Heat Index Calculator: estimate how hot outdoor conditions feel when air temperature combines with relative humidity. Enter current readings to calculate apparent temperature, compare Fahrenheit and Celsius, and understand the official heat-risk category.
Outdoor Heat Safety Tool
Calculate the heat index from air temperature and relative humidity, compare °F and °C, and review practical heat-risk guidance.
Your heat index result
Heat Index Calculator formula
The calculator uses the National Weather Service Rothfusz regression:
T is temperature in °F and R is relative humidity as a percentage. Standard low-humidity and high-humidity adjustments are applied where appropriate. Below the full-equation range, the calculator uses the simpler official approximation and explains the limitation.
Heat Index Calculator worked example
At 90°F with 60% relative humidity, the apparent temperature is about 100°F. That is roughly 10°F above the measured air temperature and falls in the “Extreme caution” range.
Heat Index Calculator safety categories
| Heat index | General risk |
|---|---|
| Below 80°F | Lower risk, though prolonged exertion can still cause heat stress. |
| 80–90°F | Caution: fatigue is possible with prolonged exposure or activity. |
| 90–103°F | Extreme caution: heat cramps and heat exhaustion are possible. |
| 103–125°F | Danger: heat cramps or heat exhaustion are likely; heat stroke is possible. |
| Above 125°F | Extreme danger: heat stroke is highly likely with continued exposure. |
These categories are general planning guidance, not medical diagnosis. Move to a cooler place and seek urgent medical help for confusion, fainting, seizures, very hot skin or suspected heat stroke.
Heat Index Calculator accuracy and limitations
Heat index estimates conditions in shade with light wind. Full sunshine can add up to about 15°F to perceived heat. The model does not account for individual health, hydration, acclimatization, clothing or workload. Use official local alerts for current decisions.
Reference: National Weather Service heat index guidance.
Frequently asked questions
Is heat index the same as temperature?
No. Air temperature is measured directly; heat index estimates how hot it feels when humidity reduces sweat evaporation.
Does the heat index apply in direct sunlight?
The standard value assumes shade. Direct sun can make conditions feel considerably hotter.
Can the heat index be lower than air temperature?
At lower humidity, the apparent temperature may be close to or slightly below the measured temperature. The full heat-index equation is most meaningful in hot, humid conditions.
Should I use indoor humidity?
Use temperature and humidity from the environment you are assessing. Outdoor forecasts may not represent indoor workplaces or vehicles.
Using heat index for outdoor planning
Check the heat index before exercise, outdoor work, travel, sports, gardening or events. Plan demanding activity for cooler hours, take more frequent breaks, drink water regularly and use shade or air conditioning. People who are not acclimatized to hot weather may need extra recovery time even when a result falls below a formal warning threshold.
Humidity matters because the body normally cools itself when sweat evaporates. In humid air, evaporation slows and the body retains more heat. A small rise in humidity can therefore cause a much larger rise in apparent temperature during hot weather. Wind can improve evaporation in some situations, but very hot moving air does not always provide meaningful cooling.
Compare results with official alerts because local agencies may consider overnight temperatures, duration, location, vulnerable populations and other conditions beyond a single reading. Employers, coaches and event organizers should also follow any applicable occupational, school or athletic heat-safety rules.
Related Everyday Calculators
Heat Index Calculator example
At 90°F with 60% relative humidity, the Heat Index Calculator estimates that it feels like approximately 100°F (38°C). This falls in the “extreme caution” range, where heat cramps and heat exhaustion become more likely with prolonged exposure.