🥵 Heat Index Calculator

What This Calculator Does

This calculator produces the heat index — summer's “feels-like” temperature — from the air temperature and relative humidity, using the National Weather Service equation behind every US heat advisory. Enter the afternoon's numbers and see what your body is actually being asked to handle.

The heat index is the mirror image of wind chill: where wind steals heat faster than the thermometer admits, humidity blocks your cooling more than the thermometer shows. Both convert an invisible physiological load into one comparable number — and both save people from underestimating conditions.

The NWS Equation and the Physiology

The operational formula — the Rothfusz regression, adopted by the NWS in 1990 — is a polynomial fit to Robert Steadman's 1979 “apparent temperature” model, which simulated a human body's heat balance: metabolism, clothing, sweating, and the physics of evaporation into air of a given humidity. The regression compresses that model into a computable equation in temperature and relative humidity, with small NWS adjustments at the very humid and very dry edges. At 90°F and 70% RH it returns about 105°F; at 95°F and 60%, about 114°F.

The physiology it encodes: evaporating sweat is the body's primary heat pump, and evaporation slows as the air fills with moisture. High humidity does not add heat — it disables cooling, so core temperature climbs at air temperatures that dry climates shrug off. The index answers “what dry-air temperature would stress the body this much?”, which is why a Gulf Coast 92°F can be more dangerous than a desert 105°F, and why the number belongs in every summer forecast where humidity lives.

Reading the Danger Categories

The NWS pairs the index with exposure categories that are worth knowing cold. 80-90°F: caution — fatigue possible with prolonged exposure and activity. 90-103°F: extreme caution — heat cramps and heat exhaustion possible. 103-124°F: danger — heat exhaustion likely, heat stroke possible with continued activity. 125°F+: extreme danger — heat stroke likely. Advisories and excessive-heat warnings key off these bands, adjusted regionally for what populations are acclimatized to.

The categories assume an average person; the thresholds arrive earlier for children, older adults, outdoor workers, athletes mid-exertion, and anyone on medications that impair heat regulation. The practical responses are unglamorous and effective: shift exertion to morning, hydrate ahead of thirst, take shade breaks on a schedule rather than by feel (heat illness famously degrades judgment first), and know the escalation signs — cramps and heavy sweating, then cool clammy skin and nausea (exhaustion: stop, cool, hydrate), then hot dry skin and confusion (stroke: emergency, cool aggressively, call for help). The index's job is to trigger that seriousness a day early.

What the Index Assumes — and Does Not

Like wind chill, the heat index has boundaries that casual use ignores. It is computed for shade and light wind: full sun can raise the effective value by up to 15°F, which is why a “98 feels like 108” forecast can undersell a treeless afternoon. It applies from roughly 80°F and 40% humidity upward — below those, evaporation runs freely and the index adds little (this calculator flags out-of-range inputs). And it describes a healthy adult at light activity; exertion effectively moves you categories upward.

Two related numbers complete the summer picture. Dew point measures humidity absolutely (dew points above 70°F feel oppressive at any temperature) and many meteorologists prefer it for describing mugginess. Wet-bulb globe temperature (WBGT) — used by the military, OSHA guidance, and athletics — folds in sun and wind directly and governs formal work-rest standards; when institutions set heat policy, WBGT is usually the instrument. For everyday judgment, though, the heat index remains the number on the forecast and the one this calculator gives you — an honest translation of humid heat into terms a body understands.

Quick Tips

• 90°F + 70% humidity ≈ 105°F heat index; sun can add up to 15°F more.
• Danger bands: 90-103 extreme caution, 103-124 danger, 125+ extreme danger.
• Humidity blocks sweating — hydrate ahead of thirst and schedule shade breaks.
• Hot dry skin and confusion = heat stroke = emergency. Cool first, transport fast.

Frequently Asked Questions

How is the heat index calculated?

With the National Weather Service's Rothfusz regression — a polynomial in temperature (°F) and relative humidity that models how humid air impairs sweating. At 90°F and 70% humidity, the heat index is about 105°F.

Why does humidity make heat feel hotter?

Your body sheds heat mainly by evaporating sweat, and humid air accepts less evaporation. The heat index expresses the combination as the dry-air temperature that would stress the body equally — 90°F at 70% humidity taxes you like 105°F dry.

What heat index is dangerous?

NWS categories: 80-90°F caution (fatigue possible), 90-103°F extreme caution (heat cramps and exhaustion possible), 103-124°F danger (heat exhaustion likely, heat stroke possible), 125°F+ extreme danger (heat stroke likely). Prolonged exposure and exertion move every threshold earlier.

When does the heat index apply?

The formula is designed for temperatures of about 80°F and above with humidity 40%+ (the calculator notes when inputs fall outside it). It also assumes shade and light wind — full sun can add as much as 15°F to the effective value.

The Official Number, Free to Use

This calculator runs entirely in your browser, applying the NWS equation the moment you type. It needs no account or download and works on any device. Like every tool here, it is free to use as often as you like.

Related Tools

For the other side of the year, the wind chill calculator handles winter’s feels-like, the temperature converter switches °F and °C, and the water intake calculator covers the hydration side.