Heat index calculator

Calculate the NWS apparent temperature from air temperature and relative humidity.

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Air temperature (degrees F)

Relative humidity (%)

Answer

…

Calculate the NWS apparent temperature from air temperature and relative humidity.

More results

Precision and calculation record

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Rounding changes displayed numbers, not the calculation. Text, fractions, matrices and CSV schedules keep their own formatting. Automatic keeps up to 12 significant digits, with money shown to cents where applicable.

Saved scenario

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Formula and assumptions

Uses the NWS simple estimate below 80 F apparent temperature and the Rothfusz regression with humidity adjustments above it. For shade and light wind. This implementation rejects results above 137 F, rather than extrapolating extreme combinations. It does not forecast exposure limits or individual heat illness.

Automatic precision shows numbers to 12 significant digits. Thousands separators such as 1,000 are accepted; use a dot for decimals.

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Updated 2026-09-29

Using Heat index calculator

Calculate the NWS apparent temperature from air temperature and relative humidity.

Next: Temperature converter

A worked example

At 90 °F and 70% relative humidity the heat index is about 106 °F.

Before you use the result

Example results for air temperature (degrees f)

These examples use Relative humidity (%): 70. They are reference calculations, not recommended settings.

Air temperature (degrees F)	Heat index (degrees F)
45	43.745
90	105.922021

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