Heat Index Calculator

The heat index combines air temperature and relative humidity into a single feels-like number. The math behind it is the Rothfusz regression — a 9-term polynomial in T (temperature, °F) and R (relative humidity, %) derived in 1990 by NWS meteorologist Lans Rothfusz from the 1979 Steadman heat-balance tables. The full equation runs whenever the simple Steadman heat index would exceed 80 °F. Below 80 °F, your body's evaporative cooling stays ahead of humidity load, so the heat index effectively equals the air temperature and the regression is skipped. Two correction terms refine the result: when humidity is very low (under 13%) and temperatures are 80-112 °F, a low-humidity adjustment is subtracted; when humidity is very high (over 85%) and temperatures are 80-87 °F, a high-humidity adjustment is added. Both corrections bring the regression result in line with the published NWS heat-index chart. Our calculator applies all three pieces — simple heat index, full Rothfusz, and the two adjustments — exactly as the NWS does.

The NWS heat-index chart divides apparent temperatures into four caution bands. Caution (80-90 °F / 27-32 °C): fatigue possible with prolonged exposure or activity; healthy adults tolerate it routinely. Extreme Caution (90-103 °F / 32-39 °C): heat cramps and heat exhaustion possible; sunstroke possible with prolonged exposure. Danger (103-125 °F / 39-51 °C): heat exhaustion likely; heatstroke possible with continued exposure. Extreme Danger (125 °F+ / 51 °C+): heatstroke imminent for active or vulnerable people. These bands are calibrated to healthy adults at rest in shade — multiply risk for outdoor workers, athletes, infants, the elderly, and people on heat-impairing medications. The NWS issues a Heat Advisory when heat index will hit 100 °F+ for 2 hours and an Excessive Heat Warning at 105 °F+ for 2 consecutive days.

Air temperature is what your thermometer reads; heat index is what your body feels. The gap between them is driven entirely by relative humidity, because humidity controls how fast sweat evaporates from your skin — and sweat evaporation is your body's primary cooling mechanism. At 90 °F with 30% humidity, heat index is about 90 °F (no effect). At 90 °F with 80% humidity, heat index jumps to 113 °F (a 23-degree difference from humidity alone). At 100 °F with 80% humidity, heat index reaches 158 °F — a 58-degree gap. This is why humid afternoons in Miami, Houston, and Mumbai feel dramatically hotter than the thermometer suggests, and why desert heat at the same air temperature is more tolerable. Wind helps slightly by speeding evaporation, but the Rothfusz formula assumes light wind only.

These three terms often refer to the same thing — but the precise meaning depends on which country you're in. Apparent temperature is the technical name for any combined-effect feels-like value (Steadman, Rothfusz, humidex, AT, WBGT all qualify). Heat index is the US-specific Rothfusz-based apparent temperature used by NWS. Feels-like temperature is the marketing term used by weather apps and TV; in summer it equals heat index, in winter it equals wind chill. Humidex (Canada) and BOM apparent temperature (Australia) are mathematically distinct but produce comparable numbers in typical conditions. Wet Bulb Globe Temperature (WBGT) is a different, more comprehensive measurement that adds sun and wind effects to humidity and temperature. For everyday outdoor-safety decisions in the US, UK, and Australia, heat index and feels-like are interchangeable; use WBGT for sports, military, and industrial heat-management protocols.

The Rothfusz formula is defined in degrees Fahrenheit, so to compute heat index in Celsius our calculator converts your °C input to °F, runs the regression, then converts the result back to °C. This is the same approach used by Environment Canada and the UK Met Office when reporting heat index in metric units. Quick reference: 80 °F = 26.7 °C (start of Caution band); 90 °F = 32.2 °C (start of Extreme Caution); 103 °F = 39.4 °C (start of Danger); 125 °F = 51.7 °C (start of Extreme Danger). NWS Heat Advisory threshold of 100 °F equals 37.8 °C; Excessive Heat Warning at 105 °F equals 40.6 °C. Always check which unit a weather report uses — the same number means very different conditions in °F vs °C.

OSHA's heat-illness prevention guidance ties work intensity to heat index levels. For acclimatized outdoor workers in light to moderate work: heat index below 91 °F — normal work. 91-103 °F (Extreme Caution): 15 minutes of rest per hour in shade, water every 15-20 min, eye on coworkers for symptoms. 103-115 °F (Danger): 30 min rest per hour, mandatory buddy system, ice towels available, supervisor heat-illness training current. Above 115 °F (Extreme Danger): minimize outdoor exertion; if work must continue, 45+ min rest per hour with active cooling. For unacclimatized workers (first 5 days on the job or first hot week of season), reduce all thresholds by 10 °F. Provide cool potable water within 20 m of work areas, shaded rest space, and ensure first-aid trained personnel are on site. These rules also work for athletic practices and military training.

