Heat Index Calculator

The heat index is how hot it feels in the shade when humidity is added to the air temperature

The heat index is the “feels like” temperature the National Weather Service (NWS) computes from air temperature and relative humidity. Sweat cools you only when it evaporates, and humid air slows evaporation, so the same temperature feels hotter on a muggy day. At 90°F and 60% relative humidity the heat index is 100°F, which the NWS puts in the ‘Extreme Caution’ category. Drop the humidity to 40% and the same 90°F gives 91°F.

Type the air temperature and the humidity (or the dew point, if your forecast gives that instead) and the calculator returns the heat index in degrees Fahrenheit, its NWS category, the wet-bulb temperature, and the highest value you could see in direct sun. The map and the scale below the results show how the number moves as humidity changes. Everything on this page uses the NWS formula and the NWS categories (80, 90, 103 and 125°F).

When the heat index is the number to check

• Reading a forecast: the NWS uses the heat index as one of its heat tools, but a plain temperature forecast does not tell you how muggy it will be. Enter the forecast high and humidity to see which category the afternoon falls in.
• Converting a dew point: the current-conditions box on an NWS local forecast page lists both humidity and dew point. If you only have the dew point, switch the humidity box to ‘Dew point’ and the calculator works out the relative humidity itself (a 70°F dew point at 90°F air is about 52%).
• Checking a chart by hand: a printed chart has fixed rows and columns. This calculator gives the value between them, such as 86°F at 64%, which is 93°F.
• Dry heat: the NWS Amarillo office notes that when humidity is low, the heat index can be lower than the air temperature. At 100°F and 15% humidity it is 96°F, and the calculator shows that difference instead of assuming that hotter air always feels hotter.
• Planning around sun: the NWS says chart values are for shade and that full sunshine can add up to 15°F. The result list shows that upper value so you can see the worst case for a sunny field, court or parking lot.

How to use it

1. Enter the air temperature. The default unit is °F; pick °C in the unit box if your source uses it. For below zero, type a minus sign (-5).
2. Enter the relative humidity in percent, or change the humidity box to ‘Dew point’ and enter the dew point instead. The dew point cannot be higher than the air temperature.
3. The result card shows the heat index, how many degrees it is above the air temperature, and its NWS category. The sentence below it quotes what the NWS says about that category.
4. The list underneath adds the wet-bulb temperature, the dew point (or the relative humidity, if you entered a dew point) and the heat index in direct sun.
5. On the temperature and humidity map, click or drag to set both values at once. Colors are the NWS categories, and the sentence under the map says how much hotter or how much drier it would have to get to change category.
6. The scale compares the air temperature (open dot) with the heat index (filled dot); the hatched stretch to the right is the extra heat possible in direct sun. ‘Copy result’ copies the numbers and ‘Copy link’ copies an address that holds your current values.

How the heat index is calculated

The calculator follows the steps the NWS Weather Prediction Center publishes. The formula works in °F, so a value entered in °C is converted first and the answer is converted back.

1. Cold air: at 40°F (4.4°C) or below, the heat index is just the air temperature.
2. Simple formula: HI = 0.5 × {T + 61.0 + [(T − 68.0) × 1.2] + (RH × 0.094)}, with T in °F and RH in percent. If the result is 79°F or lower, that is the heat index. For example, 75°F at 50% gives 74.55, shown as 75°F, which is below the lowest NWS category.
3. Rothfusz regression: if the simple value is above 79°F, the NWS applies HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T·RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²·RH + 0.00085282T·RH² − 0.00000199T²·RH². The NWS describes this as the step for heat index values of 80°F or higher.
4. Dry-air adjustment: if RH is 13% or lower and T is between 80 and 112°F, subtract [(13 − RH) ÷ 4] × √{[17 − |T − 95|] ÷ 17}.
5. Very humid adjustment: if RH is above 85% and T is between 80 and 87°F, add [(RH − 85) ÷ 10] × [(87 − T) ÷ 5].

Wet-bulb temperature, dew point and rounding

The wet-bulb row uses Stull’s 2011 approximation, which is meant for relative humidity between 5% and 99% and air temperatures from −20 to 50°C, with errors from −1 to +0.65°C in that range. At 90°F and 60% it gives 78.8°F. When you enter a dew point, the calculator turns it into relative humidity with the saturation vapor pressure formula 6.112 × exp[17.62t ÷ (243.12 + t)] hPa (the WMO formula for vapor pressure over water) and uses the same heat index steps.

