Heat Index Calculator – How Hot It Really Feels
Add the humidity to the temperature — the heat your body actually experiences.
The weather
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Advanced options — unit select, dew point and wet-bulb
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Worked out with the Magnus formula from the temperature and humidity above. When the air cools to this temperature, dew forms.
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Worked out with the Stull formula. It is the lowest temperature evaporation alone can reach, and the reason humid heat is harder to shed than dry heat.
The dew point and wet-bulb figures below are already worked out from the temperature and humidity above — open Advanced options to see them, and use the unit select to switch the whole calculator between Fahrenheit and Celsius.
Unit select, dew point and wet-bulb are already included in the result with their default values — open Advanced options to change them.
What it feels like
The numbers behind it
Heat check: Danger band (103–124°F)
At a heat index of 106°F, heat exhaustion is likely with continued exposure and heat stroke becomes possible, according to National Weather Service guidance. Move hard work to the cool hours, take shade and air-conditioned breaks, and drink water before you feel thirsty — heavy sweating, nausea or a headache means stop and cool down now.
Temperature is only half the weather. Add the humidity and this calculator gives you the heat index — the temperature your body actually experiences — worked out the National Weather Service way: the simple estimate first, the full regression when the air gets heavy, and the danger band your number falls into. Too cold for heat to be the problem? It says so honestly instead of inventing a number.
Key takeaways
- The heat index combines air temperature and relative humidity into one feels-like number, worked out here the National Weather Service way.
- Humidity, not temperature alone, decides the band: a humid 90°F afternoon can feel like 106°F, well into the Danger band.
- The chart values assume shade and a light wind, so full sun can push the effective figure up to 15°F higher.
Contents
- See where it lands
- Pairs well with this calculation
- Heat index in three steps
- How the heat index calculator works
- Going deeper
- Heat index chart: temperature against humidity
- Dew point and wet-bulb: what the same two numbers also tell you
- Sun, shade and activity: adjusting the number before you act on it
- Heat risk: the page’s bands and the forecast’s HeatRisk
- Frequently asked questions
See where it lands
One scale, from a mild day on the left to the top of the official chart on the right. The shaded bands are the National Weather Service categories; the two markers are your air temperature and the temperature your body feels.
Heat index 106°F — inside the Danger band (103–124°F), where heat exhaustion is likely and heat stroke is possible.
Why the heat index can read lower than the air temperature
In very dry air the formula can honestly return a number below the thermometer — 100°F at 10% humidity feels like 94°F, because sweat evaporates so fast that your body sheds heat more easily than the still, shaded reference conditions the index assumes. It is not a glitch: desert heat and humid heat are different kinds of hard, and the index is built to say so.
Pairs well with this calculation
The rest of the weather shelf — what the air holds, and what the day will do with it.
Heat index in three steps
Enter the air temperature
The plain thermometer reading, from a forecast or the gauge outside — ideally the shade temperature, since that is what the official method assumes. Pick Celsius first if that is how you think.
Add the relative humidity
The percentage from the same forecast. Humidity decides how fast sweat can evaporate, and evaporation is your body coolant — which is why this one number moves the answer so far.
Read the band, not just the number
The result lands in one of five National Weather Service bands, from comfortable to extreme danger. The check under your result says what the band means for work, sport and health — and when to stop.
How the heat index calculator works
Step 1 — simple estimate: HI = 0.5 × (T + 61 + (T − 68) × 1.2 + R × 0.094), averaged with T
T is the air temperature in °F and R the relative humidity in percent. If the simple estimate stays under 80°F, that is the answer. Otherwise the full nine-term Rothfusz regression runs — the polynomial the National Weather Service fits to Steadman’s apparent-temperature model — followed by two humidity adjustments for very dry and very saturated air.
The example on this page. Air 90°F, humidity 70%. The simple step gives 92.0, and averaged with the air temperature itself, 91.0 — already over the 80°F line, so the full regression runs and returns 105.92. Rounded the National Weather Service way, the heat index is 106°F: the Danger band, a full 16 degrees hotter than the thermometer.
