Use our Heat Index Calculator to find the feels-like temperature based on air temperature and relative humidity. This quick tool helps you gauge heat stress risk so you can plan outdoor work, training, and recreation more safely.
What is the heat index?
The heat index, sometimes called the feels-like temperature, blends the effects of air temperature and relative humidity to estimate how hot conditions feel to the human body. When humidity is high, sweat evaporates more slowly, making it harder for the body to cool itself. As a result, the perceived temperature can be much higher than the actual air temperature. The heat index is most accurate in shady locations with light wind. Direct sunlight and low wind can increase heat stress beyond the stated index.
- Air temperature: the ambient temperature measured in a shaded, well-ventilated area.
- Relative humidity: the amount of moisture in the air compared to its capacity at that temperature.
- Feels-like temperature: an effect of reduced evaporative cooling on the skin.
How to use the Heat Index Calculator
- Enter the air temperature and choose the correct unit (°F or °C).
- Input the relative humidity as a percentage from 0 to 100%.
- Click Calculate to get the heat index (feels-like) temperature and a safety category.
- Use the result to plan hydration breaks, shade, and activity intensity.
Behind the calculation
This calculator uses the National Weather Service Rothfusz regression to estimate heat index for temperatures typically above 80 °F (27 °C). For lower temperatures, a simplified Steadman formula is used. The algorithm is computed in Fahrenheit and converted back to your preferred unit for display. It also applies standard adjustments for unusually low humidity at high temperatures and very high humidity at moderate temperatures, both of which meaningfully change perceived heat.
While the heat index is a practical guide, local factors—direct sun, dark clothing, high metabolic rates, and low wind—can make conditions feel even hotter. Consider adding extra caution on bright, still days or when working hard outdoors.
Safety categories and what they mean
- 80–90 °F (27–32 °C): Caution—fatigue possible with prolonged exposure and activity.
- 90–103 °F (32–39 °C): Extreme Caution—heat cramps and heat exhaustion possible.
- 103–124 °F (39–51 °C): Danger—heat cramps and heat exhaustion likely; heat stroke possible.
- 125+ °F (52+ °C): Extreme Danger—heat stroke highly likely with continued exposure.
Tips to reduce heat stress
- Hydrate early and often—don’t wait for thirst. Include electrolytes for long efforts.
- Schedule the most intense work or training during cooler parts of the day.
- Seek shade or air conditioning for recovery breaks; use cooling towels or misting.
- Wear light-colored, breathable fabrics and a hat that shades your face and neck.
- Acclimatize gradually over 1–2 weeks, especially for strenuous outdoor work.
- Watch for signs of heat illness: cramps, dizziness, rapid pulse, confusion. Act fast.
Limits and best practices
The heat index is calibrated for shaded, light-wind conditions. In direct sun, add extra caution—conditions may feel 10–15 °F (?6–8 °C) hotter than indicated. Very dry, windy environments may not be perfectly captured by the index either; in those cases, sweat evaporation can be more efficient than the standard model assumes. For athletes and safety managers, pairing the Heat Index Calculator with real-time observations (such as wet-bulb temperature or WBGT) can provide a more complete picture of heat stress risk.
Remember that personal factors—fitness, hydration status, clothing, medications, and age—affect heat tolerance. When in doubt, slow down, cool down, and check on teammates or family members. The calculator gives you a strong starting point to make smarter, safer decisions in the heat.