Heat Index Calculator

How the Heat Index Works

The heat index formula, developed by R.G. Steadman in 1979 and adapted by the NWS, uses a regression equation with temperature and relative humidity as inputs.^[1] It models the human body's thermal regulation system, accounting for how humidity impairs sweat evaporation. The index is most meaningful above 80°F, where humidity begins to meaningfully affect perceived temperature. Use the Water Intake Calculator to plan hydration in hot conditions.

Heat Index Risk Levels

What the Heat Index Measures

The heat index — often called the "feels like" temperature — combines air temperature and relative humidity to express how hot it actually feels to the human body. When humidity is high, sweat evaporates more slowly, reducing the body's primary cooling mechanism and making it feel significantly hotter than the thermometer reads. At 90°F with 30% humidity, the heat index is approximately 90°F (no significant effect). At 90°F with 70% humidity, the heat index jumps to roughly 106°F — a dangerous level where heat exhaustion can develop within 30–60 minutes of strenuous activity. This calculator helps you assess the true heat exposure risk for outdoor work, exercise, and daily activities based on current conditions.

Heat Index Chart

Notice how humidity has a minimal effect at 80°F but an extreme effect at 95°F+. The relationship is exponential, not linear — a 10% increase in humidity at 95°F raises the heat index far more than the same increase at 80°F. This non-linear relationship is why humid subtropical climates (Gulf Coast, Southeast Asia) present greater heat danger than dry desert climates at the same temperature.

Heat Danger Zones and Health Risks

Heat-related illness progresses through stages. Heat cramps (muscle spasms during exertion) are the earliest warning. Heat exhaustion produces heavy sweating, weakness, nausea, and cool/clammy skin — the body is still attempting to cool itself but failing. Heat stroke occurs when the body's cooling system fails entirely: body temperature rises above 103–104°F, sweating may stop, and confusion or loss of consciousness develops. Heat stroke is a medical emergency requiring immediate cooling and 911 activation — untreated, it can cause organ damage or death within 30–60 minutes.

Humidity and the Human Cooling System

The body maintains its core temperature of 98.6°F primarily through evaporative cooling — sweating. When sweat evaporates from the skin, it absorbs heat energy and cools the blood flowing through surface capillaries. This system works efficiently in dry air where evaporation is rapid. At 100% relative humidity, evaporation essentially stops — sweat drips off without cooling. The body produces up to 1–1.5 liters of sweat per hour during vigorous exercise, requiring significant fluid replacement. At heat index values above 110°F, even healthy, hydrated individuals cannot sustain outdoor activity safely for extended periods because the cooling system simply cannot dissipate heat fast enough, regardless of fitness level.

Heat Index for Outdoor Workers and Athletes

Occupational heat exposure affects millions of workers annually. OSHA recommends mandatory work-rest cycles based on heat index: at 91–103°F heat index, provide water every 15 minutes and shaded rest breaks every hour. Above 103°F, reduce physical workload and increase rest frequency. Above 115°F, only essential emergency work should continue outdoors. For athletes, the American College of Sports Medicine recommends canceling outdoor practices when the Wet Bulb Globe Temperature (WBGT) — a more comprehensive heat stress measure that includes wind and radiant heat — exceeds 86°F for acclimatized athletes. Pre-season football practices are particularly dangerous because players wear heavy equipment while not yet acclimatized, and the majority of heat-related deaths in sports occur during the first two weeks of practice. Acclimation takes 10–14 days of gradually increasing heat exposure.

Regional Heat Index Patterns

Geography dramatically affects typical heat index values. The US Gulf Coast (Houston, New Orleans, Miami) regularly experiences heat indices of 105–115°F during summer because temperatures in the mid-90s combine with humidity of 60–80%. The Desert Southwest (Phoenix, Las Vegas) reaches higher air temperatures (110–120°F) but lower humidity (10–20%), often producing comparable or even lower heat indices. The Midwest and Northeast experience periodic heat waves where heat indices spike above 105°F when Gulf moisture pushes northward. Globally, the Persian Gulf, South Asia, and the Amazon Basin experience some of the most extreme sustained heat index conditions, with values regularly exceeding 130°F during peak months. Climate models project that parts of these regions may approach the limits of human survivability (sustained heat index above 160°F) by mid-century without intervention. See our Temperature Converter to convert between Fahrenheit and Celsius for international heat index data.

Protecting Yourself in High Heat Index Conditions

Effective heat protection follows the principle of reducing heat gain and maximizing heat loss. Hydration is the most critical factor — drink before you feel thirsty, targeting 16–24 ounces per hour of outdoor activity. Electrolyte replacement becomes important after 60+ minutes of sweating, as water alone can dilute blood sodium to dangerous levels (hyponatremia). Clothing should be lightweight, loose-fitting, and light-colored to reflect solar radiation. Wetting clothing or wearing evaporative cooling vests enhances cooling in dry climates but provides little benefit in high humidity. Schedule outdoor activities during early morning (before 10 AM) or evening (after 6 PM) when heat index is typically 10–20°F lower than afternoon peaks. Air conditioning is the most effective protection — cooling centers and air-conditioned public spaces become life-saving resources during extreme heat events for vulnerable populations.

The Heat Index Formula

The National Weather Service heat index uses the Rothfusz regression equation, a polynomial formula with nine terms that combines temperature and humidity. The simplified version: HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T² − 0.05481717R² + 0.00122874T²R + 0.00085282TR² − 0.00000199T²R², where T is temperature in Fahrenheit and R is relative humidity as a percentage. This equation was derived empirically from extensive studies of human thermal comfort and heat exchange. The formula includes adjustment factors for very low humidity (below 13%) and very high humidity (above 85%) that modify the base calculation. The full computation involves conditional branches that this calculator handles automatically, providing the adjusted heat index value the NWS would report.

Heat Index vs. Wind Chill — Understanding Both Extremes

While the heat index measures thermal stress from heat and humidity, the wind chill index measures thermal stress from cold and wind. Both translate actual weather conditions into a "feels like" temperature that reflects the body's thermal experience. Wind chill matters because moving air strips heat from exposed skin faster than still air — a 20°F day with 30 mph winds has a wind chill near −2°F. Interestingly, wind actually helps in hot conditions by enhancing evaporative cooling (unless humidity is at 100%), which is why fans provide relief in hot weather even though they do not reduce air temperature. The heat index calculation assumes light winds (under 6 mph) and shade — direct sun exposure can add 10–15°F to the apparent temperature, and strong winds in dry heat can reduce the effective heat index by 3–5°F through enhanced evaporation.

How to Use This Calculator

1. Enter air temperature — Actual temperature in °F or °C.
2. Enter relative humidity — From weather reports or a hygrometer.
3. Review heat index — Feels-like temperature and NWS risk category.

Tips and Best Practices

See also: Wind Chill · Temp Converter · Water Intake · Calories Burned