When the air is saturated. At 86°F with 90% humidity, the regression alone gives 105.29. Because the humidity is over 85% and the temperature sits in the 80–87°F window, the method adds a muggy-air correction: ((90 − 85) / 10) × ((87 − 86) / 5) = 0.5 × 0.2 = +0.10, landing on 105.39 — a heat index of 105°F, nearly twenty degrees above the air.
When the air is desert-dry. At 100°F with 10% humidity, the regression gives 94.75. With humidity under 13%, a dry-air correction comes off: ((13 − 10) / 4) × √((17 − 5) / 17) = 0.75 × 0.840 = 0.63, leaving 94.12 — a heat index of 94°F, six degrees below the air temperature. Your sweat works so well in desert air that the felt temperature honestly drops.
When the simple step is enough. At 75°F with 60% humidity, the simple step gives 75.02, averaged with the temperature to 75.01 — under the 80°F line, so the method stops there. Heat index 75°F: on a mild day the humidity adds nothing you can feel, and the calculator does not pretend otherwise.
How the numbers are rounded. Results are rounded to the nearest whole degree, the National Weather Service convention — its published chart is all whole degrees, and the values on this page were checked against that chart. The unrounded figure is shown in the results panel, so nothing is hidden by the rounding.
Going deeper
The Rothfusz regression of 1990 fits the heat index beautifully across the hot, humid middle of the chart — but at the edges it drifts. In very dry air it overstates the heat, because a real body in desert air cools better than the fitted curve allows. So when the relative humidity falls below 13% and the temperature is between 80 and 112°F, the method subtracts ((13 − R) / 4) × √((17 − |T − 95|) / 17): a correction that is largest near 95°F, where the desert effect bites hardest, and fades to nothing at the window edges.
At the other edge, saturated air in the low-to-mid 80s feels worse than the regression says, so when humidity rises above 85% and the temperature is between 80 and 87°F, the method adds ((R − 85) / 10) × ((87 − T) / 5). Both corrections are small — the largest either can reach is about two degrees — but they are the difference between matching the official chart and merely resembling it, which is why this page implements them exactly rather than skipping to the headline formula.
The heat index is defined for a person resting in the shade with a light wind. National Weather Service guidance notes that full sunshine can add up to 15°F to the felt heat — so a shaded 106°F reading can behave like 121°F on an open field at noon. Wind cuts the other way: a real breeze speeds evaporation and softens the number, at least until the air gets hotter than skin, when moving air starts to act like a convection oven instead.
For athletes, soldiers and outdoor workers, safety bodies use a stricter instrument — wet-bulb globe temperature, which folds sunshine and wind into the reading and drives work-and-rest schedules. The heat index stays the right number for planning a day, a run or a shift schedule in the shade; it is not a clearance to keep going. Age, fitness, alcohol, medication and poor sleep all lower the point at which heat turns dangerous, and hot nights remove the recovery window entirely — when overnight lows stay near 80°F, risk compounds day after day.
Heat index chart: temperature against humidity
The table below is not copied from a poster. Every cell was produced by running this page’s own calculator over the temperature and humidity shown, so the chart and the tool above can never disagree. Read across a row to see how fast humidity moves the feels-like number once the air is warm: at 80°F the whole row spans only 6 degrees, while at 100°F the same humidity range spans 67 degrees. That widening is the whole story of humid heat.
