Heat Index Calculator — Feels Like Temperature & Humidity
The heat index tells you what the temperature actually feels like to your body when humidity is factored in. Enter the air temperature and relative humidity to get the apparent temperature, a heat danger level, and the difference from the actual reading — using the same formula published by the U.S. National Weather Service.
Enter temperature and humidity to calculate the heat index.
Heat Danger Levels
| Level | °C | °F |
|---|---|---|
| Caution | 27–32°C | 80–91°F |
| Extreme Caution | 32–41°C | 91–103°F |
| Danger | 41–54°C | 103–124°F |
| Extreme Danger | ≥54°C | ≥124°F |
How it works
What the heat index is and how the NWS formula works
The heat index — sometimes called the apparent temperature or 'feels like' temperature — measures how hot the air feels to the human body when relative humidity is combined with actual air temperature. At high humidity, sweat evaporates slowly from the skin, reducing the body's ability to shed heat. The result is that 35°C (95°F) at 60% RH feels closer to 46°C (115°F) to most people.
The standard formula used in the United States is the Rothfusz regression equation, published by Robert Steadman in 1979 and later adopted by the National Weather Service (NWS). It is a multi-variable polynomial that fits Steadman's original human biometeorological model across the range where humidity has a significant effect: roughly 27°C (80°F) and above, with relative humidity at 40% or higher. Below those thresholds, the formula loses accuracy and the heat index is not reported.
The NWS also applies two adjustments. When humidity is very low (below 13% RH) and temperature is between 80°F and 112°F, the heat index is slightly reduced because dry air still allows efficient evaporative cooling. When humidity is very high (above 85% RH) and temperature is between 80°F and 87°F, a small upward correction is applied. These adjustments keep the formula accurate across the full range of summer weather conditions encountered in the continental United States.
Heat-related illness: symptoms, thresholds, and prevention
Heat-related illness follows a progression tied closely to the heat index. Heat cramps — painful muscle spasms, usually in the legs or abdomen — are the mildest form and typically occur with sustained physical exertion above 27°C (80°F). They signal that the body is losing water and electrolytes faster than it is replacing them.
Heat exhaustion develops when the heat index rises above 32°C (90°F) and the body can no longer maintain a safe core temperature. Symptoms include heavy sweating, cold or clammy skin, fast or weak pulse, nausea, and dizziness. The skin may appear pale rather than flushed. Move the person to a cool environment immediately, loosen clothing, apply cool damp cloths, and have them sip water. Heat exhaustion can progress to heat stroke within 30 minutes if untreated.
Heat stroke is a medical emergency. Core body temperature reaches 40°C (104°F) or higher, sweating may stop entirely, and the person can become confused, lose consciousness, or have seizures. Call emergency services immediately. While waiting, use any available cooling method: cold water immersion is the most effective, followed by ice packs on the neck, armpits, and groin. Heat stroke can cause permanent organ damage or death. Prevention is straightforward: stay hydrated (water or electrolyte drinks every 15–20 minutes during outdoor work), avoid peak sun hours between 10 a.m. and 4 p.m., wear light-colored loose clothing, and schedule strenuous activity for the morning or evening when heat index values are lower.
Heat index vs. humidex vs. apparent temperature: what each measures
The heat index (NWS), the humidex (Canada), and Steadman's apparent temperature are three different scales that all try to answer the same question — how hot does it actually feel? — but they use different biometeorological models and report different numbers for the same conditions.
The humidex, developed by Canadian meteorologists J.M. Masterton and F.A. Richardson in 1979, is based on the dew point rather than relative humidity. Dew point is more physically direct: it is the temperature at which air becomes saturated. The humidex formula is simpler than Rothfusz and tends to give higher apparent values than the NWS heat index for the same conditions. Environment Canada issues humidex advisories when the value reaches 40, and extreme warnings above 45.
Steadman's apparent temperature, used by the Australian Bureau of Meteorology, goes further than both. It incorporates wind speed alongside temperature and humidity, and distinguishes between conditions in the shade and direct sunlight. Direct sun can add 6–10°C to the apparent temperature. This makes it more physically complete but also harder to communicate to the public. The NWS heat index remains the most widely recognized index in the United States and is the basis for most public heat advisories and workplace safety guidelines issued by OSHA and NIOSH.
Frequently asked questions
›Why does the calculator show an out-of-range message at low temperatures?
The Rothfusz formula is only valid when air temperature is at least 27°C (80°F) and relative humidity is at least 40%. Below those values, sweat evaporates quickly enough that humidity has little measurable effect on perceived temperature, so reporting a heat index figure would be misleading.
›Is 60% humidity at 30°C (86°F) dangerous?
At 30°C and 60% RH, the heat index is roughly 34°C (93°F), which falls in the Extreme Caution range. Prolonged exposure or physical activity can lead to heat cramps or heat exhaustion. Drink water regularly and limit outdoor exertion during the hottest part of the day.
›What is the difference between relative humidity and dew point?
Relative humidity is how much moisture the air contains as a percentage of the maximum it could hold at that temperature. Dew point is the actual temperature at which the air would become saturated. A dew point above 21°C (70°F) is considered oppressive by most people, regardless of the air temperature.
›Why does the same temperature feel hotter near the coast than inland?
Coastal air typically has higher relative humidity because of proximity to the ocean. Higher humidity slows sweat evaporation and raises the heat index, making 35°C by the sea feel significantly hotter than 35°C in a dry inland desert.
›Does wind affect the heat index?
The NWS heat index formula assumes light wind or shade conditions. Wind speeds above about 8 km/h (5 mph) can lower the apparent temperature by increasing evaporative cooling. The Australian apparent temperature scale does account for wind; the NWS heat index does not. In very hot, dry conditions, a strong hot wind (like a desert sirocco) can actually increase heat stress by preventing evaporation.
›How do I convert the heat index to the humidex used in Canada?
They are different scales with no direct conversion formula, because humidex is calculated from dew point rather than relative humidity, and uses a different biometeorological model. As a rough guide, humidex values run about 5–8 units higher than the NWS heat index for the same conditions, but the difference varies. For Canadian weather, use the humidex directly rather than trying to convert.
›At what heat index should outdoor sports be canceled?
Most sports medicine organizations recommend modifying activity when the heat index exceeds 32°C (90°F) and suspending outdoor practice when it exceeds 41°C (105°F). OSHA uses the NWS heat index thresholds for workplace safety guidance: above 33°C (91°F) is high risk; above 39°C (103°F) is very high risk; above 51°C (125°F) is extreme risk.
›Is the heat index the same as the wet-bulb temperature?
No. Wet-bulb temperature is a physical measurement — the temperature of a thermometer covered with a wet cloth and exposed to air movement. It is the theoretical lower limit of evaporative cooling and is used by physiologists to define survivability limits. The NWS heat index is a perceived-comfort index derived from a regression model of human thermal sensation; it is not a direct physical measurement. Wet-bulb temperature above 35°C is considered the upper survivability threshold for healthy adults.
Related tools
Last updated: