Heat index: the temperature it feels like in the heat
The heat index tells how hot it feels when humidity is added to the air temperature. At 90 °F (32 °C) with 50% relative humidity, the heat index reaches 95 °F (35 °C).
Updated November 7, 2026 · Sources
How it is calculated
The National Weather Service first applies a simple formula. When the result reaches 80 °F (27 °C), it switches to a longer regression, then adjusts it for very dry or very humid air. Our calculator follows that full procedure.
Humidity slows the cooling of the body by sweat, so humid heat feels hotter than the thermometer says.
Heat index chart
Read the row of the air temperature, then the column of the relative humidity. Values are in °F, the unit of the National Weather Service.
| Air | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|
| 80 °F (27 °C) | 80 | 81 | 82 | 83 | 84 | 86 |
| 84 °F (29 °C) | 83 | 85 | 88 | 90 | 94 | 98 |
| 88 °F (31 °C) | 88 | 91 | 95 | 100 | 106 | 113 |
| 92 °F (33 °C) | 94 | 99 | 105 | 112 | 121 | 131 |
| 96 °F (36 °C) | 101 | 108 | 116 | 126 | 138 | 152 |
| 100 °F (38 °C) | 109 | 118 | 129 | 143 | 158 | 176 |
| 104 °F (40 °C) | 119 | 131 | 145 | 161 | 181 | 202 |
Danger categories
The National Weather Service ranks heat index values into four categories:
- Caution: 80 °F (27 °C) … 89 °F (32 °C).
- Extreme caution: 90 °F (32 °C) … 102 °F (39 °C).
- Danger: 103 °F (39 °C) … 124 °F (51 °C).
- Extreme danger: 125 °F (52 °C) and above.
Heat index or humidex?
The heat index is American and uses relative humidity; the Canadian humidex uses the dew point and has no unit. Do not compare one with the other.