Heat Index Calculator
Find out how hot it actually feels using the official NOAA heat index formula. Enter the air temperature and relative humidity to see the "feels like" temperature and heat danger level.
Conditions
Heat Index Formula (NOAA/Rothfusz)
Rothfusz Regression (°F, valid above ~80°F)
+ 0.00123T²R + 0.000852TR² − 0.00000199T²R²
Where T = temperature in °F, R = relative humidity (%)
Worked Example
Applying the Rothfusz regression:
Heat Index ≈ 113.1°F
This falls in the "Danger" category.
NOAA Heat Index Danger Categories
90-103°F (32-39°C): Extreme Caution
103-124°F (39-51°C): Danger
124°F+ (51°C+): Extreme Danger
Common Conditions
Frequently Asked Questions
Heat Index Calculator - Know How Hot It Actually Feels
Heat index explains why a humid 32°C afternoon can feel far more oppressive than a dry 38°C day in the desert. It combines air temperature and relative humidity into a single "feels like" number that reflects how hot the human body actually perceives the conditions - which is why weather reports highlight heat index alongside the raw temperature during summer months.
Why Humidity Makes Heat Feel Worse
The human body's primary cooling mechanism is evaporating sweat from the skin - as sweat evaporates, it carries heat away from the body. High humidity means the surrounding air is already close to saturated with water vapor, which slows down that evaporation dramatically. Your sweat glands keep producing sweat, but it just sits on your skin rather than evaporating efficiently, leaving your body's cooling system working overtime with far less effect - which is exactly why the same temperature feels so much hotter as humidity climbs.
NOAA's Heat Index Danger Categories
The US National Weather Service classifies heat index values into four risk categories. Caution (80-90°F / 27-32°C) suggests fatigue is possible with prolonged exposure or physical activity. Extreme Caution (90-103°F / 32-39°C) raises the risk of heat cramps and heat exhaustion. Danger (103-124°F / 39-51°C) makes heat cramps and heat exhaustion likely, with heat stroke possible. Extreme Danger (above 124°F / 51°C) makes heat stroke highly likely with continued exposure - this level of heat index is considered a genuine medical emergency risk.
Heat Index Assumes Shade - Sunlight Makes It Worse
The standard heat index formula is calculated for conditions in the shade with light wind. In full, direct sunlight, the actual perceived temperature can run 8°C (15°F) or more higher than the calculated heat index, since direct sun exposure adds a substantial additional heat load to the body that the formula doesn't account for. This is an important reason to treat heat index as a baseline rather than an absolute figure when working or exercising outdoors in direct sun.
Practical Ways to Stay Safe in High Heat Index Conditions
- Increase fluid intake well before you feel thirsty, since thirst is a delayed signal of dehydration.
- Limit strenuous outdoor activity to early morning or evening hours when both temperature and humidity are typically lower.
- Take breaks in air-conditioned or shaded spaces to give your body's cooling systems a chance to recover.
- Watch for early heat exhaustion symptoms - heavy sweating, weakness, dizziness, and nausea - and move to a cooler environment immediately if they appear.
The Winter Counterpart: Wind Chill
Just as heat index shows how much hotter humid weather feels, the Wind Chill Calculator shows how much colder windy weather feels, since wind strips away the warm air layer your body maintains near the skin.
How this calculator works, and where the numbers come from
The Heat Index Calculator applies the standard formula for this calculation to the values you enter and updates the result as you type. The calculation itself happens in your browser, and the page explains the method so you can check any result by hand.
Please note: Results are provided for general information and are calculated from the values you enter.