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Heat index lookup
Air temperature plus humidity — the feel-like temperature and its danger band.
REFERENCE TABLE · WEATHER · EVERYDAY
Heat Index Chart: What It Really Feels Like
When the air is 100°F and the humidity is 55%, it does not feel like 100°F — it feels like 124°F, well into the Danger band. The table below runs the National Weather Service’s real heat-index formula over air temperatures from 80 to 110°F and humidity from 40 to 100%, and the two lookups below it compute your exact feel-like temperature — or work backward from the heat index to the air temperature. Tip: the table prints cleanly — Ctrl/Cmd+P before a heat wave.
Heat index table (feel-like °F)
Rows are air temperature, columns are relative humidity. Values are whole degrees, computed with the NWS Rothfusz regression including both official adjustments.
| Air temp | 40% | 45% | 50% | 55% | 60% | 65% | 70% | 75% | 80% | 85% | 90% | 95% | 100% |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 80°F | 80 | 80 | 81 | 81 | 82 | 82 | 83 | 84 | 84 | 85 | 86 | 88 | 89 |
| 82°F | 81 | 82 | 83 | 84 | 84 | 85 | 86 | 88 | 89 | 90 | 92 | 94 | 96 |
| 84°F | 83 | 84 | 85 | 86 | 88 | 89 | 90 | 92 | 94 | 96 | 98 | 101 | 104 |
| 86°F | 85 | 87 | 88 | 89 | 91 | 93 | 95 | 97 | 100 | 102 | 105 | 109 | 112 |
| 88°F | 88 | 89 | 91 | 93 | 95 | 98 | 100 | 103 | 106 | 110 | 113 | 117 | 121 |
| 90°F | 91 | 92 | 95 | 97 | 100 | 103 | 106 | 109 | 113 | 117 | 122 | 127 | 132 |
| 92°F | 94 | 96 | 99 | 101 | 105 | 108 | 112 | 116 | 121 | 126 | 131 | 137 | 143 |
| 94°F | 97 | 100 | 103 | 106 | 110 | 114 | 119 | 124 | 129 | 135 | 141 | 148 | 155 |
| 96°F | 101 | 104 | 108 | 112 | 116 | 121 | 126 | 132 | 138 | 145 | 152 | 160 | 168 |
| 98°F | 105 | 109 | 113 | 117 | 123 | 128 | 134 | 141 | 148 | 155 | 164 | 172 | 181 |
| 100°F | 109 | 114 | 118 | 124 | 129 | 136 | 143 | 150 | 158 | 167 | 176 | 185 | 195 |
| 102°F | 114 | 119 | 124 | 130 | 137 | 144 | 152 | 160 | 169 | 179 | 189 | 199 | 210 |
| 104°F | 119 | 124 | 131 | 137 | 145 | 153 | 161 | 171 | 181 | 191 | 202 | 214 | 226 |
| 106°F | 124 | 130 | 137 | 145 | 153 | 162 | 172 | 182 | 193 | 204 | 216 | 229 | 243 |
| 108°F | 130 | 137 | 144 | 153 | 162 | 172 | 182 | 193 | 205 | 218 | 231 | 245 | 260 |
| 110°F | 136 | 143 | 152 | 161 | 171 | 182 | 194 | 206 | 219 | 233 | 247 | 262 | 278 |
Danger bands (apply to the feel-like value): 80–90 Caution — fatigue possible with long exposure; 90–103 Extreme caution — heat cramps and heat exhaustion possible; 103–124 Danger — heat cramps or exhaustion likely, heatstroke possible; 125+ Extreme danger — heatstroke highly likely. These values assume shade and light wind: full sunshine can add up to 15°F to the feel-like temperature.
Read the table: 100°F at 55% humidity → 124°F, Danger (the weather.gov reference example). The same 100°F at 40% humidity feels like only about 109°F — humidity, not heat alone, drives the danger. Below 80°F feel-like, values use the simpler Steadman average per NWS guidance.
