Calculation details
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What does this calculator estimate?
A heat index calculator computes the apparent (feels-like) temperature from air temperature and relative humidity, using the National Weather Service formula. Enter the temperature and humidity to see the heat index.
- Heat index combines temperature + humidity
- NWS formula valid above 80°F and 40% RH
- Above ~103°F is dangerous; ~125°F is extremely dangerous
What the heat index measures
High humidity slows sweat evaporation, so your body cools less effectively — 90°F at 60% humidity can feel like 100°F. The heat index converts temperature and humidity into the 'feels like' number for that condition.
Limitations to watch for
The formula is calibrated for shade conditions — direct sun can add up to 15°F. It's designed for temperatures above 80°F and humidity above 40%; outside that range the tool returns the air temperature. Wind and individual heat tolerance aren't modeled.
How to use it in practice
Check the heat index before outdoor work or exercise, not just the air temperature. Above 90°F, take more breaks, hydrate, and watch for heat illness symptoms. Above 103°F (NWS 'danger'), limit strenuous activity and seek shade/AC.
['Enter the air temperature in °F.', 'Enter the relative humidity as a percent.', 'The tool computes the heat index and its risk zone.']
How this calculator works
Formula
Heat index (NWS regression, valid for T ≥ 80°F and RH ≥ 40%): HI = −42.379 + 2.04901523×T + 10.14333127×RH − 0.22475541×T×RH − 0.00683783×T² − 0.05481717×RH² + 0.00122874×T²×RH + 0.00085282×T×RH² − 0.00000199×T²×RH². Below those thresholds, HI = T.
Worked example
At 90°F with 60% relative humidity, the heat index is about 100°F — the temperature it feels like when humidity slows evaporative cooling.
Assumptions to verify
- The NWS heat-index regression is used.
- Conditions are in the shade.
- Temperature is in Fahrenheit.
Frequently asked questions
What is the heat index?
The apparent temperature — how hot it feels when relative humidity is factored in with the air temperature. Humidity reduces sweat evaporation, making it feel hotter.
How is it calculated?
The National Weather Service uses a regression formula based on temperature and relative humidity, valid above 80°F and 40% humidity. Below those, the heat index equals the air temperature.
Why is humidity important?
Your body cools by evaporating sweat. High humidity slows evaporation, so the same temperature feels hotter and is harder on your body.
When is heat index dangerous?
NWS categories: 90–103°F caution, 103–124°F danger (cramps and heat exhaustion likely), 125°F+ extreme danger (heat stroke possible).
Does direct sun matter?
Yes — the heat index is for shade. Direct sunlight can add up to 15°F to the feels-like temperature.
Does wind change it?
Wind helps evaporative cooling at moderate temperatures, but the standard heat index doesn't include it — the wind chill index is the cold-weather counterpart.
Is heat index the same as feels-like temperature?
Roughly — 'feels like' in weather apps often combines heat index (summer) or wind chill (winter) with other factors.
Cite this tool
BoringToolsKit. “Heat Index Calculator.” boringtoolskit.com/heat-index-calculator/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
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