in this guide

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