Dew Point Calculator – Dew Point From Temperature and Humidity
Enter temperature and humidity — get the dew point, the saturation gap and a comfort reading.
Air temperature and humidity
A phone weather app, a home weather station or a room hygrometer all report the two numbers you need: the air temperature right now and the relative humidity beside it.
🔒 Everything is calculated in your browser — nothing you type leaves your device.
Advanced options — unit select, dew point in Kelvin, wet-bulb temperature
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Worked from the unrounded Magnus dew point plus 273.15, so it can differ by 0.1 from adding 273.15 to the rounded display above.
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Worked out with the Stull formula from the same temperature and humidity. It sits between the dew point and the air temperature, and marks the limit of evaporative cooling.
The Kelvin figure and the wet-bulb figure 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 Celsius and Fahrenheit.
Unit select, dew point in Kelvin, wet-bulb temperature are already included in the result with their default values — open Advanced options to change them.
Your dew point
Enter the air temperature and the relative humidity, and this calculator works out the dew point — the temperature the air would have to cool to before dew, fog or condensation appears — together with the gap between the air and saturation, an honest comfort reading, and how much water the air is actually holding.
Key takeaways
- The dew point is the temperature the air must cool to before water condenses out of it, worked out here with the Magnus formula.
- Unlike relative humidity, the dew point barely moves through the day for the same air mass, which is why forecasters trust it as the mugginess gauge.
- Condensation appears wherever a surface is at or below the dew point, which is why windows, pipes and cold walls drip first.
Contents
- How close is the air to saturation?
- Keep exploring
- How it works in three steps
- The anchors, worked through
- Going deeper
- Dew point chart: temperature against humidity
- The Magnus formula, its frost-point variant, and altitude
- Comfort, condensation and mould: reading the number in a house
- Dew point beside heat index and wet-bulb: one afternoon, three numbers
- Frequently asked questions
How close is the air to saturation?
The gauge puts the air temperature and the dew point on one scale. The distance between the markers is the spread: cool the air by exactly that much and it saturates. Below it, the comfort strip shows where your dew point lands on the scale weather services use — the same air can feel dry in the morning and muggy by afternoon purely because the dew point moved.
The air sits at 30.0 °C and the dew point at 18.4 °C — a gap of 11.6 °C. Cool the air by that gap and the excess water condenses out as dew, fog or cloud.
Your dew point of 18.4 °C falls in the Sticky band. Most people start to feel the humidity here, even when the air temperature itself is not extreme.
Keep exploring
The neighbours of this page: the all-purpose unit converter for temperature scales, the density tool that shares this page’s physics, and the snow day estimator that turns weather into a day off.
How it works in three steps
Read the air where you are
Take the air temperature and the relative humidity from a weather app, a weather station or a hygrometer. Both numbers describe the same parcel of air at the same moment — a forecast high for this afternoon paired with this morning’s humidity will skew the answer.
The Magnus formula does the physics
The calculator runs the Magnus formula, the approximation weather services use worldwide: it converts your temperature and humidity into the vapour pressure the air is carrying, then asks at what lower temperature that same vapour pressure would be the saturation point. That temperature is the dew point.
Read the gap, not just the number
The dew point alone tells you how the air feels. The spread — air temperature minus dew point — tells you what happens next: a small spread means dew, fog or window condensation is close; a wide spread means the air has room to cool a long way first.
The anchors, worked through
The opening example, by hand: air at 30 °C and 50 percent relative humidity. First the humidity term: ln(50 ÷ 100) is ln(0.5), which is −0.693. Then the temperature term: 17.27 times 30 is 518.1, divided by (237.7 + 30) — 267.7 — giving 1.935. Add them: alpha is −0.693 + 1.935, which is 1.242.
Dew point 18.4 °C: 237.7 times 1.242 is 295.3, and 17.27 minus 1.242 is 16.03. Divide: 295.3 ÷ 16.03 is 18.42, which displays as 18.4 °C. In Fahrenheit the unrounded figure converts to 65.2 °F. Some tables quote 65.1 °F for this case — that figure comes from converting the already-rounded 18.4 °C; the difference is rounding, not method.
Spread 11.6 °C: 30.0 minus 18.4 is 11.6. That is how far this air can cool before dew forms — a whole cool night’s worth of cooling in most climates, which is why the gauge above reads “well short of saturation”.
Absolute humidity 15.1 g/m³: the same Magnus curve gives the saturation vapour pressure at 30 °C as about 42.3 hPa; at 50 percent humidity the air carries half of that, 21.2 hPa. Absolute humidity is 216.7 times the vapour pressure, divided by the temperature in kelvin: 216.7 × 21.2 ÷ 303.15 is 15.1 grams of water in every cubic metre. Saturated air at 30 °C holds about 30.3 g/m³ — the widely published figure for this temperature is 30.4, a rounding-level difference between formula variants.
A cooler, damper check: at 20 °C and 65 percent humidity, ln(0.65) is −0.431 and the temperature term is 345.4 ÷ 257.7, which is 1.340, so alpha is 0.910. The dew point is 237.7 × 0.910 ÷ (17.27 − 0.910) — 216.2 ÷ 16.36 — which is 13.2 °C, squarely in the Comfortable band.
