Heat Index Calculator

About the Heat Index Calculator

A heat index calculator determines the "feels like" temperature by accounting for humidity's effect on the body's ability to cool through perspiration. When relative humidity is high, sweat evaporates more slowly, reducing the cooling effect of perspiration and making air feel significantly hotter than the thermometer reading. Heat is the deadliest weather phenomenon in the United States, killing more people annually than hurricanes, tornadoes, floods, and lightning combined — more than 700 deaths per year. Understanding heat index enables individuals, event organisers, coaches, and public health officials to make informed safety decisions before dangerous heat becomes life-threatening. Our calculator uses the official NWS Rothfusz regression equation, includes the NWS heat index danger categories, and shows frostbite's opposite — heat stroke risk timing thresholds.

How It Works

NWS Heat Index uses the Rothfusz regression equation (valid when T ≥ 80°F and RH ≥ 40%): HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541×T×RH − 0.00683783×T² − 0.05481717×RH² + 0.00122874×T²×RH + 0.00085282×T×RH² − 0.00000199×T²×RH², where T = temperature °F and RH = relative humidity %. For T < 80°F or simpler estimates, a Steadman approximation is used: HI ≈ T + 0.33×e − 4.0, where e = water vapour pressure. Example: 95°F and 65% humidity: HI ≈ 116°F — dangerously hot, in the NWS "Danger" category. At the same 95°F with 30% humidity: HI ≈ 100°F — significantly more comfortable.

Tips & Best Practices

• ✓NWS heat danger categories: Caution: HI 80-90°F (fatigue possible with prolonged exposure). Extreme Caution: 90-103°F (heat cramps/exhaustion possible). Danger: 103-124°F (heat exhaustion/stroke likely). Extreme Danger: 125°F+ (heat stroke highly likely).
• ✓Heat stroke emergency: core body temperature > 104°F (40°C) is a medical emergency. Signs include hot dry skin (no longer sweating), rapid confused breathing, loss of consciousness. Call 911 and cool the person immediately with ice water or cold wet cloths.
• ✓Hydration: in hot and humid conditions, the body can lose 1-2 litres of sweat per hour during exercise. Replace fluids before you feel thirsty — thirst is already a sign of mild dehydration.
• ✓Vulnerable populations: the elderly, children under 4, people with chronic illness (heart disease, diabetes), and those on certain medications (diuretics, antihistamines, beta-blockers) are at significantly higher risk at lower heat index values.
• ✓Night-time minimum temperatures: urban heat islands can keep overnight temperatures dangerously high, preventing the body's recovery from daytime heat stress. Prolonged multi-day heat without overnight relief is more dangerous than a single hot day.
• ✓Direct sun adds 10-15°F to the effective heat index: the NWS formula uses air temperature in the shade. Full sun exposure increases the effective temperature substantially above the calculated heat index.
• ✓WBGT (Wet Bulb Globe Temperature): sports organisations and military use WBGT, which accounts for temperature, humidity, wind, and radiant heat in a single measurement. Heat index correlates with but differs from WBGT.
• ✓Acclimatisation: the body takes 10-14 days to physiologically adapt to heat (improved sweating efficiency, cardiovascular adaptation). This period is when heat-related illness risk is highest for unacclimatised individuals.

Who Uses This Calculator

Public health officials issuing heat advisories and opening cooling centres. Athletic coaches deciding whether to modify or cancel outdoor practices. Outdoor event organisers assessing spectator safety in summer heat. Construction and agricultural supervisors monitoring worker heat exposure. Parents making decisions about outdoor activities for children. Emergency managers planning response to extreme heat events. Sports medicine professionals monitoring athlete heat acclimatisation.