Heat Index Calculator
Calculate heat index instantly in your browser — the published formula, worked locally with no data sent anywhere.
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What this tool does
Calculate heat index instantly in your browser — the published formula, worked locally with no data sent anywhere. The Rothfusz regression is the US National Weather Service standard, fitted for shade with light wind; full sun can add up to 15 °F. It is defined for temperatures of 80 °F and above — below that a simple Steadman average is shown.
How to use the Heat Index Calculator
- Enter or select air temperature (°f).
- Enter or select relative humidity (%).
- Read the calculated result; change any measurement to compare alternatives.
Formula
NWS Rothfusz regression: HI = −42.379 + 2.04901523·T + 10.14333127·R − 0.22475541·T·R − 0.00683783·T² − 0.05481717·R² + 0.00122874·T²·R + 0.00085282·T·R² − 0.00000199·T²·R² (T in °F, R in %), with the NWS low- and high-humidity adjustments; below 80 °F the simpler Steadman formula is used.
- temp
- Air temperature (°F)
- rh
- Relative humidity (%)
The Rothfusz regression is the US National Weather Service standard, fitted for shade with light wind; full sun can add up to 15 °F. It is defined for temperatures of 80 °F and above — below that a simple Steadman average is shown.
Worked example
For heat index calculator, the following measurements illustrate the exact method: Air temperature (°F): 90; Relative humidity (%): 70.
Inputs
- Air temperature (°F)90
- Relative humidity (%)70
Result
- Heat index (°F)105.92
- Heat index (°C)41.07
- Air temperature (°F)90
- NWS risk categoryDanger — heat cramps and exhaustion likely, heat stroke possible
Results explained
- Heat index (°F)
- Heat index (°F) from the formula above. The Rothfusz regression is the US National Weather Service standard, fitted for shade with light wind; full sun can add up to 15 °F. It is defined for temperatures of 80 °F and above — below that a simple Steadman average is shown.
- Heat index (°C)
- Heat index (°C) from the formula above. The Rothfusz regression is the US National Weather Service standard, fitted for shade with light wind; full sun can add up to 15 °F. It is defined for temperatures of 80 °F and above — below that a simple Steadman average is shown.
- Air temperature (°F)
- Air temperature (°F) from the formula above. The Rothfusz regression is the US National Weather Service standard, fitted for shade with light wind; full sun can add up to 15 °F. It is defined for temperatures of 80 °F and above — below that a simple Steadman average is shown.
- NWS risk category
- NWS risk category from the formula above. The Rothfusz regression is the US National Weather Service standard, fitted for shade with light wind; full sun can add up to 15 °F. It is defined for temperatures of 80 °F and above — below that a simple Steadman average is shown.
Frequently asked questions
NWS Rothfusz regression: HI = −42.379 + 2.04901523·T + 10.14333127·R − 0.22475541·T·R − 0.00683783·T² − 0.05481717·R² + 0.00122874·T²·R + 0.00085282·T·R² − 0.00000199·T²·R² (T in °F, R in %), with the NWS low- and high-humidity adjustments; below 80 °F the simpler Steadman formula is used.
The Rothfusz regression is the US National Weather Service standard, fitted for shade with light wind; full sun can add up to 15 °F. It is defined for temperatures of 80 °F and above — below that a simple Steadman average is shown.
Your body cools by evaporating sweat. Humid air is already near saturation, so sweat evaporates slowly and core temperature climbs — the heat index is the temperature that would produce the same stress in dry reference conditions.
Every field is labelled with its unit — enter values in exactly the labelled unit and read the result in the labelled output unit. Fields with a unit symbol also support the site-wide imperial/metric switch, which converts before the formula runs.
It uses double-precision arithmetic and the published constants and formulas named on this page. The last displayed digit may be rounded, and real conditions can differ from the idealized model described in the assumptions.
Yes. Every calculation runs entirely in your browser; nothing you enter is uploaded, stored or shared.