Heat Index Calculator

The heat index is the apparent temperature you feel when air temperature and humidity are combined. It is calculated using the NWS Steadman formula and reflects how hot it feels to a healthy adult walking in shade. At 95 F with 60% humidity, the heat index is about 114 F, meaning your body experiences thermal stress equivalent to a dry 114 F day. The chart assumes shade and light wind; direct sun adds another 15 F.

Heat index matters more for health decisions. Temperature alone misses humidity, which is the key variable in heat stress because sweat cools you by evaporating, and high humidity slows evaporation. A 90 F day at 70% humidity (HI 106 F) is more dangerous than a 100 F day at 15% humidity (HI 100 F). When in doubt, check the heat index, not the thermometer.

NWS defines four risk bands. Caution: 80-90 F (fatigue possible). Extreme Caution: 90-103 F (heat cramps and exhaustion possible). Danger: 103-125 F (heat cramps and exhaustion likely, heat stroke possible with continued activity). Extreme Danger: 125 F+ (heat stroke highly likely). Heat advisories are typically issued in the Extreme Caution range; excessive heat warnings in the Danger range or above.

The NWS uses the Steadman regression formula: HI = -42.379 + 2.04901523*T + 10.14333127*RH - 0.22475541*T*RH - 0.00683783*T*T - 0.05481717*RH*RH + 0.00122874*T*T*RH + 0.00085282*T*RH*RH - 0.00000199*T*T*RH*RH, where T is air temperature in F and RH is relative humidity as percent. Adjustments apply for very low humidity (< 13%) and very high humidity (> 85%). Below 80 F, a simpler formula is used.

Sweat cools you by evaporating off your skin. Each gram of water evaporating removes 580 calories of heat. Dry air has plenty of room for water vapor, so sweat evaporates fast and cooling is efficient. Humid air is already loaded with water vapor, so evaporation slows. Your sweat sits and drips instead of cooling you. Above 95 F wet-bulb temperature, evaporation stops entirely and no healthy adult can survive prolonged exposure.

Heat exhaustion: core body temperature 100-103 F, heavy sweating, weakness, nausea, but the person is lucid. Treat with shade, cool water in sips, cool wet cloths. Most cases improve in 20-30 minutes. Heat stroke: core temperature above 104 F, plus altered mental status (confusion, slurred speech, agitation). Skin can be hot and dry, or still sweating profusely. Call 911 immediately and begin aggressive cooling, ice water immersion if available, otherwise ice packs on neck, armpits, groin.

OSHA recommends 1 cup (8 oz) every 15-20 minutes during outdoor work at heat index 80-90 F, totaling 1 quart per hour. Above heat index 90 F, the same frequency plus mandatory 15-minute shade rest each hour. For sustained activity above HI 90 F, switch from plain water to a sports drink with 100-200 mg sodium per 8 oz, pure water at that volume risks hyponatremia. Pre-hydrate 17-20 oz two hours before activity.

Dry heat reads lower on the heat index chart at the same air temperature because sweat evaporates faster. But dehydration risk is much higher in dry climates because you stop noticing you are sweating, it evaporates instantly. Tourists in places like Phoenix or Las Vegas die more from dehydration than heat stroke. Humid heat is more immediately life-threatening at the same temperature, but desert heat catches people off guard.

Wet-bulb is the lowest temperature evaporative cooling can achieve under current humidity. It is measured by a thermometer wrapped in a wet cloth. At 100% humidity, wet-bulb equals air temperature (no evaporation possible). The 95 F wet-bulb threshold is widely cited as the physiological survival limit for healthy adults. Most of the world rarely approaches this. WBGT (Wet-Bulb Globe Temperature) extends the concept by adding sun and wind, it is what athletic governing bodies and the US military use for activity decisions.

Heat index 80-90 F is fine for most healthy adults; vulnerable groups (elderly, infants, people on medications affecting thermoregulation) should reduce intensity. HI 90-103 F: limit strenuous activity, move workouts to early morning or evening. HI 103-125 F: cancel non-essential outdoor activity. HI 125 F+: stay indoors with AC. Add 15 F if you will be in direct sun for an extended period, the NWS chart assumes shade.

Heat index calculation does not change with time of day, but it is computed from air temperature and humidity, both of which usually fall after sunset. Most dangerous indoor heat happens 4-8 hours after peak outdoor temperature, when unconditioned homes are still warm but outside has cooled, people open windows expecting relief and instead trap heat inside. During heat waves, nighttime cooling is reduced and the cumulative heat exposure builds.

Higher risk: adults over 65, infants and young children, pregnant people, people with heart disease, diabetes, kidney disease, or obesity. People on diuretics, beta blockers, antihistamines, antipsychotics, anticholinergics have reduced thermoregulation. People in their first 14 days of heat-exposed work or sports practice have not acclimatized and are at elevated risk. Healthy acclimatized adults handle 5-10 F more heat than the chart suggests.

The NWS chart assumes shade with a light wind. Direct sun adds approximately 15 F to the effective heat index. So a 95 F shaded HI 99 F becomes an effective HI 114 F in direct sun, Danger band. This is why people who feel worse than the heat index suggests are usually in sun while reading the shaded number. For outdoor activity decisions, add the sun correction.

Use it with extra caution. Dogs cool primarily through panting, which becomes inefficient above 85 F. Brachycephalic breeds (pugs, bulldogs, boxers) overheat at temperatures other dogs handle. Also check the surface temperature: asphalt at 130 F (which happens at air 90 F in direct sun) causes paw burns in 60 seconds. The 7-second back-of-hand test: if you cannot hold your hand on pavement, do not walk your dog on it.

The NWS heat index is the US standard. Canada and parts of the UK use it. Other regions use similar but different formulas: Australia uses an Apparent Temperature formula by Steadman that includes wind; many countries use Humidex (Canada) which incorporates dew point; the WMO recommends UTCI (Universal Thermal Climate Index) for international comparison. For practical purposes, the heat index numbers translate roughly across systems within 2-3 degrees.