The frost case: at 0 °C and 50 percent humidity, alpha is −0.693 and the dew point comes out at −9.2 °C — below freezing. Strictly, moisture settling there forms frost rather than liquid dew, and the exact frost point differs slightly from this water-based formula; the calculator shows the frost note whenever your dew point lands below 0 °C (32 °F).
The saturated case: at 100 percent relative humidity the logarithm term is ln(1), which is zero, and the formula returns the air temperature itself, exactly: 25 °C at 100 percent gives a dew point of 25 °C. The air is already saturated, the spread is zero, and any surface at air temperature is already at the condensation line.
Going deeper
Relative humidity is a ratio, not an amount: it says how much water the air holds compared with how much it could hold at this temperature. Warm air can hold far more water than cold air, so 50 percent humidity on a hot afternoon carries much more water than 50 percent on a cold morning — which is why relative humidity alone is a poor guide to how muggy a day feels.
The dew point fixes that by folding the temperature in. It is effectively an absolute measure in temperature clothing: air with a dew point of 20 °C carries the same amount of water vapour wherever it is found. Absolute humidity, the third tile on this page, states the amount directly as grams per cubic metre. The three numbers describe the same air — this calculator shows all three so you can use whichever your situation speaks: comfort charts use the dew point, drying and ventilation work uses absolute humidity, and weather reports lead with relative humidity.
On a clear night, grass, car roofs and window panes radiate heat to the sky and cool faster than the air around them. When a surface drops to the dew point, water vapour condenses onto it as liquid dew. If the whole layer of air near the ground cools to its dew point instead, the water condenses in the air itself as fog. Both are the same physics at different addresses, and both are predicted by the spread: the smaller tonight’s spread, the earlier dew or fog appears.
When the dew point itself is below freezing, the deposit is frost — feathery ice crystals rather than liquid drops. Strictly, the temperature at which frost forms is the frost point, and it sits a little above the water-based dew point because saturation over ice differs from saturation over water. The difference is small (a degree or so in typical conditions), and this calculator flags the frost case rather than pretending the distinction does not exist.
Cool the air, and it tells you exactly how much water it was hiding. ⭐
Dew point chart: temperature against humidity
Every cell in this table was produced by running this page’s own calculator, so the chart and the tool above cannot disagree. Two patterns are worth reading out of it. Down a column, the dew point rises with temperature at fixed humidity, because warm air can hold more water: at 50% humidity the dew point climbs from 9.3°C at 20°C air to 23.0°C at 35°C air. Across a row, the dew point rises with humidity at fixed temperature, because more of the air’s capacity is filled: at 30°C it climbs from 10.5°C at 30% humidity to 28.2°C at 90%.
| Air temperature | 30% humidity | 50% humidity | 70% humidity | 90% humidity |
|---|---|---|---|---|
| 20°C | 1.9°C | 9.3°C | 14.4°C | 18.3°C |
| 25°C | 6.2°C | 13.8°C | 19.1°C | 23.2°C |
| 30°C | 10.5°C | 18.4°C | 23.9°C | 28.2°C |
| 35°C | 14.8°C | 23.0°C | 28.7°C | 33.1°C |
The gap between air temperature and dew point, called the spread, is the other number to watch. A spread of 11.6°C, as in the default case of 30°C air at 50% humidity, means comfortable, unsaturated air with room to cool before anything condenses. A spread of 1.8°C, as in 25°C air at 90% humidity, means fog and dew are one small cooling step away. Forecasters read the spread the way a driver reads following distance: large is safe, small means something is about to happen.
The Magnus formula, its frost-point variant, and altitude
The calculator uses the Magnus formula over water: with air 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. The constants 17.27 and 237.7 are the standard over-water pair, and they reproduce the saturation vapour pressure curve closely across ordinary weather. The same formula can be written in vapour-pressure form: saturation vapour pressure at temperature T is about 6.112 × exp(17.67 × T ÷ (T + 243.5)) hectopascals. At 30°C that gives about 42.4 hPa; at 50% humidity the actual vapour pressure is half of that, about 21.2 hPa; and the temperature at which saturation vapour pressure equals 21.2 hPa is 18.4°C, exactly the calculator’s answer. The two forms are the same physics, and checking one against the other is a good way to catch a slip.
Below freezing, a second variant matters. Over ice, the Magnus constants change to 21.875 and 265.5, and the temperature at which frost forms is the frost point, not the dew point. For the same vapour pressure the frost point sits slightly above the dew point, by roughly a degree in typical winter conditions, because saturation over ice is lower than saturation over supercooled water. This calculator flags the frost case rather than pretending the distinction does not exist: if the air is below freezing, read the result as a frost-point-style figure and expect frost, not liquid dew, on cold surfaces.
