Heat Index Calculator: How Hot It Feels When Humidity Climbs

What is the heat index?

The heat index is what the temperature feels like to your body once humidity is counted in. Sweat cools you only when it evaporates. In humid air it evaporates slowly, so your body sheds less heat and the day feels hotter than the thermometer says. On a dry day the reverse can happen, and the heat index comes out a little below the air temperature.

The National Weather Service figures assume shade and a light breeze. Its heat index page notes that full sunshine can push the value up by as much as 15°F, so a roofer, a farmworker or a kid at an afternoon practice is facing more than the number shows.

The Weather Service formula, step by step

The calculator follows the method published by the NWS Weather Prediction Center on its heat index equation page. T is the temperature in °F and RH the relative humidity in percent.

First comes Steadman's simple formula:

HI = 0.5 × (T + 61 + (T - 68) × 1.2 + RH × 0.094)

If the average of that result and the temperature is below 80°F, that is the answer. Otherwise the calculator switches to the Rothfusz regression, a fit to Steadman's original tables:

HI = -42.379 + 2.04901523 T + 10.14333127 RH - 0.22475541 T RH
     - 0.00683783 T² - 0.05481717 RH² + 0.00122874 T² RH
     + 0.00085282 T RH² - 0.00000199 T² RH²

Last come the two NWS corrections. A small amount is subtracted when humidity is under 13 percent and the temperature is 80 to 112°F, and a little is added when humidity is over 85 percent and the temperature is 80 to 87°F.

Worked example: 92°F at 55 percent humidity

These are the numbers the calculator opens with, a typical humid July afternoon in much of the South and Midwest.

The simple formula gives 93.5°F. Averaged with 92 that is well above 80, so the regression takes over. With 55 percent humidity neither correction applies, and the result is 101.4°F. The calculator rounds it to 101°F (39°C) and places it in Extreme caution, where heat cramps and heat exhaustion become possible with long exposure or activity.

Small changes matter. At 90°F and 70 percent humidity the heat index is about 106°F, which crosses into Danger. A desert afternoon at 100°F and 10 percent humidity comes out near 94°F, because the low-humidity correction pulls it down. That dry heat still dehydrates you fast; it just feels less oppressive.

What each warning level asks of you

Use the level to change the day, not just to describe it. Move errands, runs and yard work to early morning. Drink water before you feel thirsty, and check on older neighbors, babies and anyone with heart disease.

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The reading inside your home matters as much as the forecast, especially in a top-floor apartment or an older building that holds heat overnight. A simple indoor thermometer and hygrometer gives you both inputs for this calculator. Fans have limits: the CDC's heat guidance says to use them only while indoor temperatures stay below 90°F, because above that a fan can raise body temperature. When the room gets hotter than that, go somewhere with air conditioning. Libraries, community centers and official cooling centers are free, and dialing 2-1-1 connects you to local listings in most of the country.

Heat stroke is a medical emergency. If someone is confused, has stopped making sense or passes out in the heat, call 911 and start cooling them while you wait.

Renters have more options than they may think. A growing number of cities and counties set a maximum indoor temperature that landlords must meet, and some state energy assistance programs help with cooling bills, not only heating. Your local housing office or 2-1-1 can tell you what applies where you live.

Our selections rest on manufacturer specifications and published reviews and owner feedback rather than on our own trials.

Heat does not land evenly across a city. Our story on redlining and urban heat shows why some blocks run several degrees hotter than others. Planning a hike on a warm day? The hiking time calculator adjusts the water to carry for hot weather.

Questions

What is the heat index?

The heat index is how hot the air feels to the human body when relative humidity is combined with the air temperature. High humidity slows the evaporation of sweat, so the body cools less and the heat index rises above the actual temperature. The National Weather Service values assume shade and light wind.

What heat index is dangerous?

The National Weather Service treats a heat index of 103 to 125°F as Danger, when heat exhaustion is likely and heat stroke is possible with long exposure or activity. From 90 to 103°F is Extreme caution, and 125°F or higher is Extreme danger. People who are older, very young, pregnant, ill or working hard outdoors can be harmed at lower values.

Does the heat index include direct sun?

No. Heat index values are calculated for shady, lightly breezy conditions. The National Weather Service says full sunshine can raise the heat index by up to 15°F.

Can the heat index be lower than the temperature?

Yes. In very dry air sweat evaporates quickly, so the heat index can come out a few degrees below the thermometer reading. For example, 100°F at 10 percent humidity gives a heat index near 94°F, though dry heat still dehydrates the body quickly.

How do I find a cooling center near me?

In most of the United States you can dial 2-1-1 to hear the cooling centers and air-conditioned public spaces open near you. Local health departments and emergency management offices also post cooling center lists during heat waves, and public libraries are often open as cool places to sit.