| Air temperature | 40% humidity | 50% humidity | 60% humidity | 70% humidity | 80% humidity | 90% humidity |
|---|---|---|---|---|---|---|
| 80°F | 80°F | 81°F | 82°F | 83°F | 84°F | 86°F |
| 85°F | 84°F | 86°F | 89°F | 93°F | 97°F | 102°F |
| 90°F | 91°F | 95°F | 100°F | 106°F | 113°F | 122°F |
| 95°F | 99°F | 105°F | 113°F | 123°F | 134°F | 147°F |
| 100°F | 109°F | 118°F | 129°F | 143°F | 158°F | 176°F |
| 105°F | 121°F | 134°F | 149°F | 166°F | 187°F | 209°F |
Two cautions belong with this chart. First, the National Weather Service chart itself stops where conditions become extreme, and the largest corner values here are regression extrapolations: above 125°F every cell sits in the same Extreme danger band, and the band is the actionable information, not the exact figure. Second, the chart assumes shade and a light wind, which is why the next section adjusts it for sun and activity. The worked anchors check out against the published examples: 100°F at 40% humidity gives 109°F, and 96°F at 65% humidity gives 121°F, exactly as the National Weather Service examples state.
Dew point and wet-bulb: what the same two numbers also tell you
The heat index answers one question: how hot does it feel? Two related numbers answer different, equally practical questions, and both come from the same temperature and humidity you already entered. They are shown live in the Advanced options panel above, and worked here so you can check them by hand.
Dew point is the temperature the air would have to cool to, with its moisture unchanged, before dew forms. It is worked out with the Magnus formula: with temperature T in Celsius and relative humidity RH in percent, first form a = (17.27 × T ÷ (237.7 + T)) + ln(RH ÷ 100), then the dew point is 237.7 × a ÷ (17.27 − a) degrees Celsius. For the page’s default afternoon, 90°F (32.2°C) at 70% humidity, the dew point is 26.0°C, or 78.9°F. That is a muggy number on its own: when the dew point sits in the high 70s Fahrenheit, the air is holding a great deal of water, and that is why the heat index reaches 106°F. A dew point below about 55°F feels dry and pleasant; between 55°F and 65°F it starts to feel sticky; above 65°F it feels oppressive, and in the 70s it feels tropical. Unlike relative humidity, the dew point does not swing through the day just because the temperature rises: the same air mass keeps roughly the same dew point from morning to afternoon, which is why forecasters trust it as the mugginess gauge.
Wet-bulb temperature is the lowest temperature that evaporation alone can reach, and it is the number that sets the limit on sweating as a cooling method. It is estimated here with the Stull formula, which takes the same temperature and humidity and returns a value always at or below the air temperature and at or above the dew point. For 90°F at 70% humidity the wet-bulb temperature is 27.7°C, or 81.8°F: sweat can cool skin toward about 82°F, but no lower, no matter how much you sweat. Compare a drier afternoon, 100°F at 40% humidity: the dew point is only 71.3°F and the wet-bulb temperature 80.2°F, so although the air is hotter, evaporation still has room to work, and the heat index is 109°F rather than the 143°F that 100°F at 70% humidity produces. The comparison is the point: two afternoons can share a heat index yet feel different, and the dew point and wet-bulb figures explain why.
| Afternoon | Heat index | Dew point | Wet-bulb | What it means |
|---|---|---|---|---|
| 90°F, 70% humidity | 106°F | 78.9°F | 81.8°F | Humid and dangerous; sweat cools only toward 82°F |
| 100°F, 40% humidity | 109°F | 71.3°F | 80.2°F | Hotter air, drier air; evaporation still works |
| 86°F, 90% humidity | 105°F | 82.7°F | 83.5°F | Cooler air that still feels dangerous because the air is nearly saturated |
| 75°F, 60% humidity | 77°F | 60.2°F | 65.3°F | Comfortable; dew point in the low 60s, evaporation easy |
One limit to state plainly: the Magnus and Stull figures are formulas, not measurements. A sling psychrometer or a weather station measures the real wet-bulb and dew point for your exact air; the formulas reproduce them closely in ordinary summer conditions and drift a little at the extremes. Use them to understand the afternoon, and use a station reading to decide anything that depends on a single degree.
