
Heat Index Calculator
Feels-like temperature from the official NWS heat index formula
About the Calculator
Temperature alone does not tell you how hot it feels outside. Humidity slows sweat evaporation, so the same thermometer reading can feel much worse when the air is muggy. This calculator uses the National Weather Service method: Steadman's simple formula when conditions are mild, and the full Rothfusz regression with humidity adjustments when heat stress is meaningful. Enter your numbers, choose Fahrenheit or Celsius, and get a clear feels-like result with NWS-style safety guidance.
How to Calculate Manually
- 1Enter the air temperature and relative humidity (0–100%).
- 2Choose Fahrenheit or Celsius; the formula runs in °F internally, then converts back if needed.
- 3Click Calculate to see the feels-like heat index, danger level, and optional math breakdown.
- 4Use the result as a guide for outdoor activity — not as a substitute for official heat advisories or medical advice.
Enter air temperature and relative humidity to see the NWS heat index — the standard "feels like" temperature used in U.S. weather reports — plus a color-coded safety level for outdoor activity.
Range: 32–130°F
The Formulas
Heat Index Chart
The table below matches the familiar NOAA heat index chart: air temperature down the side, relative humidity across the top, and heat index values in each cell. Use it to compare conditions at a glance or to verify a result from the calculator.
Each cell is the calculated heat index (°F) for that air temperature and relative humidity, using the same NWS formulas as the calculator above. Colors follow National Weather Service heat stress categories.
| Temp °F | Relative humidity (%) | |||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 5 | 10 | 15 | 20 | 25 | 30 | 35 | 40 | 45 | 50 | 55 | 60 | 65 | 70 | 75 | 80 | 85 | 90 | 95 | 100 | |
| 80 | 78 | 78 | 78 | 79 | 79 | 79 | 79 | 80 | 80 | 81 | 81 | 82 | 82 | 83 | 84 | 84 | 85 | 86 | 88 | 89 |
| 81 | 78 | 79 | 79 | 79 | 79 | 80 | 80 | 81 | 81 | 82 | 82 | 83 | 84 | 85 | 85 | 86 | 87 | 89 | 91 | 93 |
| 82 | 79 | 79 | 80 | 80 | 80 | 80 | 81 | 81 | 82 | 83 | 84 | 84 | 85 | 86 | 88 | 89 | 90 | 92 | 94 | 96 |
| 83 | 79 | 80 | 80 | 81 | 81 | 81 | 82 | 82 | 83 | 84 | 85 | 86 | 87 | 88 | 90 | 91 | 93 | 95 | 97 | 100 |
| 84 | 80 | 81 | 81 | 81 | 82 | 82 | 83 | 83 | 84 | 85 | 86 | 88 | 89 | 90 | 92 | 94 | 96 | 98 | 101 | 104 |
| 85 | 81 | 81 | 82 | 82 | 82 | 83 | 84 | 84 | 85 | 86 | 88 | 89 | 91 | 93 | 95 | 97 | 99 | 102 | 105 | 108 |
| 86 | 81 | 82 | 83 | 83 | 83 | 84 | 85 | 85 | 87 | 88 | 89 | 91 | 93 | 95 | 97 | 100 | 102 | 105 | 109 | 112 |
| 87 | 82 | 83 | 83 | 84 | 84 | 85 | 86 | 87 | 88 | 89 | 91 | 93 | 95 | 98 | 100 | 103 | 106 | 109 | 113 | 116 |
| 88 | 83 | 84 | 84 | 85 | 85 | 86 | 87 | 88 | 89 | 91 | 93 | 95 | 98 | 100 | 103 | 106 | 110 | 113 | 117 | 121 |
| 89 | 84 | 84 | 85 | 85 | 86 | 87 | 88 | 89 | 91 | 93 | 95 | 97 | 100 | 103 | 106 | 110 | 113 | 117 | 122 | 126 |
| 90 | 84 | 85 | 86 | 86 | 87 | 88 | 89 | 91 | 92 | 95 | 97 | 100 | 103 | 106 | 109 | 113 | 117 | 122 | 127 | 132 |
| 91 | 85 | 86 | 87 | 87 | 88 | 89 | 90 | 92 | 94 | 97 | 99 | 102 | 105 | 109 | 113 | 117 | 122 | 126 | 132 | 137 |
| 92 | 86 | 87 | 88 | 88 | 89 | 90 | 92 | 94 | 96 | 99 | 101 | 105 | 108 | 112 | 116 | 121 | 126 | 131 | 137 | 143 |
| 93 | 87 | 88 | 89 | 89 | 90 | 92 | 93 | 95 | 98 | 101 | 104 | 107 | 111 | 116 | 120 | 125 | 130 | 136 | 142 | 149 |
| 94 | 87 | 89 | 90 | 90 | 91 | 93 | 95 | 97 | 100 | 103 | 106 | 110 | 114 | 119 | 124 | 129 | 135 | 141 | 148 | 155 |
| 95 | 88 | 89 | 91 | 91 | 93 | 94 | 96 | 99 | 102 | 105 | 109 | 113 | 118 | 123 | 128 | 134 | 140 | 147 | 154 | 161 |
| 96 | 89 | 90 | 92 | 93 | 94 | 96 | 98 | 101 | 104 | 108 | 112 | 116 | 121 | 126 | 132 | 138 | 145 | 152 | 160 | 168 |
| 97 | 90 | 91 | 93 | 94 | 95 | 97 | 100 | 103 | 106 | 110 | 114 | 119 | 125 | 130 | 136 | 143 | 150 | 158 | 166 | 174 |
