Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Heat index (feels like)
109
NWS risk category
Danger
Degrees hotter than air temp
14
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter air temperature (°F).
- 2Enter relative humidity (%).
- 3Read your heat index (feels like) on the right — it updates as you type.
- 4Hit Share to keep the scenario or send it to someone.
About this calculator
Heat index is what your body actually experiences when humidity slows evaporative cooling from sweat. The National Weather Service uses the Rothfusz regression, a polynomial fit to Steadman's original biometeorological model, valid for temperatures at or above 80°F and relative humidity at or above 40%. Below that range the NWS just reports actual air temperature since the correction is negligible. This tool applies the full regression plus the two standard adjustments: a downward correction for low humidity with high heat, and an upward correction for high humidity with moderate heat. Heat index drives NWS heat advisories and warnings — 90-103°F is caution territory for heat exhaustion, 103-124°F triggers extreme caution to danger, and 125°F+ is extreme danger with heat stroke likely on continued exposure.
Worked example
Using the values the calculator loads with:
Inputs
- Air temperature: 95 °F
- Relative humidity: 55 %
Results
- Heat index (feels like): 109
- NWS risk category: Danger
- Degrees hotter than air temp: 14
What each field means
Inputs
- Air temperature (°F)
- The air temperature used in the calculation, measured in °F. Starts at 95 °F so you have a working example on load. Accepted range: 40–130 °F.
- Relative humidity (%)
- The relative humidity used in the calculation, measured in %. Starts at 55 % so you have a working example on load. Accepted range: 0–100 %.
Results
- Heat index (feels like)
- Returned as a whole number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- NWS risk category
- Returned as a plain value. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Degrees hotter than air temp
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Why doesn't heat index apply below 80°F?
The Rothfusz regression was fit to data in the 80-110°F, 40-100% RH range, and below 80°F humidity's effect on perceived temperature is small enough that the NWS just reports the actual air temperature instead of a barely-different corrected number.
Is heat index the same as the heat category used for outdoor sports?
No. Athletic and military heat categories (like WBGT — wet bulb globe temperature) also factor in direct sun load and wind, so they run hotter-feeling at the same air temp and humidity than NWS heat index, which assumes shade and light wind.
What humidity makes the biggest difference?
Between 40-70% RH at temperatures near 90-100°F the curve is steepest — each 10-point jump in humidity can add 5-10°F to the feels-like number, which is why muggy 92°F days feel more dangerous than dry 100°F days.
How accurate is this for direct sunlight?
Add roughly 10-15°F to the result if you're in direct sun rather than shade; the NWS heat index formula assumes shaded conditions with light wind around 5 mph.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Wind Chill Calculator
Feels-like cold temperature from air temp and wind speed using the NWS 2001 formula.
Dew Point Calculator
Compute dew point and comfort level from temperature and relative humidity.
WBGT Heat Stress Calculator
Estimate Wet Bulb Globe Temperature and safe work/exercise guidance in the sun.
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Heat Index Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/heat-index-calculator
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/heat-index-calculator" target="_blank" rel="noopener">Heat Index Calculator — RevenueLab</a> (2026).</p>
Source: [Heat Index Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/heat-index-calculator) (2026).