♨️ Heat Index Calculator
Enter the air temperature and relative humidity to get the ‘feels like’ heat index in °F and °C, plus the National Weather Service risk category — so you know how dangerous the heat really is.
🌡️ Temperature Meets Humidity
What is a Heat Index Calculator?
It converts a plain thermometer reading and the humidity into the apparent temperature your body actually experiences. High humidity blunts sweating — your main cooling system — so a 90°F afternoon at 70% humidity can feel like 105°F. The tool applies the National Weather Service equation and sorts the result into a clear risk band.
Use it alongside your local weather service's heat forecast and alerts, not instead of them. The figure is computed for shade and light wind; full sun can raise it by up to 15°F. It's a documented estimate for general awareness, not medical advice.
Heat illness: the warning signs
Heat stroke is an emergency
The CDC lists these symptoms of heat stroke:
- Confusion, altered mental status, slurred speech
- Loss of consciousness (coma)
- Hot, dry skin or profuse sweating
- Seizures
- Very high body temperature
Call 911 (or your local emergency number) for emergency medical care. The CDC's first-aid steps: stay with the person until help arrives, move them to a shaded, cool area, remove outer clothing, and cool them quickly with cold water or cold wet cloths.
Heat exhaustion
The CDC lists these symptoms of heat exhaustion:
- Headache
- Nausea
- Dizziness
- Weakness
- Irritability
- Thirst
- Heavy sweating
- Elevated body temperature
- Decreased urine output
The CDC says to get the person to a clinic or emergency room for medical evaluation, and to call 911 if medical care is unavailable.
Never leave children or pets in a parked car
The National Weather Service: “Never leave children, disabled adults or pets in parked vehicles!” It adds that hyperthermia “can occur even on a mild day with temperatures in the 70s.”
Fans have a limit
The CDC: “Use fans, but only if indoor temperatures are less than 90°F. In temperatures above 90°F, a fan can increase body temperature.” It points to air conditioning, or a cooling center you can find by dialing 2-1-1 or calling your local health department.
Sources: NWS heat index, CDC heat-related illnesses, CDC heat and your health, NWS children, pets and vehicles. General awareness, not medical advice.
❓ Frequently Asked Questions
What is the heat index?
The heat index — often called the 'feels like' temperature — combines air temperature and relative humidity into a single number that reflects how hot it actually feels to the body. When humidity is high, sweat evaporates more slowly, so your body can't cool itself as well and the air feels hotter than the thermometer reads.
How is the heat index calculated here?
This tool follows the U.S. National Weather Service algorithm published by its Weather Prediction Center. It first applies Steadman's simple formula; when that result averaged with the temperature is 80°F or higher, it uses the Rothfusz regression instead, then subtracts the NWS low-humidity adjustment (humidity under 13% at 80 to 112°F) or adds the high-humidity adjustment (humidity over 85% at 80 to 87°F). The result is the apparent temperature in °F, also shown in °C.
What do the risk categories mean?
The NWS groups heat-index values into bands: Caution 80°F to 90°F, Extreme Caution 90°F to 103°F, Danger 103°F to 124°F, and Extreme Danger 125°F or higher. Where two bands share an edge this tool reports the higher band, and it never rounds a reading down into a lower one. Below 80°F the tool says Below Caution, not safe: heat illness can still happen with hard work, full sun or a closed car.
Does direct sun make the heat index higher?
Yes. The National Weather Service says: “Since heat index values were devised for shady, light wind conditions, exposure to full sunshine can increase heat index values by up to 15°F.” Treat the figure here as a shade figure. This is general awareness information, not medical advice.