# Heat Index Calculator

Use this Heat Index Calculator to calculate the heat index (HI) based on air temperature (T) and relative humidity (RH).

How to use this calculator

1. Choose the temperature unit between degrees Fahrenheit, degrees Celsius, or Kelvin
2. Enter the air temperature
3. Enter the relative humidity and click the "Calculate" button to determine the heat index temperature (i.e., how hot it feels).

## Heat Index Formula

This calculator uses the following basic formula to calculate the heat index:

HI (Heat Index, °F) = C1 + C2 T + C3 RH + C4 T RH + C5 T2 + C6 RH2 + C7 T2 RH + C8 T RH2 + C9 T2 RH2 + CF

Where:

HI is the heat index (°F),

T is the air temperature in Fahrenheit (°F),

RH is the relative humidity (%),

CF is the correction factor,

C1 = -42.379,

C2 = 2.04901523,

C3 = 10.14333127,

C4 = -0.22475541,

C5 = -6.83783 × 10 -3,

C6 = -5.481717 × 10 -2,

C7 = 1.22874 × 10 -3,

C8 = 8.5282 × 10 -4,

C9 = -1.99 × 10 -6.

If 80°F ≤ T ≤ 112°F, RH ≤ 13%; CF (Correction Factor) = - ([13 - RH] / 4) × ([17 - |T - 95|] / 17)0.5

If 80°F ≤ T ≤ 87°F, RH > 85%; CF (Correction Factor) = 0.02 × (RH - 85) × (87 - T)

Example: If the temperature is 90°F and the relative humidity is 80%, the heat index is:

HI (Heat Index, °F) = -42.379 + 2.04901523 × 90 + 10.14333127 × 80
- 0.22475541 × 90 × 80 - 6.83783 × 10 -3 × 902 - 5.481717 × 10 -2 × 802
+ 1.22874 × 10 -3 × 902 × 80 + 8.5282 × 10 -4 × 90 × 802 - 1.99 × 10 -6 × 902 × 802

Heat Index = -42.379 + 184.4113707 + 811.4665016
- 1618.238952 - 55.386423 - 350.829888
+ 796.22352 + 491.22432 - 103.1616 = 113.3298493 degrees Fahrenheit.

You may also like to try our Wind Chill Calculator or Dew Point Calculator

Heat Index (HI) Calculator

°F

%

Heat Index Calculator Results
Results
Fahrenheit (°F) Celsius (°C) Kelvin (K)
Heat Index (HI): - - -

## Heat Index Chart (Apparent Temperature)

Temperature (°F) Relative Humidity (%)
40 45 50 55 60 65 70 75 80 85 90 95 100
110 136 143 152 161 171 182 194 206 219 233 247 262 278
108 130 137 144 153 162 172 182 193 205 218 231 245 260
106 124 130 137 145 153 162 172 182 193 204 216 229 243
104 119 124 131 137 145 153 161 171 181 191 202 214 226
102 114 119 124 130 137 144 152 160 169 179 189 199 210
100 109 114 118 124 129 136 143 150 158 167 176 185 195
98 105 109 113 117 123 128 134 141 148 156 164 172 181
96 101 104 108 112 116 121 126 132 138 145 152 160 168
94 97 100 102 106 110 114 119 124 129 135 141 148 155
92 94 96 99 101 105 108 112 116 121 126 131 137 143
90 91 93 95 97 100 103 106 109 113 117 122 127 132
88 88 89 91 93 95 98 100 103 106 110 113 117 121
86 85 87 88 89 91 93 95 97 100 102 105 108 112
84 83 84 85 86 88 89 90 92 94 96 98 100 103
82 81 82 83 84 84 85 86 88 89 90 91 93 95
80 80 80 81 81 82 82 83 84 84 85 86 86 87
 Classification: Very Warm Hot Very Hot Extremely Hot

Use this Heat Index Calculator to calculate the heat index (HI) based on air temperature (T) and relative humidity (RH).

How to use this calculator

1. Choose the temperature unit between degrees Fahrenheit, degrees Celsius, or Kelvin
2. Enter the air temperature
3. Enter the relative humidity and click the "Calculate" button to determine the heat index temperature (i.e., how hot it feels).

## Heat Index Formula

This calculator uses the following basic formula to calculate the heat index:

HI (Heat Index, °F) = C1 + C2 T + C3 RH + C4 T RH + C5 T2 + C6 RH2 + C7 T2 RH + C8 T RH2 + C9 T2 RH2 + CF

Where:

HI is the heat index (°F),

T is the air temperature in Fahrenheit (°F),

RH is the relative humidity (%),

CF is the correction factor,

C1 = -42.379,

C2 = 2.04901523,

C3 = 10.14333127,

C4 = -0.22475541,

C5 = -6.83783 × 10 -3,

C6 = -5.481717 × 10 -2,

C7 = 1.22874 × 10 -3,

C8 = 8.5282 × 10 -4,

C9 = -1.99 × 10 -6.

If 80°F ≤ T ≤ 112°F, RH ≤ 13%; CF (Correction Factor) = - ([13 - RH] / 4) × ([17 - |T - 95|] / 17)0.5

If 80°F ≤ T ≤ 87°F, RH > 85%; CF (Correction Factor) = 0.02 × (RH - 85) × (87 - T)

Example: If the temperature is 90°F and the relative humidity is 80%, the heat index is:

HI (Heat Index, °F) = -42.379 + 2.04901523 × 90 + 10.14333127 × 80
- 0.22475541 × 90 × 80 - 6.83783 × 10 -3 × 902 - 5.481717 × 10 -2 × 802
+ 1.22874 × 10 -3 × 902 × 80 + 8.5282 × 10 -4 × 90 × 802 - 1.99 × 10 -6 × 902 × 802

Heat Index = -42.379 + 184.4113707 + 811.4665016
- 1618.238952 - 55.386423 - 350.829888
+ 796.22352 + 491.22432 - 103.1616 = 113.3298493 degrees Fahrenheit.

You may also like to try our Wind Chill Calculator or Dew Point Calculator

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