Weather

Heat Index Calculator

Estimate the heat index: how hot it feels when humidity slows sweat evaporation. Based on the US National Weather Service Rothfusz regression.

Calculate

Measurements convert when you switch units

°C
%

Updates as you type

Result

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The formula

The heat index combines air temperature and relative humidity into a shade “feels like” value. The NWS publishes a multiple-regression equation in Fahrenheit; we convert inputs and outputs when you use metric units.

HI = Rothfusz regression (°F, RH%) — converted for metric display

Worked example

  1. 32 °C air temperature (~90 °F)
  2. 70% relative humidity
  3. Heat index ≈ 41 °C

Result: Feels like ≈ 41 °C

How the heat index is calculated

High humidity slows evaporative cooling, so the same air temperature can feel much hotter on a muggy day.

Rothfusz regression

The NWS heat index is a regression fit (often called the Rothfusz equation) relating temperature and relative humidity to apparent temperature under standardized assumptions (shade, light wind).

Valid range

The full regression is most meaningful near or above about 27 °C / 80 °F with non-trivial humidity. Cooler or very dry conditions may not need a heat-index adjustment.

Sun and exertion

Published values assume shade. Full sun can feel several degrees hotter. Heavy work, dark clothing, and poor acclimatization raise personal risk beyond the index alone.

Safety categories

NWS heat-index charts map ranges to caution, extreme caution, danger, and extreme danger. Follow local heat advisories — they incorporate forecasts and regional criteria.

Interesting facts

Valid in hot, humid air

The full NWS regression is most accurate near or above ~27 °C / 80 °F with meaningful humidity.

Caution levels rise quickly

Above roughly 32 °C (90 °F) heat index, prolonged exposure raises heat-stress risk — prioritize shade, water, and rest.

Shade vs sun

Heat index assumes shade and light wind. Full sun can feel several degrees hotter than the reported index.

Humidity’s role

Moisture slows sweat evaporation, so muggy days feel worse at the same thermometer reading.

Individuals differ

Age, medications, heart disease, and exertion change risk even at the same heat-index value.

Frequently asked questions

Enter air temperature and relative humidity (%). We evaluate the NWS Rothfusz regression and show a “feels like” temperature.

Relative humidity from 0–100%, from a weather app or station — not dew point (use the dew-point calculator for that).

Shade. Full sun can feel several degrees hotter than the index.

Use NWS category charts and local alerts. As a rough cue, prolonged exposure above ~90 °F / 32 °C heat index warrants extra precautions for many people.

References

  1. Heat index equation and chart — U.S. National Weather Service Official heat-index methodology and caution categories.
  2. Extreme heat health guidance — U.S. Centers for Disease Control and Prevention (CDC) Public-health context for heat-related illness prevention.