Heat Index Calculator (Feels-Like Temperature)
Combine air temperature and humidity into the heat index — the “feels-like” temperature the National Weather Service uses to gauge heat risk.
Calculator
At 90 °F and 70% RH it feels like 106 °F (NWS heat index) — extreme caution / danger.
The heat index, or “feels-like” temperature, captures why a humid 90 °F day is far more punishing than a dry one. When the air is humid, sweat evaporates slowly, so the body sheds heat less effectively and the air feels hotter than the thermometer reads. The National Weather Service quantifies this with the Rothfusz regression, and this tool applies it to your temperature and humidity to return a single feels-like number plus its NWS risk band.
It is the standard behind heat advisories and the reason employers and coaches watch humidity, not just temperature, on hot days. The formula is defined for warm conditions; below about 80 °F it falls back to a simpler estimate, since the heat-index concept only matters when it is genuinely hot.
Formula
For temperatures at or above 80 °F, the NWS Rothfusz regression (T in °F, RH in %):
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
Two adjustments refine the extremes: a subtraction for very low humidity (RH < 13% between 80–112 °F) and an addition for very high humidity (RH > 85% between 80–87 °F). Below 80 °F the tool uses the simpler NWS averaging form.
Worked example
At 90 °F and 70% relative humidity, plugging into the regression gives a heat index of about 106 °F. In other words, that humid 90-degree afternoon stresses the body like a dry 106-degree one — squarely in the NWS “extreme caution / danger” band where heat cramps and heat exhaustion become likely with prolonged exposure.
Drop the humidity to 40% at the same 90 °F and the heat index falls to roughly the low 90s — the same air temperature, but a very different strain on the body. That gap is exactly what the heat index exists to show. For the moisture side of comfort, see the dew point tool.
Reading the risk bands
The NWS groups the heat index into bands: below 90 °F is generally safe; 90–103 °F is “caution” (fatigue possible); 103–125 °F is “extreme caution / danger” (heat cramps and exhaustion likely, heat stroke possible); and above 125 °F is “extreme danger” with heat stroke highly likely. These assume shade and light wind — in full sun the effective heat index can be up to 15 °F higher.
The heat index is a comfort-and-safety indicator, not medical advice. Hydration, exertion level, age, acclimatization and direct sun all change real risk. Use it to plan outdoor work and cooling, and remember that indoors the fix is air conditioning plus dehumidification — humidity is half the equation, which is why the dehumidifier and ventilation tools sit alongside this one.
For strenuous outdoor work, agencies such as OSHA lean on WBGT (wet-bulb globe temperature), which also folds in sun load and wind, rather than the heat index alone. Acclimatization matters too: someone used to desert heat tolerates a given heat index far better than a visitor on their first hot day. Treat the number here as a planning gauge for scheduling breaks, hydration and cooling — not as a personal safety threshold. When the feels-like climbs into the danger bands, shift heavy activity to the cooler hours and get out of direct sun.
Reference table
NWS heat-index risk bands (in shade):
| Heat index | Risk level |
|---|---|
| < 90 °F | No elevated risk |
| 90–103 °F | Caution: fatigue possible |
| 103–125 °F | Extreme caution / danger |
| ≥ 125 °F | Extreme danger |
Full sun can raise the effective heat index by up to 15 °F. Not medical advice.