Free heat index calculator using the NWS Rothfusz formula. Find the apparent feels-like temperature from air temperature and humidity instantly.
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The heat index, also known as apparent temperature, tells you how hot it actually feels when humidity is factored in with the air temperature. High humidity prevents sweat from evaporating, making it harder for your body to cool down. Our calculator uses the official NWS formula.
Heat index (apparent temperature) combines air temperature and relative humidity to estimate how hot the air feels in the shade to the human body. When humidity is high, sweat evaporates more slowly, so your body cools less effectively and the air feels hotter. This calculator uses the official NWS Rothfusz regression (1990), valid at 80°F (27°C) and above. At 90°F with 70% humidity, it can feel like 106°F.
NWS Rothfusz Heat Index Formula
OSHA recommends monitoring heat index for worker safety protocols.
Coaches use heat index to determine safe practice conditions.
Plan outdoor events with attendee comfort in mind.
Monitor conditions for heat-sensitive individuals.
Know when outdoor conditions become dangerous for work or exercise.
Schedule outdoor activities during safer heat index periods.
Protect vulnerable individuals from heat-related illness.
A heat index of 103°F (39°C) or higher is considered dangerous. Heat exhaustion and heat stroke become likely with prolonged exposure. Above 125°F (52°C) is extremely dangerous.
Your body cools itself through sweating. When humidity is high, sweat evaporates more slowly, reducing your body's cooling efficiency. This makes the air feel hotter than it actually is.
The NWS heat index applies at 80°F (27°C) and above, and is most accurate when relative humidity is 40% or higher. Below 80°F this calculator shows the air temperature, since the heat index is not meaningful there. The formula also includes an adjustment for very dry air (humidity under 13%).
Heat index values assume shade. Full sun can raise the perceived temperature by up to 15°F (8°C), but that solar load is not part of the heat index — the WBGT index is what accounts for sun and wind. Seek shade when the heat index is high.
Heat index estimates how hot it feels in the shade from temperature and humidity alone. WBGT (wet-bulb globe temperature) also adds direct sun and wind, so it is used for sports and military activity guidelines. The wet-bulb temperature is the lowest temperature reachable by evaporation and is a survivability limit — a wet-bulb near 35°C (95°F) marks the threshold beyond which the body can no longer cool itself.