The heat index is shown to the whole degree in °F and to one decimal in °C, and the category is always judged on the whole-degree °F value, so switching to °C never changes it. The NWS writes its category ranges with shared end points (80–90, 90–103), so this calculator puts a boundary value in the higher category: 90°F is Extreme Caution and 103°F is Danger. In °C the category changes between about 26.4 and 26.7, 31.9 and 32.2, 39.2 and 39.4, and 51.4 and 51.7°C, depending on where the °F value rounds.

Heat index by temperature and humidity (°F)

Each cell is the heat index for that air temperature and relative humidity, in °F. The color is the NWS category: Caution starts at 80°F, Extreme Caution at 90°F, Danger at 103°F and Extreme Danger at 125°F.

A ‘—’ marks a cell where the heat index would be above 137°F (58.3°C). The NWS regression is not valid for extreme temperature and humidity combinations beyond the range of data Steadman considered, so the calculator does not fill those in on the chart.

What the NWS says about each category

These descriptions assume shade. A heat index of 100°F is not a safe number in the sun, where the NWS says the value can be up to 15°F higher. At 90°F and 60% that would be 115°F, in the Danger category.

Heat index and wet bulb globe temperature

The heat index uses only temperature and humidity. The NWS describes wet bulb globe temperature (WBGT) as a measure that also takes wind and solar radiation into account, and calls it a particularly effective indicator of heat stress for active people such as outdoor workers and athletes. It also notes that the American College of Sports Medicine bases its practice-intensity guidelines on WBGT, and that OSHA recommends measures for outdoor work such as shade, work/rest schedules and acclimatization. This calculator does not compute WBGT. For sport or outdoor work, use the heat index as a quick screen and look at a WBGT forecast as well.

Common mistakes

• Reading a shade value as a sun value: the NWS chart is for shade and light wind. Direct sunlight can add up to 15°F (about 8.3°C), not 15 degrees Celsius.
• Entering the dew point as humidity: a 74°F dew point is not 74% humidity. At 90°F, a dew point of 74.2°F is 60% relative humidity. Switch the humidity box to ‘Dew point’ instead.
• Putting °C into the °F formula: the regression is built on °F. Use the unit box and let the calculator convert.
• Assuming dry air is always easier: at 100°F and 15% humidity the heat index is 96°F, lower than the air, but that only reflects faster evaporation of sweat. The NWS also warns that strong winds, particularly with very hot, dry air, can be extremely hazardous.
• Comparing with another app’s ‘feels like’: this number is the NWS heat index only. A weather app’s ‘feels like’ value may use a different formula, so the two can differ.

Limits

• The heat index assumes shade and light wind. Sun, wind, clothing, how hard you are working, age, health conditions and medications all change how much heat the body actually takes on. This calculator does not diagnose anything.
• The regression equation has an error of about ±1.3°F, according to the NWS. A result of 103°F could sit on either side of the Danger boundary.
• The regression is not valid for extreme temperature and humidity combinations beyond Steadman’s data. The calculator marks results above 137°F (58.3°C) or with air above 110°F (43.3°C) as rough values.
• The heat index is one input to NWS heat products. It does not replace an official heat advisory or warning for your area.

Formula and categories: NWS Weather Prediction Center and NWS Amarillo. Wet bulb: Stull (2011). Dew point conversion: WMO saturation vapor pressure formula. All checked against the original pages on October 21, 2026.

Sources

• 01NWS Weather Prediction Center: The Heat Index EquationSimple formula, Rothfusz regression coefficients, adjustments for RH below 13% and above 85%, validity note
• 02NWS Weather Prediction Center: Heat Index CalculatorThe calculator code: air temperature of 40°F or below is returned unchanged, and the regression is used when the simple-formula value is above 79°F
• 03NWS Amarillo: What is the heat index?Four categories and their effects, shade basis, up to 15°F more in direct sun, ±1.3°F regression error, examples at 100°F
• 04NWS: Heat Safety ResourcesHeat index defined for shady, light-wind conditions, hazard of strong wind with very hot dry air, WBGT description, 96°F at 65% example
• 05NWS local forecast page (example)Current conditions list relative humidity and dew point side by side
• 06Stull (2011): Wet-Bulb Temperature from Relative Humidity and Air TemperatureJournal of Applied Meteorology and Climatology 50: 2267–2269; valid for RH 5–99% and −20 to 50°C
• 07NCAR/EOL: Water Vapor Pressure FormulationsWMO (2008) saturation vapor pressure over water, used to convert dew point and humidity

Frequently asked questions

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