About

The heat index — sometimes called the "feels-like" temperature or humiture — is a single number that combines air temperature and relative humidity to describe the thermal stress your body experiences. High humidity slows evaporation of sweat, the body's primary cooling mechanism, so humid air at a moderate temperature can feel dramatically hotter than dry air at the same reading.

For example, 90 °F air at 90 % humidity produces a heat index near 122 °F — territory where heat stroke becomes a serious risk within minutes of strenuous activity. The thermometer alone would never tell you that.

The heat index is most meaningful above 80 °F (27 °C) and 40 % relative humidity. Below those thresholds the body copes well and the simple air temperature is a good enough guide.

This site uses the Rothfusz regression published by the U.S. National Weather Service, the same equation used in official NWS heat-index tables and forecasts:

where T is air temperature in °F and RH is relative humidity in percent. Two correction terms adjust the result when humidity is very low (below 13 %) or very high (above 85 %) and temperature falls in a specific range, improving accuracy at the extremes.

For conditions below the threshold where the full regression applies — roughly when the simple average of temperature and a basic heat-index estimate is under 80 °F — the calculator automatically falls back to a simpler blended formula so the output stays sensible even on mild days.

If you enter a dew point instead of relative humidity, the calculator first converts it to RH using the Magnus approximation before running the heat-index model.

The manual heat index calculator at the top of the home page lets you punch in any air temperature and either a relative humidity percentage or a dew point. It supports Fahrenheit, Celsius, and Kelvin inputs and updates instantly as you type — no button to press.

The result card shows the feels-like temperature alongside a risk classification drawn from the NWS heat-index scale:

• Comfortable — below 80 °F heat index; no meaningful heat stress.
• Caution — 80–90 °F; fatigue possible with prolonged exertion.
• Extreme Caution — 90–105 °F; heat cramps and exhaustion are possible.
• Danger — 105–130 °F; heat exhaustion likely; heat stroke probable with activity.
• Extreme Danger — above 130 °F; heat stroke is imminent.

The "Check by location" section beneath the manual calculator fetches live conditions — temperature and humidity — from Open-Meteo for any city, region, or country you type, or for your GPS coordinates if you tap the location button. It then runs the same NWS heat-index model on that live data and shows the result instantly. No account or API key is required.

Once you load a location, a heat index chart appears below the result cards. It plots the hourly heat index for the past seven days, pulling the same Open-Meteo data source used for current conditions. The chart makes it easy to spot the daily temperature cycle — cooler nights, peak afternoon heat — and to see whether today's conditions are unusually hot or typical for the recent stretch of weather at that location.

The chart respects whichever temperature unit (°F, °C, or K) you have selected, and hovering or tapping any point shows the exact feels-like temperature and timestamp for that hour.

Live weather data is provided by Open-Meteo, a free and open-source weather API that aggregates forecasts from national meteorological services around the world. Conditions update hourly. No data is stored on our servers — every calculation happens in your browser.

The heat index is a guide, not a medical instrument. Individual factors — sun exposure, physical fitness, medications, age, and acclimatization — all shift the threshold at which heat becomes dangerous. When in doubt, seek shade, hydrate, and follow guidance from your local health authority.