Heat Index Calculator

1. Enter the air temperature, in Fahrenheit or Celsius.
2. Enter the relative humidity, as a percentage from 0 to 100.
3. Read the heat index, the temperature it actually feels like.
4. See how many degrees hotter it feels, and the health risk category.

Here is what each result means:

The heat index is the temperature your body actually feels, not what a thermometer reads. When the air is humid, sweat evaporates more slowly, so your body cools itself less effectively and a hot day feels even hotter.

The heat index is always calculated for a specific combination of air temperature and relative humidity. It assumes shade and a light wind, so standing in direct sunlight can feel up to 15 degrees F hotter than the value shown.

It applies the official National Weather Service heat index formula, a regression fitted to studies of how the human body loses heat.

1. Convert the temperature to Fahrenheit if entered in Celsius.
2. Run a simple average formula first, to check whether it feels warm enough for the heat index to matter.
3. When it does, apply the full regression, combining temperature (T) and relative humidity (R).
4. Apply the published low-humidity and high-humidity adjustments where they apply, then convert back to your display unit.

This formula is calibrated for temperatures at or above about 80 degrees F. Below that, sweat evaporates freely and the heat index is close to the air temperature.

Find the heat index at 90 degrees F with 70 percent relative humidity.

Calculation: feeding T = 90 and R = 70 into the regression works out to about 106 degrees F. Even though the thermometer reads 90 degrees, the humid air makes your body feel as hot as it would at 106 degrees in dry conditions, about 16 degrees hotter.

Higher heat index values raise the risk of heat-related illness, especially during physical activity or prolonged exposure. The bands below follow the official National Weather Service heat index chart.

Relative Humidity

Humidity is the key driver. The more moisture in the air, the slower sweat evaporates, and the hotter a given temperature feels. On a dry day the heat index can be lower than the air temperature.

Air Temperature

The heat index only rises meaningfully above about 80 degrees F. Below that, the body cools efficiently and the felt temperature stays close to the actual reading.

Sun and Wind

The standard formula assumes shade and a light breeze. Direct sunlight can add up to 15 degrees F to the felt temperature, while a strong dry wind can lower it. Neither is part of the formula.

Planning Outdoor Activity

Check the heat index before exercising, working, or spending long periods outside in summer to judge the real heat-illness risk.

Protecting Vulnerable People

Older adults, young children, and anyone with a health condition feel high heat index values more sharply. The number helps you decide when to stay indoors or hydrate.

Reading a Weather Forecast

Forecasts often report the heat index alongside the air temperature. Knowing how the two differ helps you plan your day correctly.

1. Ignoring Direct Sunlight

The heat index assumes shade. In full sun it can feel up to 15 degrees F hotter than the value shown, so treat the number as a shaded minimum.

2. Using It in Cool Weather

Below about 80 degrees F the formula is not meaningful. The body cools freely, so the heat index stays close to the air temperature.

3. Confusing Humidity With Dew Point

Relative humidity changes as the air warms and cools through the day. The heat index formula uses relative humidity at the current temperature, not the dew point.

4. Confusing Heat Index With Wind Chill

Heat index measures how hot it feels in hot, humid weather. Wind chill measures how cold it feels in cold, windy weather. They use different formulas for opposite conditions.

The regression is exact within its valid range; only the displayed decimals are rounded.

The heat index uses the National Weather Service Rothfusz regression, which combines air temperature and relative humidity. At 90 degrees F with 70 percent humidity, the heat index works out to about 106 degrees F.

Air temperature is what a thermometer reads. The heat index is how hot it actually feels once humidity is factored in, because humid air slows the evaporation of sweat and reduces your body's ability to cool itself.

The heat index formula is calibrated for temperatures at or above about 80 degrees F. Below that, sweat evaporates freely, so the heat index stays close to the air temperature.

Your body cools itself by evaporating sweat. When the air is already full of moisture, sweat evaporates more slowly, so you lose heat less efficiently and the same temperature feels hotter.

No. The standard formula assumes shade and a light wind. Standing in full sunshine can feel up to 15 degrees F hotter than the heat index value shown here.

They are similar in purpose but not identical. The heat index is the official National Weather Service measure using temperature and humidity, while some "real feel" figures add extra factors like sun and wind using private formulas.

No. The heat index measures how hot it feels in hot, humid weather. Wind chill measures how cold it feels in cold, windy weather. They use different formulas for opposite conditions.

No. The calculation runs in your browser and nothing you enter is stored or sent anywhere, unless you choose Save, which keeps the result only on this device.

• The Heat Index Equation (NOAA Weather Prediction Center).
• Heat Index (National Weather Service).