What the heat index measures
When you step outside on a humid day, the temperature on a thermometer doesn't tell the whole story. Your body's ability to cool itself through sweat evaporation depends heavily on how much moisture is already in the air. The heat index—also called apparent temperature—combines actual air temperature and relative humidity to show what the conditions feel like to your body.
On a dry day, sweat evaporates quickly and efficiently, so 85°F feels manageable. On a humid day at the same temperature, sweat can't evaporate as readily, and your body retains more heat. That same 85°F might feel like 95°F or hotter. This calculator uses the official U.S. National Weather Service formula to compute that "feels like" value.
The formula
HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541(T×RH) − 0.00683783(T²) − 0.05481717(RH²) + 0.00122874(T²×RH) + 0.00085282(T×RH²) − 0.00000199(T²×RH²)
Where T is temperature in °F and RH is relative humidity as a percentage. This polynomial regression model was developed by the National Weather Service and is the standard for U.S. weather reporting. It's most accurate when temperature is 80°F or above and humidity is 40% or higher; below those thresholds, the actual temperature is a better guide to how hot it feels.
Worked example
Imagine a summer afternoon: the thermometer reads 92°F and the relative humidity is 65%.
Step 1: Set T = 92 and RH = 65.
Step 2: Calculate the squared and product terms:
- T² = 92² = 8,464
- RH² = 65² = 4,225
- T × RH = 92 × 65 = 5,980
- T² × RH = 8,464 × 65 = 550,160
- T × RH² = 92 × 4,225 = 388,700
- T² × RH² = 8,464 × 4,225 = 35,770,400
Step 3: Substitute into the formula:
- −42.379 + (2.049 × 92) + (10.143 × 65) − (0.225 × 5,980) − (0.0068 × 8,464) − (0.0548 × 4,225) + (0.00123 × 550,160) + (0.00085 × 388,700) − (0.00000199 × 35,770,400)
Step 4: Work through each term:
- −42.379 + 188.5 + 659.3 − 1,345.5 − 57.6 − 231.5 + 676.0 + 330.4 − 71.1 = 106.1°F
So at 92°F and 65% humidity, the heat index is approximately 106°F—meaning your body experiences conditions equivalent to about 14 degrees hotter than the actual air temperature.
Heat stress and safety
The heat index matters for health and safety. When the heat index exceeds 103°F, heat exhaustion becomes a genuine risk for prolonged outdoor activity. Above 125°F, heat stroke can develop rapidly, even in fit individuals. Vulnerable groups—young children, elderly people, and those with heart or respiratory conditions—face danger at lower thresholds.
On high heat index days, limit strenuous outdoor work to early morning or evening, drink water frequently (not caffeine or alcohol), and take regular breaks in shade or air conditioning. Outdoor workers, athletes, and parents should monitor heat index forecasts just as they would temperature warnings.
Note: This calculator provides an estimate based on the standard formula and current conditions. For heat-related health decisions, consult local weather alerts and medical guidance.
Common mistakes
Forgetting humidity matters most in the danger zone. At 75°F, humidity barely affects how hot it feels. But jump to 95°F, and the same 10-point increase in humidity can add 5–8 degrees to the heat index.
Assuming the formula works below 80°F. The model was calibrated for hot, humid conditions. When temperature or humidity is low, the actual thermometer reading is more reliable than the calculated index.
Confusing heat index with wind chill. Wind chill applies only to cold conditions and describes how wind speeds up heat loss from skin. Heat index is the opposite: it shows how humidity slows your body's cooling.