This heat index calculator computes the “feels-like” temperature from air temperature and relative humidity using the National Weather Service’s Rothfusz regression — the same numbers behind heat advisories — with the danger category spelled out.
Why Humidity Steals Your Cooling
Humans cool by evaporation; humid air is already near saturation, so sweat stops working and core temperature climbs — the heat index prices that failure. The Rothfusz regression fits Steadman's physiological model across the hot-humid grid the NWS actually warns on (80 °F and up). Humidity's other public face is the Dew Point: many people read it more honestly than RH% (a 70 °F dew point is oppressive at any temperature). Unit sanity via the Temperature Converter. Part of our Other Calculators collection.
Worked Example (Verified)
- 90 °F at 70% RH → heat index 106 °F — deep in the DANGER band: the widget's defaults, a classic Gulf Coast afternoon
- Same 90 °F at 40% RH → ≈ 91 °F — humidity contributed 15 degrees of the misery in the first case, the whole point of the index
- 100 °F at 40% → ≈ 109 °F; at 60% → ≈ 129 °F (extreme danger) — dry-ish deserts and humid subtropics reach the same danger by different roads
- Below 80 °F the index fades out (the simple Steadman blend this calculator uses) — cool air can't be made dangerous by humidity alone
The NWS Category Ladder
| Heat index | Category | Meaning |
|---|---|---|
| Below 80 °F | — | Normal caution only |
| 80–90 °F | Caution | Fatigue possible with prolonged exertion |
| 90–103 °F | Extreme caution | Cramps/exhaustion likely; stroke possible |
| 103–124 °F | Danger | Exhaustion likely; stroke probable with exposure |
| 125 °F+ | Extreme danger | Heat stroke imminent |
Related Tools
- Wind Chill Calculator — the cold-season twin
- Dew Point Calculator — humidity in its most honest unit
- Other Calculators — the full collection
Frequently Asked Questions
How is the heat index calculated?
By the Rothfusz regression on temperature (°F) and relative humidity — a nine-term polynomial fitted to Steadman's heat-balance model, with small adjustments for very dry and very humid extremes. 90 °F and 70% RH evaluate to ≈ 106 °F.
Why does it "feel" hotter than the heat index sometimes?
The index assumes shade and a light breeze. Direct sun adds up to 15 °F of perceived heat, exertion generates internal heat, and dark pavement radiates — the official number is a shaded-baseline, and forecasters say so for a reason.
Heat index vs dew point — which should I watch?
Heat index for danger decisions (it integrates temperature), dew point for comfort forecasting (it's stable through the day while RH% swings with temperature). A dew point above 65 °F feels muggy; above 70 °F, oppressive — regardless of what RH% says at noon vs dawn.
At what heat index should outdoor work or sport stop?
Guidelines (OSHA/ACSM families) escalate rest-and-water mandates from ~91 °F, with the strictest regimes suspending strenuous exertion in the 104–125 °F bands depending on acclimatization. The category table above is the public-facing version of the same ladder.
Can the heat index be lower than the temperature?
Yes, slightly — in hot, very dry air (e.g., 100 °F at 10% RH computes ≈ 95 °F), evaporation works better than the baseline model assumes. The dry adjustment in the formula handles it, and desert residents confirm the phenomenon ("dry heat" is real, within limits).
Is this heat index calculator free?
Yes — free, instant, no sign-up.