Sun, shade and activity: adjusting the number before you act on it
The heat index is defined for shade and a light wind. Step into full sun and the National Weather Service states that exposure can increase heat index values by up to 15°F. That single sentence changes how the chart should be read: a shade figure of 95°F can behave like 110°F in direct sun, which crosses from Extreme caution into Danger without the thermometer moving at all. The table below applies the full-sun allowance to a few shade figures so the size of the shift is visible at a glance. Treat the sun column as an upper allowance, not a promise: cloud, wind, clothing and surface heat all move the real figure, and the shade number is the measured one.
| Heat index in shade | In full sun, up to | Band change |
|---|---|---|
| 90°F | 105°F | Extreme caution to Danger |
| 95°F | 110°F | Extreme caution to Danger |
| 100°F | 115°F | Extreme caution to Danger |
| 106°F | 121°F | Danger, deeper into the band |
| 115°F | 130°F | Danger to Extreme danger |
Activity matters as much as sun. The heat index model describes a person at rest in the shade; working, running or playing sport adds internal heat that the index does not count. That is why sports and workplace guidance keys off a different number, the wet-bulb globe temperature, once exertion is involved, and why the practical advice below is organised by effort rather than by temperature alone. It is general safety guidance, not medical advice, and local heat advisories always override a rule of thumb.
| Effort | Shade heat index 90–102°F | Shade heat index 103°F and up |
|---|---|---|
| Light: walking, light garden work | Drink before you feel thirsty, rest in shade every hour | Rest in shade every 30 minutes, keep water close, watch for cramps and dizziness |
| Moderate: brisk walking, manual work, cycling | Schedule shade breaks, pace the work, pair up so someone notices symptoms | Move the work to cooler hours if possible, otherwise shorten shifts and lengthen breaks |
| Strenuous: running, sport, heavy labour | Consider moving the session to morning or evening, and treat the sun-adjusted figure as the real one | Postpone or restructure the session; the sun-adjusted figure is well into Danger |
Hot nights deserve a line of their own. The heat index describes an afternoon, but consecutive nights with lows near 80°F remove the recovery window: the body never fully sheds the day’s heat, and risk compounds day after day. When a forecast shows several hot nights in a row, that sequence is the warning, even if no single afternoon looks record-breaking.
Heat risk: the page’s bands and the forecast’s HeatRisk
The calculator sorts every result into one of five bands, and the thresholds are the same ones the National Weather Service publishes. Below 80°F the air is in the Normal range and humidity adds little. From 80°F to 89°F is Caution: fatigue is possible with prolonged exposure and activity. From 90°F to 102°F is Extreme caution: heat cramps and heat exhaustion are possible, and continued activity can lead to heat stroke. From 103°F to 124°F is Danger: heat cramps and heat exhaustion are likely, and heat stroke is possible with prolonged exposure and activity. At 125°F and above is Extreme danger: heat stroke is highly likely with continued exposure. The default afternoon on this page, 90°F at 70% humidity, lands at 106°F, squarely in Danger, which is why the verdict line says so rather than merely printing the number.
| Band | Heat index range | What it means |
|---|---|---|
| Normal | Below 80°F | Humidity adds little to how the air feels |
| Caution | 80–89°F | Fatigue possible with prolonged exposure and activity |
| Extreme caution | 90–102°F | Heat cramps and exhaustion possible; continued activity can lead to heat stroke |
| Danger | 103–124°F | Cramps and exhaustion likely; heat stroke possible with prolonged exposure |
| Extreme danger | 125°F and above | Heat stroke highly likely with continued exposure |
A second, different forecast product is worth knowing about because its name invites confusion with this page. The National Weather Service HeatRisk forecast rates how unusual and medically risky the forecast heat is for a particular place over the coming days, on a five-colour scale from green through yellow, orange and red to magenta. It is not a temperature and it is not the heat index: a HeatRisk red day in a normally cool city can arrive at a lower heat index than a HeatRisk yellow day in a desert city, because HeatRisk measures risk relative to what local people and infrastructure are used to, and it folds in how long the heat lasts. Use the heat index to judge this afternoon’s air, and use HeatRisk to judge whether the coming days are exceptional for your area. When the two disagree, believe both: the air can feel merely hot while the stretch of heat is still dangerous, and the air can feel extreme on a day that is unremarkable for that place.
