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Heat Index Calculator (Feels-Like Temperature)

How hot does it actually feel at this temperature and humidity?

Single calculationv1.0.0Verified 2026-11-01Runs in your browser

Spec sheet

nws_heat_index · v1.0.0

Method

NWS heat index algorithm

01Simple: HI = 0.5 (T + 61 + (T − 68)·1.2 + 0.094·RH); if average with T ≥ 80 °F use Rothfusz
02Rothfusz: −42.379 + 2.049T + 10.143RH − 0.2248T·RH − … (9 terms)
03Low-RH and high-RH adjustments per NWS

Limits — read before relying on it

Assumes shade and light wind; full sun can add up to 15 °F.
Valid mainly above 80 °F (27 °C).

Sources

NWS Weather Prediction Center — The Heat Index Equation ↗standard

Worked example

computed by the same formula

90 °F at 70 % RH

Inputs

Air temperature: 90
Unit: °F
Relative humidity: 70 %

Result

Heat index: 106 °F
Heat index: 41.1 °C
NWS level: Danger

Questions

How is the heat index calculator calculated?

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NWS heat index algorithm. Simple: HI = 0.5 (T + 61 + (T − 68)·1.2 + 0.094·RH); if average with T ≥ 80 °F use Rothfusz · Rothfusz: −42.379 + 2.049T + 10.143RH − 0.2248T·RH − … (9 terms) · Low-RH and high-RH adjustments per NWS

Example: 90 °F at 70 % RH?

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Heat index: 106 °F; heat index: 41.1 °C.

What does this calculator not cover?

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Assumes shade and light wind; full sun can add up to 15 °F. Valid mainly above 80 °F (27 °C).

Where do the numbers come from?

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Sources: NWS Weather Prediction Center — The Heat Index Equation. The formula is versioned (nws_heat_index v1.0.0) and covered by automated tests.

Is my data stored?

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No. The calculation runs in your browser and nothing is sent unless you choose to save a shareable link.

Also searched asheat index calculator · feels like temperature · heat index chart · humidex · how hot does it feel

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