Heat Index Calculator

Your body's primary cooling system is evaporation: sweat absorbs about 580 calories per gram as it vaporizes. Evaporation rate depends on the vapor-pressure gap between your skin and the air — high humidity narrows that gap until sweat drips instead of evaporating, removing almost no heat. At 95°F and 70% RH the index hits 124°F because that's the dry-air temperature producing the same net heat load on a body that can barely cool itself.

NWS bands: 80–90°F caution (fatigue with prolonged exposure), 90–103 extreme caution (heat cramps and exhaustion possible), 103–124 danger (heat exhaustion likely, stroke possible with exertion), 125+ extreme danger (stroke imminent). Risk multiplies with exertion, direct sun (add up to 15°F), age, and several medication classes. Most US heat deaths occur at index values the chart colors orange, not crimson — duration and lack of overnight relief kill more than single-afternoon peaks.

Below roughly 80°F, ordinary humidity doesn't meaningfully impede the body's heat shedding — the skin-to-air temperature gradient and modest sweating cope regardless of RH, so 'feels like' ≈ actual temperature and the regression's assumptions break. (Other systems behave differently: Canada's humidex starts registering in the 70s°F because it's a pure vapor-pressure formula rather than a physiological model — our humidex tool shows the contrast.)

The formula does — the assumptions mostly don't: it presumes a 5-knot breeze and shade. Still indoor air with no fan feels worse than the computed index (no convective assist), while a fan restores and improves on it UNTIL air temperature passes ~95°F, beyond which a fan blows heat INTO you faster than evaporation removes it — the reason health agencies warn against fans as the sole cooling in extreme heat. Indoor heat-safety guidance uses WBGT or simple temperature thresholds instead.