Heat Index Calculator (NWS / Rothfusz Approximation)
Heat index is how hot it feels when humidity slows sweat evaporation. This page follows the US National Weather Service procedure: a Steadman-style simple formula below about 80°F, and the Rothfusz multiple-regression above, with the published dry-air and very-humid adjustments. It is an outdoor apparent-temperature sketch, not a medical monitor, indoor WBGT, or wet-bulb globe.
Heat index40.4 °C
Heat index104.7 °F
NWS-style bandDanger (103–125°F)
How this calculation works
Convert temperature to Fahrenheit if you entered Celsius. Relative humidity is percent 0–100.
Simple branch: HI = 0.5 × (T + 61.0 + (T−68)×1.2 + RH×0.094). If the average of that HI and T is under 80°F, that simple value is used.
Otherwise Rothfusz: HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T×RH − 6.83783×10⁻³T² − 5.481717×10⁻²RH² + 1.22874×10⁻³T²×RH + 8.5282×10⁻⁴T×RH² − 1.99×10⁻⁶T²×RH², with NWS adjustments when RH < 13% (80–112°F) or RH > 85% (80–87°F).
Bands (HI in °F): under 80 none, 80–90 caution, 90–103 extreme caution, 103–125 danger, 125+ extreme danger. Shade, activity, and full sun change real risk; NWS itself warns the formula is for shade and light wind.
Worked example
90°F and 70% RH → heat index about 106°F (danger band). 32.2°C is the same 90°F. 80°F and 40% RH stays near 80°F on the simple branch.
Frequently asked questions
Is this wet-bulb globe temperature (WBGT)?
No. WBGT includes sun, wind, and black-globe temperature. Heat index is air temperature plus humidity in a shade/light-wind regression.
Why can 70% humidity feel worse than the air temperature?
Sweat cannot evaporate as easily, so the body sheds less heat. That is the whole point of heat index.
Indoor air conditioning?
If you type indoor T and RH you get an indoor “feels like” in the same formula, but NWS categories were built for outdoor summer warnings.
Source?
NWS heat-index equation (Rothfusz 1990 / NWS liaisons). We do not invent a 2026 replacement. If NWS publishes a new official equation, prefer that.
Medical emergency?
Heat stroke is a clinical emergency. A calculator number is not a diagnosis. Seek local emergency care when someone is confused, not sweating, or collapsing in the heat.