Finally, a note on where the heat index stops being useful. It is not defined for cold air, and this calculator says so honestly instead of inventing a number when the temperature is too low. It also does not describe wind: a strong breeze in hot, dry air can add heat to the body rather than removing it, and it does not describe radiant heat from hot surfaces. For cold air, the companion wind chill page is the right tool; for sun, use the full-sun allowance above; for exertion, use wet-bulb globe temperature guidance from your sport or workplace. Each number answers one question well, and none of them answers all of them.
Wet-bulb temperature: the number behind the number
The guide above explains the heat index, but the question readers keep asking is what happens when sweat itself stops working. That is the wet-bulb temperature: the coldest temperature evaporation can reach in the air around you. At 90°F and 70% humidity it sits near 83°F, leaving only about 12°F of evaporative headroom between the air and your skin. The narrower that gap, the less cooling each drop of sweat buys you.
Two practical consequences follow. First, a lower thermometer reading can be the more dangerous day: 88°F at 85% humidity (wet-bulb near 84°F) strains the body more than 105°F at 10% humidity (wet-bulb near 70°F). Second, fans help only while the gap is open — a fan in saturated air moves hot, wet air across skin that cannot evaporate into it. If the wet-bulb temperature is in the mid-80s°F, shade and water are necessary but not sufficient for hard work; the work itself has to slow down or move indoors.
Sun, wind, and the 15°F the chart does not show
The heat index describes a person in the shade with a light breeze, and both assumptions fail in exactly the situations where heat hurts people. Full sun can add up to 15°F to what the heat feels like, and still air removes the breeze that carries saturated air away from the skin. A forecast heat index of 100°F therefore behaves like roughly 115°F for a roofer in direct sun, and worse again if the work is heavy.
The honest way to use the number is as a floor, not a ceiling: start from the heat index, add for sun and exertion, and switch to wet-bulb globe temperature whenever you are responsible for other people’s exposure. The calculator on this page shows the shaded figure; your judgment has to supply the rest.
Frequently asked questions
What is the heat index?
The heat index is the temperature it feels like to the human body when relative humidity is combined with the air temperature. Your body cools itself by evaporating sweat, and humid air slows that evaporation down, so heat builds up faster than the thermometer suggests. On this page, a humid 90°F afternoon at 70% humidity feels like 106°F.
How is the heat index calculated?
The National Weather Service method works in two steps. First a simple estimate: 0.5 × (T + 61 + (T − 68) × 1.2 + R × 0.094), averaged with the air temperature T, where R is the relative humidity. If that estimate reaches 80°F, a nine-term regression (the Rothfusz equation) replaces it, with a small correction subtracted in very dry air and added in very humid air. The answer is rounded to the nearest whole degree: 90°F at 70% humidity gives 106°F.
What heat index is dangerous?
The National Weather Service bands run: below 80°F, no heat stress expected; 80–89°F, caution — fatigue is possible with prolonged exposure and activity; 90–102°F, extreme caution — heat cramps and heat exhaustion are possible; 103–124°F, danger — heat exhaustion is likely and heat stroke is possible; and 125°F or higher, extreme danger — heat stroke is highly likely. The band matters more than the exact degree: once you are in the danger band, the advice changes, not just the number.
Why can the heat index be lower than the air temperature?
In very dry air, sweat evaporates so efficiently that your body sheds heat more easily than it does in the still, shaded reference conditions the index is built on. The method even subtracts a small dry-air correction when humidity falls below 13%. So 100°F at 10% humidity feels like 94°F — genuinely more bearable than a humid 90°F, which is exactly the comparison the heat index exists to make.
Is there a heat index in Celsius?
