Heat Index Calculator: How Hot Does It Really Feel?
Enter the air temperature and relative humidity (or dew point) to find out what the heat index is - the temperature it actually feels like to the human body. The calculator uses the official NOAA / NWS Rothfusz regression and flags your result against the four NOAA danger categories. Switch freely between Fahrenheit and Celsius.
Formula
Worked example
Air temperature 95 degF, relative humidity 60%: the Rothfusz equation gives HI = -42.379 + 2.049(95) + 10.143(60) - 0.225(95)(60) - 0.00684(95^2) - 0.0548(60^2) + ... = approximately 114 degF. That is 19 degF above the air temperature and falls in the Danger category.
What is the heat index?
The heat index, sometimes called the apparent temperature or feels-like temperature, is the temperature the human body actually perceives when air temperature and relative humidity are combined. High humidity slows the evaporation of sweat, our body's primary cooling mechanism. When sweat cannot evaporate efficiently, the body retains more heat and feels hotter than the thermometer reads. The formula was originally developed by R.G. Steadman in 1979 and later refined by Robert Rothfusz for the National Weather Service (NWS) into the nine-term regression equation still used today.
How the NWS Rothfusz equation works
The NWS uses a two-tier approach. For air temperatures below 80 degF or relative humidity below 40%, the simple Steadman linear formula is applied: HI = 0.5 * (T + 61 + (T - 68) * 1.2 + RH * 0.094). Above those thresholds, the full nine-term Rothfusz polynomial is used: HI = -42.379 + 2.049*T + 10.143*RH - 0.225*T*RH - 0.00684*T^2 - 0.0548*RH^2 + 0.00123*T^2*RH + 0.000853*T*RH^2 - 0.0000002*T^2*RH^2. Two small corrections are applied at the edges: a downward adjustment when RH is below 13% and the temperature is 80-112 degF, and an upward adjustment when RH exceeds 85% and the temperature is 80-87 degF. The equation carries an uncertainty of about 1.3 degF.
Dew point mode: an alternative input
Many weather stations report dew point rather than relative humidity. The dew point is the temperature at which air becomes saturated - the higher it is relative to the air temperature, the more humid it feels. This calculator converts dew point to relative humidity using the August-Roche-Magnus approximation, then feeds that value into the Rothfusz formula. As a rule of thumb, a dew point above 60 degF (15.5 degC) starts to feel oppressive; above 70 degF (21 degC) it is very uncomfortable; above 75 degF (24 degC) it is dangerous.
Heat danger categories and health risks
The NWS divides heat index values into four categories. "Caution" (80-90 degF): fatigue is possible with prolonged activity. "Extreme Caution" (91-103 degF): heat cramps and heat exhaustion are possible. "Danger" (103-125 degF): heat cramps and exhaustion are likely; heatstroke is possible. "Extreme Danger" (above 125 degF): heatstroke is highly likely with continued exposure. Direct sunlight can raise the perceived temperature by an additional 10-15 degF beyond the heat index value. People at elevated risk include those over 65, children under five, pregnant women, outdoor workers, and anyone taking medications that affect hydration or heat response.
NOAA Heat Index Danger Categories
| Heat Index (degF) | Heat Index (degC) | Category | Health Risk |
|---|---|---|---|
| Below 80 | Below 27 | Comfortable | No heat-related risk |
| 80-90 | 27-32 | Caution | Fatigue possible with prolonged exposure |
| 91-103 | 32-39 | Extreme Caution | Heat cramps or exhaustion possible |
| 103-125 | 39-52 | Danger | Heat cramps or exhaustion likely; heatstroke possible |
| Above 125 | Above 52 | Extreme Danger | Heatstroke highly likely with continued exposure |
National Weather Service classification used by emergency managers across the United States. Values are heat index readings, not raw air temperature.
Frequently asked questions
What temperature and humidity make the heat index dangerous?
The NOAA "Danger" zone starts at a heat index of 103 degF (39.4 degC). A common combination that reaches this level is around 95 degF (35 degC) air temperature with 60% relative humidity. At 100 degF and 90% humidity the heat index exceeds 130 degF, well into "Extreme Danger." Always check both the temperature and humidity together, because 100 degF in dry desert air feels far less stressful than 90 degF in a humid swamp.
Is the heat index the same as the apparent temperature?
The terms are often used interchangeably in the United States, but they come from different formulas. The heat index uses the Rothfusz / Steadman equations developed for the NWS and focuses on heat combined with humidity. The Australian apparent temperature additionally accounts for wind speed and solar radiation. The Canadian Humidex is another related metric. All aim to answer the same question: what temperature does it feel like to the human body?
Why does humidity make heat feel worse?
The body cools itself mainly by sweating. When sweat evaporates it carries heat away from the skin. At high relative humidity the air is already close to saturated, so sweat evaporates slowly and the cooling effect is reduced. The body must then pump more blood to the skin and work harder to maintain a safe core temperature. Sustained exposure in high-humidity heat raises the risk of heat cramps, heat exhaustion, and heatstroke.
How accurate is the heat index formula?
The NWS Rothfusz equation is accurate to about plus or minus 1.3 degF (0.7 degC) for temperatures between 80 and 120 degF and humidity between 40 and 100%. It assumes a person is in the shade, so direct sunlight typically adds 10-15 degF to the perceived temperature. The formula also assumes light wind; strong wind, which enhances evaporative cooling, can make conditions feel significantly cooler than the heat index suggests.
What is the difference between heat index and dew point?
Air temperature and relative humidity together determine the heat index. The dew point is a separate measure: the temperature at which the air would become saturated. Because dew point does not change when air temperature changes (unlike relative humidity), many meteorologists prefer it as a direct indicator of how muggy the air feels. A dew point above 70 degF (21 degC) is commonly described as oppressive. You can enter dew point directly in this calculator and it will convert it to relative humidity for you.