Heat Index Calculator

How to use

1. 1Pick your unit system: Fahrenheit (the NWS default) or Celsius. The result panel shows both units regardless.
2. 2Choose how you want to enter humidity: Relative humidity % for the most common case, or Dew point if that is the reading you already have.
3. 3Type the air temperature and the humidity (or dew point). The heat index, OSHA risk band, and a difference from air temperature update on every keystroke.
4. 4Use the quick-pick humidity chips (20%, 30%, ..., 90%) to flip through scenarios fast, or read the full NWS heat-index reference table at the bottom for any combination.
5. 5Click Copy report to grab a plain-text summary including the air temperature, humidity, dew point, heat index, and risk advisory for a ticket, email, or safety briefing.

About this tool

Heat Index Calculator turns an air temperature plus a humidity reading into the apparent temperature the National Weather Service publishes in summer advisories. The calculation uses Lans P. Rothfusz's 1990 regression of Robert Steadman's full apparent-temperature model (the exact formula behind the official NWS heat-index web calculator), and applies both standard adjustments: the low-humidity correction when relative humidity is below 13 percent in the 80 F to 112 F range, and the high-humidity correction when relative humidity is above 85 percent near 80 F. When the simple Steadman screening value is below 80 F, the tool reports no heat-index effect and shows the air temperature directly, which matches how the NWS chart treats cooler weather. You can type temperature and humidity in either Fahrenheit or Celsius; the result panel always shows both units and reports the dew point, relative humidity, and difference from the dry-bulb air temperature alongside the headline value. Switch the humidity input to dew point and the tool derives relative humidity from the Magnus-Tetens approximation before running the regression, which lets aviation and HVAC users plug in the dew point reading they already have without doing a hand conversion. The result is colour-coded against the four standard NWS bands (Caution 80 to 89 F, Extreme Caution 90 to 102 F, Danger 103 to 124 F, Extreme Danger 125 F and above), and a complete heat-index reference chart at the bottom shows every cell from 80 F to 112 F across 40 to 100 percent humidity so you can sanity-check a single point against the official table. Useful for outdoor workers and athletes checking heat stress before training, OSHA-style workplace safety planning for construction or warehouse crews, parents deciding when to keep kids inside, HVAC techs sizing cooling loads, event planners scheduling around afternoon heat, gardeners watering plants in peak humidity, dog walkers timing summer walks, and anyone who has typed 'what does 95 degrees with 70 humidity feel like' into a search bar. All math runs locally in your browser using standard JavaScript arithmetic, so the temperatures and weather data you type here never leave your device.

Free to use. Works in your browser. No signup, no login.