| 98 | 91 | 92 | 94 | 95 | 97 | 99 | 102 | 105 | 109 | 113 | 117 | 123 | 128 | 134 | 141 | 148 | 155 | 164 | 172 | 181 |
| 99 | 92 | 93 | 95 | 96 | 98 | 101 | 104 | 107 | 111 | 115 | 120 | 126 | 132 | 138 | 145 | 153 | 161 | 170 | 179 | 188 |
| 100 | 93 | 94 | 96 | 97 | 100 | 102 | 106 | 109 | 114 | 118 | 124 | 129 | 136 | 143 | 150 | 158 | 167 | 176 | 185 | 195 |
| 101 | 93 | 95 | 97 | 99 | 101 | 104 | 108 | 112 | 116 | 121 | 127 | 133 | 140 | 147 | 155 | 164 | 173 | 182 | 192 | 203 |
| 102 | 94 | 96 | 98 | 100 | 103 | 106 | 110 | 114 | 119 | 124 | 130 | 137 | 144 | 152 | 160 | 169 | 179 | 189 | 199 | 210 |
| 103 | 95 | 97 | 99 | 101 | 104 | 108 | 112 | 116 | 122 | 127 | 134 | 141 | 148 | 157 | 165 | 175 | 185 | 195 | 206 | 218 |
| 104 | 96 | 98 | 100 | 103 | 106 | 110 | 114 | 119 | 124 | 131 | 137 | 145 | 153 | 161 | 171 | 181 | 191 | 202 | 214 | 226 |
| 105 | 97 | 99 | 102 | 104 | 108 | 112 | 116 | 121 | 127 | 134 | 141 | 149 | 157 | 166 | 176 | 187 | 198 | 209 | 221 | 234 |
| 106 | 98 | 100 | 103 | 106 | 109 | 114 | 119 | 124 | 130 | 137 | 145 | 153 | 162 | 172 | 182 | 193 | 204 | 216 | 229 | 243 |
| 107 | 99 | 101 | 104 | 107 | 111 | 116 | 121 | 127 | 134 | 141 | 149 | 157 | 167 | 177 | 188 | 199 | 211 | 224 | 237 | 251 |
| 108 | 100 | 102 | 105 | 109 | 113 | 118 | 123 | 130 | 137 | 144 | 153 | 162 | 172 | 182 | 193 | 205 | 218 | 231 | 245 | 260 |
| 109 | 100 | 103 | 107 | 110 | 115 | 120 | 126 | 133 | 140 | 148 | 157 | 167 | 177 | 188 | 200 | 212 | 225 | 239 | 254 | 269 |
| 110 | 101 | 104 | 108 | 112 | 117 | 122 | 129 | 136 | 143 | 152 | 161 | 171 | 182 | 194 | 206 | 219 | 233 | 247 | 262 | 278 |
| 111 | 102 | 106 | 109 | 114 | 119 | 125 | 131 | 139 | 147 | 156 | 166 | 176 | 187 | 199 | 212 | 226 | 240 | 255 | 271 | 288 |
| 112 | 104 | 107 | 111 | 115 | 121 | 127 | 134 | 142 | 150 | 160 | 170 | 181 | 193 | 205 | 219 | 233 | 248 | 263 | 280 | 297 |
| 113 | 104 | 108 | 112 | 117 | 123 | 129 | 137 | 145 | 154 | 164 | 175 | 186 | 198 | 211 | 225 | 240 | 256 | 272 | 289 | 307 |
| 114 | 105 | 109 | 113 | 119 | 125 | 132 | 140 | 148 | 158 | 168 | 179 | 191 | 204 | 218 | 232 | 247 | 264 | 280 | 298 | 317 |
| 115 | 106 | 110 | 115 | 121 | 127 | 134 | 143 | 152 | 162 | 173 | 184 | 197 | 210 | 224 | 239 | 255 | 272 | 289 | 308 | 327 |
| 116 | 107 | 111 | 116 | 122 | 129 | 137 | 146 | 155 | 166 | 177 | 189 | 202 | 216 | 231 | 246 | 263 | 280 | 298 | 317 | 337 |
| 117 | 108 | 112 | 118 | 124 | 132 | 140 | 149 | 159 | 170 | 181 | 194 | 208 | 222 | 237 | 253 | 271 | 289 | 307 | 327 | 348 |
| 118 | 108 | 113 | 119 | 126 | 134 | 142 | 152 | 162 | 174 | 186 | 199 | 213 | 228 | 244 | 261 | 279 | 297 | 317 | 337 | 358 |
| 119 | 109 | 114 | 121 | 128 | 136 | 145 | 155 | 166 | 178 | 191 | 204 | 219 | 235 | 251 | 268 | 287 | 306 | 326 | 347 | 369 |
| 120 | 110 | 116 | 122 | 130 | 138 | 148 | 158 | 170 | 182 | 196 | 210 | 225 | 241 | 258 | 276 | 295 | 315 | 336 | 358 | 380 |
| 121 | 111 | 117 | 124 | 132 | 141 | 151 | 162 | 174 | 187 | 200 | 215 | 231 | 248 | 265 | 284 | 304 | 324 | 346 | 368 | 392 |
| 122 | 111 | 118 | 125 | 134 | 143 | 154 | 165 | 178 | 191 | 205 | 221 | 237 | 254 | 273 | 292 | 312 | 333 | 356 | 379 | 403 |
| 123 | 112 | 119 | 127 | 136 | 146 | 157 | 169 | 182 | 196 | 211 | 226 | 243 | 261 | 280 | 300 | 321 | 343 | 366 | 390 | 415 |
| 124 | 113 | 120 | 129 | 138 | 148 | 160 | 172 | 186 | 200 | 216 | 232 | 250 | 268 | 288 | 308 | 330 | 353 | 376 | 401 | 427 |
| 125 | 114 | 121 | 130 | 140 | 151 | 163 | 176 | 190 | 205 | 221 | 238 | 256 | 275 | 296 | 317 | 339 | 362 | 387 | 412 | 439 |
Heat index / heat disorder legend
| Category | Heat index (°F) | Heat disorder risk |
|---|---|---|
| Caution | 80–90 | Fatigue possible with prolonged exposure. Take water breaks every 20–30 minutes. |
| Extreme Caution | 91–103 | Heat cramps and heat exhaustion are possible. Limit strenuous outdoor activity. |
| Danger | 104–124 | Heat exhaustion likely. Heat stroke possible. Avoid outdoor activity — stay in cool spaces. |
