Heat Index Calculator
Feels like temperature from air temp and humidity, with heat illness guidance in F and C.
What it does
Heat index translates thermometer air plus humidity into the temperature the body feels. Damp air slows sweat evaporation, so cooling stalls and the same 90 degrees punishes far more at 60% humidity than at 20%.
The tool above runs the National Weather Service regression in both systems. Enter air temperature with relative humidity and read the feels like value with a heat risk band beside it.
The default of 90 F at 60% feels like 100 F, extreme caution territory where heat illness is possible with long effort. At 100 F with 40% the index reaches 109 F, danger territory where illness is likely.
The equation comes from the Weather Service Rothfusz technical paper with field office heat pages matching its bands. Both define it from 80 F upward and warn that direct sun adds up to 15 F on top.
How to use
- Pick imperial (F) or metric (C).
- Enter the shaded station air temperature.
- Enter relative humidity as a percent.
- Read feels like first for activity choices.
- Read the risk band before long outdoor effort.
Formula
- T
- Air temperature in F inside the formula.
- R
- Relative humidity percent.
- bands
- 80-90 caution, 90-103 extreme caution, 103-124 danger, 125+ extreme danger.
Metric converts to F first, then runs the same regression, so systems agree. Dry heat below 13% and muggy heat above 85% near 80 to 87 F get small NWS adjustments inside the result.
Worked example
Example: 90 F with 60% humidity
Imperial defaults: air 90 F, humidity 60%.
- Regression combines 90 with 60 through nine terms to 99.7.
- No dry or muggy edge adjustment applies here.
- Rounds to 100 F (38 C).
- Band: 90 to 103 reads extreme caution, illness possible.
- Tool shows 100 F, the extreme caution band, Air 90 F at 60% RH.
Reference table
| Air | Humidity | Feels like |
|---|---|---|
| 90 F | 60% | 100 F |
| 100 F | 40% | 109 F |
| 32.2 C | 60% | 38 C |
| 37.8 C | 40% | 43 C |
Limits
- Hot air only: below 80 F the index has no meaning.
- Shade assumed; direct sun adds up to 15 F.
- Still air assumed; breeze helps, exertion hurts.
- Age, clothing, and health shift personal risk around the number.
Common mistakes
- Running the index on warm mild days and quoting scary feels.
- Reading car sensors in sun as air temperature.
- Mixing C inputs with the F constants.
- Treating the index as a thermometer reading objects feel.
- Skipping water because the thermometer looks moderate.
Try it yourself
Quick checks. Answers sit tucked under each question.
100 F at 40% humidity?
109 F: danger, heat illness likely with long exposure.
70 F day entered?
Rejected: enter 80 to 120 F, the index starts at 80.
Metric defaults 32.2 C at 60%?
38 C, the same air as the 100 F example.
Questions
What is the heat index formula?
The nine term Rothfusz regression on temperature and humidity, plus small edge adjustments.
When does heat index apply?
At 80 F and above. Below that, air temperature tells the story.
Humidity or temperature: which matters more?
Both multiply. 90 F at 60% feels like 100 F; dry 90 F feels near 90 F.
Heat index versus humidex?
Cousins: the US index in F against the Canadian humidex in C. Same muggy idea.
Why does shade matter?
Sun adds radiant load up to 15 F. The chart assumes shady, light wind rest.
What helps most at 100 F feels like?
Water, shade breaks, lighter effort, and cool spaces. Symptoms need care fast.