Heat Index Calculator
See what the heat actually feels like. Enter the air temperature with either the relative humidity or the dew point, and get the heat index from the National Weather Service’s Rothfusz equation — scored against the official caution-to-extreme-danger categories.
The regression is designed for air temperatures of about 80°F+ and humidity of 40%+. In full sun, add up to 15°F to the result (NWS). Dew point entries use the same unit as the temperature.
Why humidity makes heat feel hotter
Your body sheds most of its heat in hot weather by evaporating sweat. Humid air already holds a lot of water vapor, so sweat evaporates slowly and cooling stalls — 94°F at 55% humidity stresses your body like 106°F would in dry air. That equivalent temperature is the heat index. It assumes shade and a light breeze; the NWS notes direct summer sun can add up to 15°F.
The categories matter for planning: once the index passes 90°F, heat cramps and exhaustion become likely with continued activity, and past about 103°F heat stroke moves from possible to probable. Children, outdoor workers, and anyone on fluid-restricting medication hit trouble sooner.
How it’s calculated
NWS Rothfusz regression (1990): HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T² − 0.05481717R² + 0.00122874T²R + 0.00085282TR² − 0.00000199T²R² (T °F, R %), with the NWS low-humidity and high-humidity adjustments; below about 80°F the simpler Steadman average is used, per NWS practice. Dew point mode first converts to relative humidity with the Magnus formula.
Heat tolerance varies with hydration, fitness, age, medication, and acclimatization — treat categories as guidance and take heat advisories seriously.
NWS heat index categories
| Heat index | Category | Typical effects with exposure or activity |
|---|---|---|
| 80–90°F (27–32°C) | Caution | Fatigue possible with prolonged exposure and activity |
| 90–103°F (32–39°C) | Extreme caution | Heat cramps and heat exhaustion possible |
| 103–124°F (39–51°C) | Danger | Heat cramps/exhaustion likely; heat stroke probable with continued activity |
| 125°F+ (52°C+) | Extreme danger | Heat stroke imminent |
Source: U.S. National Weather Service heat index chart.
Worked example
On a 94°F afternoon at 55% relative humidity, the heat index is 106°F (41°C) — 12°F above the thermometer and squarely in the NWS Danger band, where heat exhaustion is likely for anyone active outdoors. The matching dew point is a tropical 75°F. Using dew point mode instead: 86°F with a 70°F dew point implies 59% humidity and a heat index of about 91°F.
Common mistakes
- Using the heat index for readings taken in direct sun — official values assume shade; sun can add up to 15°F.
- Entering dew point in °C while the temperature unit is set to °F (both share one unit setting).
- Expecting big numbers below 80°F — the index only separates from air temperature in real heat.
- Ignoring overnight lows: repeated days with warm, humid nights drive heat illness even when afternoons seem manageable.
Where it is used
- Deciding when to move workouts, practices, and races to the morning.
- OSHA-style job-site heat planning: water, shade, and rest cycles by category.
- Timing pet walks and checking playground safety in a heat wave.
- Understanding NWS Heat Advisory and Excessive Heat Warning triggers.
Frequently asked questions
What formula is behind this?
The Rothfusz regression the National Weather Service uses for its heat index chart, including the official adjustments for very dry (RH < 13%) and very humid (RH > 85%) air. Below about 80°F the NWS falls back to Steadman’s simple average, and so does this calculator.
Why does 90°F sometimes feel like 105°F?
Evaporation. At high humidity your sweat can barely evaporate, so your main cooling system underperforms and your body heats up as if the air were much hotter. At 90°F, moving from 40% to 80% humidity lifts the heat index from about 91°F to 113°F.
Does shade versus sun change the number?
The heat index is defined for shady, light-wind conditions. The NWS notes exposure to full sunshine can increase the felt value by up to 15°F — so a 100°F index can behave like 110–115°F on an unshaded field.
Should I use relative humidity or dew point?
Either works — they describe the same moisture given the temperature. Dew point is handy because it doesn’t swing through the day the way RH does; above about a 70°F dew point, most people feel oppressed regardless of the RH percentage.
At what heat index should I cancel outdoor plans?
By NWS categories: use caution from 80–90°F, cut intensity and add breaks from 90–103°F (Extreme Caution), and treat 103°F+ (Danger) as a strong signal to reschedule strenuous activity — heat stroke becomes probable with continued exertion.