Heat Index Calculator: What Does It Feel Like?
The heat index combines air temperature and humidity to show how hot it feels. At 90 °F (32 °C) and 70% humidity it feels like about 106 °F (41 °C), which is in the Danger range.
It feels like
105.9°F
Danger
15.9° hotter than the air temperature
- Comfortable
- Caution
- Extreme caution
- Danger
- Extreme danger
Heat exhaustion is likely and heat stroke is possible. Limit outdoor activity and stay hydrated.
Values assume shade and light wind. Direct sunshine can make it feel up to 15 °F (8 °C) hotter. The heat index is designed for warm weather (about 80 °F / 27 °C and above). Children, older adults and people with health conditions are more at risk.
Show the math
- 1
Try the simple formula first
0.5 × (90 + 61 + (90 − 68) × 1.2 + 70 × 0.094) = 92
The weather service starts with this quick estimate.
= 92 °F
Read the steps as text
- Try the simple formula first. 0.5 × (90 + 61 + (90 − 68) × 1.2 + 70 × 0.094) = 92 The weather service starts with this quick estimate.
- Use the full heat index regression. HI = −42.379 + 2.049·T + 10.143·RH − 0.2248·T·RH − 0.006838·T² − 0.05482·RH² + 0.001229·T²·RH + 0.000853·T·RH² − 0.00000199·T²·RH² with T = 90, RH = 70 The average was 91, which is 80 or more, so the full formula applies. It was fitted to a heat-stress model.
- Read the risk level. Heat index 105.9 °F → Danger Heat exhaustion is likely and heat stroke is possible. Limit outdoor activity and stay hydrated.
Why humidity makes it feel hotter
Your body cools itself by sweating, and sweat only cools you when it evaporates. In humid air, sweat evaporates slowly, so heat stays trapped and the same temperature feels hotter. The heat index turns that effect into a single number: the temperature you would feel in dry conditions that would put the same strain on your body.
For example, 90 °F feels like 90 °F at around 40% humidity but like 106 °F at 70% humidity. That difference is enough to move from a warm day to a dangerous one.
How the heat index is calculated
This calculator follows the US National Weather Service method. It first tries a simple formula: 0.5 × (T + 61 + (T − 68) × 1.2 + RH × 0.094), with T in °F. If the average of that result and the air temperature is 80 °F or more, it switches to the full Rothfusz regression, a long polynomial in temperature and humidity fitted to a physiological heat model.
Two small corrections apply in extreme conditions: a subtraction when humidity is below 13% at 80 to 112 °F, and an addition when humidity is above 85% at 80 to 87 °F. Every step is displayed with your numbers, and if you enter Celsius the calculator converts to °F and back.
Heat index risk levels
Below 80 °F the heat index is not a concern. From 80 to 90 °F is Caution, where fatigue is possible during long exposure. From 90 to 103 °F is Extreme caution, with heat cramps and heat exhaustion possible. From 103 to 125 °F is Danger, where heat exhaustion is likely, and 125 °F or above is Extreme danger, where heat stroke is highly likely.
These values are for shade and light wind. Full sunshine can add up to 15 °F to the heat index, and children, older adults, outdoor workers and people with certain health conditions are more vulnerable. Drink water, take breaks in the shade or in air conditioning, and never leave people or pets in a parked vehicle.
Frequently asked questions
- What is the heat index?
- It is the temperature that the air feels like to the human body once humidity is taken into account. The higher the humidity, the hotter it feels.
- Is heat index the same as feels-like temperature?
- In warm weather, yes. The feels-like temperature uses the heat index when it is hot and the wind chill when it is cold.
- At what heat index is it dangerous?
- The weather service classes 103 °F (39 °C) and above as Danger, and prolonged exposure at that level can cause heat exhaustion or heat stroke.
- Does the heat index work below 80 °F?
- Not meaningfully. Below about 80 °F (27 °C) the formula is not designed to apply, so the air temperature is what matters.