HesapHanem

Heat Index Calculator

Enter the temperature and humidity to find the heat index and how hot it actually feels.

How it is calculated

The heat index is the temperature it feels like once humidity slows the evaporation of sweat. The tool converts your air temperature from °C to °F (F = C × 9/5 + 32), follows the NWS procedure with temperature and relative humidity (RH, 0 to 100%): it first uses Steadman's simple formula and, when the average of that result and the air temperature is 80 °F or more, switches to the Rothfusz regression, with the low-humidity (RH under 13%, 80 to 112 °F) and high-humidity (RH over 85%, 80 to 87 °F) adjustments. The answer is converted back to °C. The category is based on the °C result: below 27 no category (shown as —), 27 to under 32 Caution, 32 to under 41 Extreme Caution, 41 to under 54 Danger, 54 and above Extreme Danger. In mild weather the heat index stays close to the air temperature and the category shows —. Wind, direct sun and individual health are not included.

Worked example

The air is 32 °C with 70% relative humidity. In Fahrenheit that is 32 × 9/5 + 32 = 89.6 °F. Putting 89.6 °F and RH 70 into the regression gives about 104.7 °F, which is 40.4 °C. That sits in the 32 to 41 band, so the category is Extreme Caution: heat cramps and exhaustion are possible with prolonged exposure.

Try this example →

Common mistakes

  • Entering the temperature in °F; the field expects °C and the result would be absurdly high.
  • Typing humidity as a fraction (0.7) instead of a percentage (70).
  • Expecting a warning category in cool weather. Below a heat index of 27 °C the category is —, which only means no heat-stress band applies.
  • Treating the heat index as the real air temperature; it is a perceived value.
  • Forgetting that sun exposure can raise the felt heat considerably above this figure.

About this calculator

This calculator finds the NWS heat index — the apparent temperature the National Weather Service uses, calculated with the Rothfusz regression from air temperature and relative humidity — since high humidity slows sweat evaporation and makes the air feel hotter than the thermometer says.

Meteorologists and weather services use it to issue heat advisories, and coaches, outdoor workers and event organizers use it to judge real heat risk and plan hydration breaks or rescheduling.

Was this helpful?

You can pick up to 3 reactions.

Comments (1)

  • Editorial team

    Use a quick plausibility test on the output. In hot, humid weather the heat index should come out at or above the air temperature. In very dry heat it can come out lower, as for 35 °C at 10 percent humidity (31.9 °C), and below a heat index of 27 °C the category shows a dash. Neither includes direct sun, so add margin for work or sport in full sunlight. What conditions are you planning around?

Popular calculators

Loan Calculator

Monthly payment, total interest and full amortization schedule.

Percentage Calculator

Percent change, percent of, and ratio calculations.

Discount Calculator

Calculate final price, savings, and discount amount.

BMI Calculator

Body mass index from height and weight (metric or imperial).

Age Calculator

Compute your exact age in years, months and days from a birth date.

Compound Interest Calculator

Calculate the final amount and interest earned with compounding.

All Calculators