Heat Index Calculator
Find the 'feels like' temperature from heat and humidity.
Results update as you type.
How it works
The heat index, often called the 'feels like' temperature, combines air temperature and relative humidity to estimate how hot conditions actually feel to the human body. This heat index calculator uses the official National Weather Service Rothfusz regression, the same equation behind the warnings you see on hot summer days. When humidity is high, sweat evaporates slowly, so your body struggles to shed heat and the air feels far hotter than the thermometer reads. This matters for health and safety. High heat index values signal real danger of heat exhaustion and heat stroke, especially for outdoor workers, athletes, children, and older adults. Event organisers, coaches, and employers use the heat index to decide when to schedule breaks, cancel activities, or issue warnings. Knowing the apparent temperature rather than just the raw reading helps people take humidity seriously as a hazard, not just an inconvenience. The formula works because it was fitted to research on human thermoregulation, weighting temperature and moisture the way the body experiences them. It applies at or above 80°F, where humidity meaningfully impairs cooling; below that threshold this calculator returns the air temperature itself, since evaporation keeps pace. Meteorologists, planners, and anyone facing a hot, muggy forecast rely on a heat index calculator to judge the true risk of the conditions ahead.
HI = −42.379 + 2.049·T + 10.143·R − 0.225·T·R − … (NWS Rothfusz regression)
The heat index blends air temperature and relative humidity into a single apparent temperature because humid air slows the evaporation of sweat, the body's main cooling mechanism, making hot days feel hotter than the thermometer alone suggests. The National Weather Service Rothfusz regression is a multi-term polynomial fitted to detailed studies of human heat balance, so each term captures how temperature and moisture interact. The formula is only valid at or above 80°F; below that, sweat evaporation is efficient enough that the felt temperature simply equals the measured air temperature.
Source:NWS Rothfusz heat index regression
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter air temperature and relative humidity.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Heat index updates instantly as you type — no button to press.
Frequently asked questions
Why does humidity make it feel hotter?
Your body cools itself mainly by sweating, and sweat carries heat away as it evaporates from your skin. When the air is already saturated with moisture, evaporation slows dramatically, so less heat leaves your body and you feel hotter than the actual temperature. That is why a humid 90-degree day feels far more oppressive than a dry one, and why the heat index folds humidity into a single felt temperature.
Why does the calculator return the air temperature below 80°F?
The National Weather Service Rothfusz regression was developed and validated only for temperatures at or above 80 degrees Fahrenheit, where humidity meaningfully impairs the body's cooling. Below that threshold sweat evaporates efficiently regardless of moisture, so the apparent temperature is essentially the same as the measured air temperature. To stay accurate, this calculator returns the air temperature directly when you enter a value under 80 degrees.
What heat index values are dangerous?
The National Weather Service flags caution from about 80 to 90 degrees, extreme caution from 90 to 103, danger from 103 to 124, and extreme danger above 124 degrees Fahrenheit. As the index rises, the risks of heat cramps, heat exhaustion, and life-threatening heat stroke increase, especially with prolonged exposure or physical activity. Use these bands to decide when to rest, hydrate, seek shade, or stop outdoor work entirely.
Does the heat index account for sun and wind?
No. The standard heat index assumes shady, light-wind conditions, so direct sunlight can add as much as 15 degrees Fahrenheit to the felt temperature. Wind can lower it slightly by aiding evaporation, though hot wind can also increase heat stress. Because it ignores these factors, treat the heat index as a baseline and stay extra cautious when you are exposed to strong sun or still, stagnant air.
What units does this calculator use?
This calculator uses Fahrenheit for temperature and a percentage from 0 to 100 for relative humidity, matching the National Weather Service formula it is built on. If your temperature is in Celsius, convert it first by multiplying by 1.8 and adding 32. The resulting heat index is also reported in Fahrenheit, the standard unit for heat advisories issued across the United States.