| Extreme Danger | 125+ | Heat stroke highly likely. This is a medical emergency risk. Seek air conditioning immediately. |
Chart values are computed with this site's NWS implementation and rounded to whole degrees, similar to the NOAA heat index chart. Direct sun, wind, and individual health factors are not included.
Official guidance and sources
Heat index values on this page follow the National Weather Service Steadman and Rothfusz methods, including published low- and high-humidity adjustments. For official forecasts, watches, and warnings in your area, rely on your local NWS office or weather.gov.
FAQ
What is a dangerous heat index?
The NWS treats 103°F and above as Danger, and 125°F or higher as Extreme Danger. At those levels, heat exhaustion or heat stroke become much more likely during outdoor exertion. Use the safety badge on this calculator as a quick reference, and follow local heat advisories.
How is heat index calculated?
The NWS starts with a simple Steadman equation, averages it with the actual temperature, and if that average is at least 80°F, applies the Rothfusz formula with up to nine polynomial terms. Low- and high-humidity adjustments apply in specific temperature and RH ranges. This page can show each step when you turn on Show the Math.
What is the difference between heat index and feels like?
“Feels like” is a general label apps use for perceived temperature. Heat index is the specific NWS model based on temperature and relative humidity in the shade. Other “feels like” values may also fold in wind, sun, or different formulas.
Does heat index apply in the shade?
Yes. The heat index is defined for shaded, ventilated conditions. Direct sun can make it feel 5–15°F hotter than the listed index, while wind can make it feel somewhat cooler.
What was the highest heat index ever recorded?
The highest widely cited heat index is about 178°F, recorded in Dhahran, Saudi Arabia on July 8, 2003. That afternoon the air temperature was 108°F with a world-record dew point near 95°F—not 95% relative humidity. At 108°F, a 95°F dew point works out to roughly 68% RH; plug those numbers into the NWS formula and you get about 177°F, matching the reported value. If you mistakenly enter 95% humidity instead, the calculator shows about 245°F, but that does not reflect what was measured that day. Dew point (°F) and relative humidity (%) are different inputs. Other Persian Gulf heat waves have also produced heat index values well above 150°F when extreme moisture is present.
Could it ever be 125°F with 100% humidity?
It depends what you mean. A natural air temperature of 125°F at 100% relative humidity is effectively impossible on Earth: saturated surface air cannot sustain that pairing because moisture and temperature are physically linked (wet-bulb temperatures in the worst real events stay near the mid-90s °F). A heat index of 125°F at 100% humidity is plausible at lower air temperatures—for example, about 89°F with 100% RH yields a heat index near 126°F on the NWS formula. Typing 125°F and 100% RH into any heat index calculator extrapolates beyond the range the equation was built for, so treat that output as mathematical, not as realistic weather.
Tips & Strategies
Quick tip. Heat index is most meaningful at 80°F (27°C) and above with meaningful humidity.
Quick tip. Shade lowers radiant heat but does not change the heat index value — it still uses air temperature and humidity.
Quick tip. Re-check after weather changes; a small humidity shift can move the index several degrees.
Quick tip. Pair this tool with hydration and rest breaks when the danger level is Caution or higher.
Cross-check when the decision matters. Run a second scenario with rounded inputs or a different path to the same quantity so you do not rely on a single fragile chain of arithmetic.
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