Heat Index Calculator (NWS) – Free Calculator

Understanding the heat index calculator — concept and principle

The heat index, sometimes called the "feels-like" temperature, combines air temperature and relative humidity into the temperature your body perceives in the shade. High humidity slows the evaporation of sweat, so a humid 32 °C day stresses the body like a much hotter dry day. The US National Weather Service (NWS) uses it for heat advisories.

At 32 °C and 60% humidity the body feels about 37 °C — the extreme caution band.

How it is calculated

Convert temperature to Fahrenheit: F = T × 9/5 + 32.
Simple estimate: HI = 0.5 × (F + 61 + (F − 68) × 1.2 + RH × 0.094).
If the average of HI and F is 80 °F or more, use the Rothfusz regression: HI = −42.379 + 2.04901523F + 10.14333127RH − 0.22475541F·RH − 0.00683783F² − 0.05481717RH² + 0.00122874F²·RH + 0.00085282F·RH² − 0.00000199F²·RH².
NWS adjustments: subtract a term when RH < 13% and 80–112 °F; add one when RH > 85% and 80–87 °F.
Convert back: °C = (HI − 32) × 5/9.

The principle

R. G. Steadman (1979) modelled how a clothed adult walking in the shade exchanges heat with air through skin, sweat and breathing, and calculated the "apparent temperature" for each combination of temperature and humidity. Lans Rothfusz (1990) fitted a polynomial to Steadman's table so it could be computed easily; that polynomial is the regression above.

Worked example

Inputs: T = 32 °C, RH = 60%.
F = 32 × 9/5 + 32 = 89.6 °F.
Simple estimate = 91.08 °F; average with 89.6 = 90.34 ≥ 80, so the regression is used.
Rothfusz regression = 98.73371142 °F; no adjustment applies (RH is between 13% and 85%).
In Celsius: (98.734 − 32) × 5/9 = 37.07428412 °C.
98.7 °F lies between 90 and 103 °F, so the risk level is extreme caution.

Each point is the calculator re-run with only “Temperature (°C)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Temperature (°C)	Heat index (°C)
8	6.422222222
16	15.22222222
24	24.02222222
32	37.07428412
40	62.64334158
48	101.9724792
64	221.9109947

How to read the result

Where does your result fall?

NWS categories: caution 80–90 °F (about 27–32 °C), where fatigue is possible with prolonged activity; extreme caution 90–103 °F (32–39 °C), where heat cramps and heat exhaustion are possible; danger 103–124 °F (39–51 °C), where heat exhaustion is likely; extreme danger 125 °F (52 °C) and above, where heat stroke is highly likely.

Common mistakes

Using it in direct sunlight; NWS notes that full sun can raise heat index values by up to about 15 °F (8 °C).
Applying it to cold or mild weather; it is designed for hot conditions.
Comparing it directly with the Korea Meteorological Administration's summer "feels-like" temperature, which uses a different formula.

Keep in mind: The heat index is a general guide, not a medical diagnosis. Elderly people, infants, people with chronic illness and those doing heavy work can be harmed at lower values. Seek medical help for confusion, fainting or very high body temperature.

Frequently asked questions

Is the heat index the same as WBGT?

No. WBGT also accounts for sun and wind and is preferred for workers and athletes, while the heat index uses only temperature and humidity in the shade.

What is the accuracy of the regression?

NWS states an error of about ±1.3 °F against Steadman's values within its valid range.

Sources: R. G. Steadman, The Assessment of Sultriness, Part I, Journal of Applied Meteorology 18(7), 861–873 (1979) · L. P. Rothfusz, The Heat Index Equation, NWS Technical Attachment SR 90-23 (1990) · NOAA National Weather Service, Heat Index (Weather Prediction Center calculator notes)

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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