Heat exhaustion is the precursor to heatstroke and the most common heat illness above 91 °F heat index. Early symptoms: heavy sweating, weakness, cold pale clammy skin, fast weak pulse, nausea or vomiting, muscle cramps, dizziness. Response: move person to shade or AC, loosen clothing, apply cool wet cloths or cold water bath, sip cool water (not iced) over 30 minutes. Heatstroke is a medical emergency above 103-105 °F core body temperature: hot dry skin OR profuse sweating, throbbing headache, confusion or unconsciousness, body temperature 103 °F+, fast strong pulse. Response: call 911 immediately, move to coolest possible space, cool the person aggressively (ice packs to armpits/neck/groin, cold-water immersion if available), do NOT give fluids if unconscious. Heatstroke kills 600+ people per year in the US alone; rapid cooling within 30 minutes is the single biggest factor in survival.

Athletic governing bodies use heat index (or WBGT) thresholds to modify practice. NATA (National Athletic Trainers Association) guidance for football and other heavy-equipment sports: heat index under 91 °F — normal practice. 91-103 °F — 15 min rest per 45 min practice, water available continuously, equipment reduced where possible. 103-115 °F — practice in helmets and shorts only, 20 min rest per 40 min practice. Above 115 °F — cancel or postpone practice. For sports without heavy equipment, thresholds are 5 °F higher. NATA also recommends WBGT instead of heat index for sports in direct sun — heat index in shade vs WBGT in sun typically differs by 5-15 °F. The first week of summer practice is highest risk because athletes are not yet acclimatized; reduce intensity 50% for days 1-3 and ramp up over 7-14 days.

Your body maintains core temperature at 98.6 °F primarily through sweat evaporation. When sweat evaporates from your skin, it carries away heat — about 580 calories per gram of water evaporated. The rate of evaporation depends on the difference between water vapor pressure at your skin (saturated, set by your body) and water vapor pressure in the surrounding air (set by humidity). At low humidity, the gradient is steep, evaporation is fast, and you stay cool. At high humidity, the gradient is shallow, evaporation slows or stops, and heat builds up in your body. Above 95% relative humidity in hot weather, sweating becomes essentially useless as a cooling mechanism — a condition called wet-bulb saturation. Wet-bulb temperatures above 95 °F (35 °C) are theoretically lethal to healthy adults regardless of activity level because no amount of sweating can shed heat fast enough. Heat index captures this nonlinear physics in a single number.

Scenario 1 — Atlanta afternoon: 92 °F air, 65% humidity. Heat index ≈ 105 °F (Danger). NWS Excessive Heat Warning territory. Outdoor practice should be modified per NATA; OSHA buddy checks every 30 min for outdoor workers. Scenario 2 — Phoenix afternoon: 110 °F air, 12% humidity. Heat index ≈ 108 °F (Danger). Low-humidity adjustment subtracts about 2 °F. Still dangerous despite the 'dry heat' reputation — dehydration is the main risk here. Scenario 3 — London heatwave: 32 °C (89.6 °F) air, 55% humidity. Heat index ≈ 92 °F / 33 °C (Extreme Caution). Triggers UK Met Office Amber Heat-Health Alert because northern Europe isn't acclimated to these conditions. Scenario 4 — Miami summer evening: 86 °F air, 85% humidity. Heat index ≈ 100 °F (Extreme Caution, Heat Advisory). Even 'mild' overnight temperatures combined with tropical humidity produce dangerous indoor heat for non-AC households.

Heat index and WBGT both measure heat stress but differ in inputs and use case. Heat Index uses two inputs (air temperature, humidity), assumes shade with light wind, and is the standard for public-facing weather communication (NWS advisories, weather apps, general outdoor planning). WBGT uses four inputs (air temperature, humidity, wind speed, solar radiation), measures heat stress in direct sun, and is the standard for sports (NCAA, NATA, military), industrial heat-management programs, and ASHRAE guidelines. WBGT is typically 5-15 °F lower than heat index in direct sun because it accounts for sweat-evaporating wind and counts solar load explicitly. Rule of thumb: use heat index for daily life and general outdoor decisions; use WBGT for organized athletic practice, military training, scaffolding/roofing work, and any decision involving sustained activity in direct sunlight.

Heat index extremes are increasing faster than air temperature alone because warming oceans drive higher absolute humidity in tropical and subtropical regions. NOAA data shows Heat Advisory days in the US Gulf Coast and southeastern states have approximately doubled since 1980, and the proportion of days with heat index above 105 °F has tripled in some Florida and Texas counties. The 2022 Lancet Countdown report links rising heat-index extremes to 350,000+ heat-related deaths globally per year, concentrated in adults over 65. By 2050, climate models project wet-bulb temperatures above 95 °F — the theoretical survivability limit for healthy adults — will occur regularly in the Persian Gulf, eastern China, parts of India, and the US Gulf Coast in summer. Practical implications: increase your personal heat-index threshold for vigilance, plan outdoor activities in early morning or evening during summer, and check that AC reliability and cooling-center access are part of your community's heat preparedness.