Heat Index Calculator
Calculate Heat Index values for science, physics, chemistry, and engineering problems.
This heat index calculator estimates the 'feels-like' temperature by combining air temperature with relative humidity, reflecting how hot the human body actually perceives conditions to be when humidity limits sweat evaporation.
Enter the air temperature and relative humidity, and the calculator returns the heat index.
Worked Calculation Examples
| Scenario | Result | Calculation Step |
|---|---|---|
| 90°F air temperature, 70% relative humidity | Heat index ≈ 105°F | Per the NWS heat index chart, 90°F combined with 70% relative humidity produces a "feels-like" temperature around 105°F - in the "extreme caution" category. |
| 95°F air temperature, 50% relative humidity | Heat index ≈ 107°F | Per the NWS heat index chart, 95°F at 50% relative humidity produces a heat index around 107°F, also in the "extreme caution" to "danger" range. |
Why Humidity Makes Heat Feel Worse
The human body cools itself primarily through sweat evaporation. High humidity slows this evaporation, since the surrounding air is already closer to saturated with moisture, which reduces the body's ability to shed heat - this is why 90°F at high humidity feels considerably hotter than 90°F in dry air.
Heat Index Danger Levels
Heat index values are commonly grouped into caution categories: values around 80-90°F suggest caution, 90-105°F extreme caution, 105-130°F danger, and above 130°F extreme danger, with rising risk of heat exhaustion or heat stroke at the higher categories, especially with prolonged exposure or physical exertion.
How to Use the Heat Index Calculator
- Open the Heat Index Calculator and enter the values requested in the input fields.
- Check the units, percentages, dates, or time periods before reading the answer.
- Review the instant result and adjust any value to compare another scenario.
- Use the formula, example, and FAQs below to understand how the heat index calculator works.
Frequently Asked Questions
How is heat index calculated?
Heat index combines air temperature and relative humidity using a regression formula developed by the National Weather Service, reflecting how hot the body actually perceives conditions when sweat evaporation is impaired by humidity.
Why does high humidity make hot weather feel worse?
High humidity reduces the rate at which sweat can evaporate from skin, which is the body's main cooling mechanism, making the same air temperature feel significantly hotter than it would in dry conditions.
At what heat index level should I be concerned?
Values above 90°F warrant extra caution, and above 105°F is generally considered dangerous, with increasing risk of heat exhaustion or heat stroke, especially during physical activity or prolonged outdoor exposure.
How do I use this heat index calculator?
Enter the known values, review the units or settings, and the calculator updates the result instantly. The formula and example on this page show how the answer is produced.
What does the Heat Index Calculator calculate?
Calculate Heat Index values for science, physics, chemistry, and engineering problems. It is designed for fast browser-based calculations without sign-up, downloads, or manual spreadsheet setup.
What formula does this heat index use?
The formula is: Heat Index = f(Temperature, Relative Humidity) — Rothfusz regression, NWS 1990. The US National Weather Service adopted the current heat index formula, developed by researcher Lans Rothfusz in 1990, as a multiple-regression fit to earlier physiological research by Robert Steadman, who in 1979 had modeled how humidity impairs the body's evaporative cooling. The formula's real value lies in translating that underlying physiology into a single, easily communicated number for public heat-safety warnings, rather than requiring the public to interpret separate temperature and humidity readings.
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Last updated: September 27, 2026.