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Reverse lookup: heat index → air temperature
You know the feel-like temperature and the humidity — find the real air temperature.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Heat index | NWS Rothfusz regression + RH adjustments | 124°F at the default 100°F / 55% — what the air feels like to a human body. |
| Danger band | 80–90 caution · 90–103 extreme caution · 103–124 danger · 125+ extreme danger | Danger at the default — heat cramps or exhaustion likely, heatstroke possible. |
| Reverse air temperature | bisection on the formula, T in [70, 130] | 103°F heat index at 75% humidity → about 88°F of real air. |
| Below-80 handling | Steadman simple average | When the formula gives under 80°F, the simpler average is used per NWS guidance — heat index is not really defined there. |
Who uses this
- Outdoor workers and crew leads: a 95°F forecast with 60% humidity is a 116°F feel-like — Danger band, which changes break schedules and water plans.
- Runners and athletes: heat index, not the thermometer, decides whether a workout is safe — and the reverse lookup reads race-day forecasts backward.
- Parents and coaches: youth-sports heat policies key off feel-like bands; 103°F+ means practice gets modified or canceled.
- Travelers to the Sun Belt: checking whether a 100°F Phoenix afternoon or a 92°F Houston one is actually worse — humidity decides.
Accuracy & limitations
This is the National Weather Service’s published formula (the Rothfusz regression) with both official adjustments: +((R−85)/10)×((87−T)/5) for very humid mid-range heat, and −((13−R)/4)×√((17−|T−95|)/17) for dry heat. Below 80°F of result it falls back to the simpler Steadman average, per NWS guidance. Assumes shade and light wind — full sun can add up to 15°F, and strong wind or wet clothing change the real feel. It does not model age, health, clothing, or acclimatization; the bands are guidance, not medical advice. Values are rounded to whole degrees. One honest caveat: at the extreme top-right corner of the table (110°F × 100% humidity) the regression extrapolates well past observed conditions — that combination implies a dew point never recorded on Earth — so treat those corner cells as directional, not precise.
Cite this calculatorBright Side Kit. “Heat Index Chart.” brightoolbox.com, 2026.
Bright Side Kit. "Heat Index Chart." https://brightoolbox.com/heat-index-chart/ Accessed October 23, 2026.Frequently asked questions
What is the heat index?
The heat index is what hot, humid air feels like to the human body. Sweat cools you by evaporating; humidity slows evaporation, so the same air temperature feels much hotter. At 100°F and 55% humidity the heat index is 124°F — Danger band, while 100°F at 40% humidity feels like only about 109°F.
How is the heat index calculated?
The National Weather Service uses the Rothfusz regression, a nine-term formula in air temperature (°F) and relative humidity, plus two adjustments — one for very humid 80–87°F days, one for dry 80–112°F days. This page runs the exact published formula; below 80°F of result it uses the simpler Steadman average per NWS guidance.
Why does the chart say shade? What about full sun?
The formula assumes shade and light wind. In full sunshine the feel-like temperature can run up to 15°F higher — a 100°F/55% day at 124°F in the shade can feel near 139°F in direct sun. Working, running, or playing in the sun, add that margin mentally.
What do the danger bands mean?
80–90 Caution: fatigue possible with prolonged exposure. 90–103 Extreme caution: heat cramps and heat exhaustion possible. 103–124 Danger: cramps or exhaustion likely, heatstroke possible. 125+ Extreme danger: heatstroke highly likely. Youth sports and workplaces commonly modify activity from the Danger band up.
The heat index is below 80 — what does that mean?
Below about 80°F the heat index is not really meaningful — humidity barely slows cooling at comfortable temperatures. Per NWS guidance the calculator falls back to a simple average there instead of the full regression.
How does the reverse lookup work?
It runs the formula in reverse: given a heat index and a humidity, it bisects the air temperature between 70 and 130°F until the formula reproduces your heat index. 103°F of heat index at 75% humidity → about 88°F of real air.