Yes — the same National Weather Service method, with the temperature converted to Fahrenheit for the calculation and the answer converted back. The toggle at the top of this calculator does exactly that, and the danger bands convert with it: the danger band of 103–124°F is 39–51°C. On the same weather both modes agree: 32.2°C at 70% humidity gives a heat index of 41°C, the twin of the 106°F example above.
Heat index or dew point — which tells me how muggy it feels?
They measure different things. The dew point is the air’s actual moisture content: below about 55°F it feels dry, 60–65°F feels muggy, and 70°F or more feels oppressive — whatever the temperature. The heat index folds that moisture together with the temperature to estimate the strain on your body. Watch the dew point to judge how the air feels; watch the heat index to judge how hard your body is working to cope with it.
How do I work out the dew point from temperature and humidity?
Use the Magnus formula: with temperature T in Celsius and relative humidity RH in percent, form a = (17.27 × T ÷ (237.7 + T)) + ln(RH ÷ 100), then the dew point is 237.7 × a ÷ (17.27 − a) degrees Celsius. For 90°F (32.2°C) at 70% humidity that gives 26.0°C, or 78.9°F. The Advanced options panel on this page works it out live from the temperature and humidity above.
What does full sun do to the heat index?
The heat index is defined for shade and a light wind, and the National Weather Service states that exposure to full sunshine can increase heat index values by up to 15°F. A shade figure of 95°F can therefore behave like up to 110°F in direct sun, crossing from Extreme caution into Danger without the thermometer moving.
What is wet-bulb temperature, and why does it matter in humid heat?
Wet-bulb temperature is the lowest temperature that evaporation alone can reach. When it is close to the air temperature, sweat has almost nowhere to go, which is why humid heat feels so much worse than dry heat at the same thermometer reading. For 90°F at 70% humidity the wet-bulb temperature is about 81.8°F, so sweat can cool skin toward 82°F but no lower. The Advanced options panel shows the wet-bulb figure for your numbers, worked out with the Stull formula.
At what heat index should outdoor work or sport be changed?
There is no single personal threshold, and local advisories override any rule of thumb, but the bands give the shape of it: at 90–102°F (Extreme caution) build in shade breaks and pace the effort, and at 103°F and above (Danger) move strenuous work or sport to cooler hours or restructure it. In full sun, judge by the sun-adjusted figure, up to 15°F above the shade number, and for exertion, sports and workplace guidance keys off wet-bulb globe temperature rather than the heat index.
What is wet-bulb temperature in simple terms?
It is the lowest temperature evaporation can cool something to in the current air — measured with a wet cloth over a thermometer bulb. When it gets close to skin temperature (around 95°F), sweating stops cooling you, no matter how much water you drink.
Does the heat index include the effect of sunshine?
No. The heat index assumes shade and a light breeze. Full sun can add up to about 15°F to the perceived heat, so treat the shaded figure as a minimum for anyone outdoors in direct sun.
Why does a humid 88°F day feel worse than a dry 100°F day?
Because sweat evaporates freely in dry air and barely at all in saturated air. At 88°F and 85% humidity the wet-bulb temperature is near 84°F, leaving almost no evaporative headroom; at 100°F and 15% humidity it is near 70°F, so sweat still cools you effectively.
Should I use heat index or wet-bulb temperature for exercise?
For personal planning, the heat index is fine. For training sessions, matches, or work crews, use wet-bulb globe temperature measured on site — it adds sun and wind to humidity, and it is the basis of real work-rest schedules.
Related calculators
Hot air, cold air, and the units to describe both.
Wind Chill Calculator
How cold the wind makes it feel.
→Dew Point Calculator
Where air gives up its moisture.
→Snow Day Calculator
Odds the school day gets called off.
→Conversion Calculator
Units converted both ways.
→Speed Calculator
Speed, distance and time.
→Wet-Bulb vs Heat Index
Formula, chart, and the safety thresholds that matter.
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