Pressure and altitude need a careful sentence, because the common advice is easy to get backwards. The dew point itself needs no pressure correction: it is a vapour-pressure temperature, and a parcel of air keeps its dew point when the total pressure changes, as long as no water is added or removed. What altitude changes is the saturation side of the story and the fate of a rising parcel. Lift a parcel and it cools, roughly 9.8°C per kilometre while dry, and once it saturates it cools more slowly, roughly 5 to 6°C per kilometre, following a moist adiabat. The height where it first saturates is the lifting condensation level, and a handy approximation is 125 metres of lift per degree of spread: cloud base height ≈ 125 × (air temperature − dew point) metres. For the default case, a spread of 11.6°C gives a cloud base near 1,450 metres; for 25°C air at 90% humidity, a spread of 1.8°C gives a cloud base near 225 metres, which is low cloud or fog. The approximation is rough, and real cloud bases depend on the full temperature profile, but the direction is exact: small spread, low cloud; large spread, high cloud or none.
| Air and dew point | Spread | Approximate cloud base |
|---|---|---|
| 30°C air, 18.4°C dew point | 11.6°C | About 1,450 m |
| 30°C air, 23.9°C dew point | 6.1°C | About 760 m |
| 25°C air, 23.2°C dew point | 1.8°C | About 225 m |
| 20°C air, 18.3°C dew point | 1.7°C | About 210 m |
Comfort, condensation and mould: reading the number in a house
Outside, the dew point is a comfort gauge. The commonly published guide runs: below 50°F (10°C) dry and pleasant; 50–55°F (10–13°C) comfortable; 55–60°F (13–16°C) noticeably humid; 60–65°F (16–18°C) sticky; 65–70°F (18–21°C) muggy and oppressive; above 70°F (21°C) tropical. Individual tolerance varies, and people acclimatised to humid climates shrug at numbers that trouble visitors, but the ordering never reverses: a higher dew point always means more water in the air, whatever the thermometer says.
| Dew point | How it feels | What to do with it |
|---|---|---|
| Below 50°F (10°C) | Dry and pleasant | Open windows freely; static and dry skin are the likelier complaints |
| 50–55°F (10–13°C) | Comfortable | Most people notice nothing; good drying weather |
| 55–60°F (13–16°C) | Noticeably humid | Laundry dries slowly; light ventilation helps |
| 60–65°F (16–18°C) | Sticky | Expect damp towels and slow drying; ventilate after showers and cooking |
| 65–70°F (18–21°C) | Muggy and oppressive | Dehumidify or air-condition; outdoor exertion feels much harder |
| Above 70°F (21°C) | Tropical | Sweat barely evaporates; heat stress risk rises sharply with exertion |
Inside, the dew point becomes a condensation predictor, and the rule is absolute: any surface at or below the dew point collects water. A window on a winter night, a cold-water pipe in a warm basement, or an uninsulated wall corner all drip for the same reason, and the fix is always one of two things: warm the surface, or lower the dew point by removing moisture. United States Environmental Protection Agency guidance is to keep indoor humidity below 60%, and ideally between 30% and 50%, because sustained damp is what allows mould to grow: mould does not need a flood, it needs a surface that stays damp long enough. Ventilating bathrooms and kitchens at source, drying laundry outside or with extraction, and running a dehumidifier in a damp basement all lower the indoor dew point directly, which is more effective than wiping condensation after it forms. If a wall stays damp despite dry indoor air, the cause is usually a cold bridge or a leak, and no amount of ventilation will fix a surface that is colder than the dew point for other reasons.
Dew point beside heat index and wet-bulb: one afternoon, three numbers
The same temperature and humidity produce three different answers, and the table below lines them up for four afternoons so the relationships are visible. Dew points come from this calculator, heat index values from the companion heat index calculator run on the same numbers, and wet-bulb temperatures from the Stull formula shown in Advanced options. The pattern to notice is that the heat index follows the dew point more closely than it follows the thermometer: the two 86°F afternoons differ by 17 degrees of heat index, and the whole difference is the dew point.
| Afternoon | Dew point | Wet-bulb | Heat index |
|---|---|---|---|
| 86°F (30°C), 50% humidity | 18.4°C (65.1°F) | 22.3°C (72.1°F) | 88°F |
| 95°F (35°C), 70% humidity | 28.7°C (83.7°F) | 30.3°C (86.5°F) | 123°F |
| 77°F (25°C), 60% humidity | 16.7°C (62.1°F) | 19.5°C (67.1°F) | 77°F |
| 86°F (30°C), 90% humidity | 28.2°C (82.8°F) | 28.6°C (83.5°F) | 105°F |
Read the rows as a decision aid. The first row is a warm, manageable afternoon: the dew point is in the sticky range but the wet-bulb leaves room for sweat to work, and the heat index barely exceeds the thermometer. The second row is dangerous by every measure at once. The third row shows why a dew point in the low 60s Fahrenheit is the sweet spot for summer comfort: the air is warm, the moisture is moderate, and the heat index adds nothing. The fourth row is the trap: the thermometer reads the same 86°F as the first row, but the dew point is ten degrees higher, the wet-bulb is six degrees higher, and the heat index is seventeen degrees higher. Anyone who plans by temperature alone will be caught by the fourth row and comfortable in the first, and the dew point is the number that tells them apart before they step outside.
Wet-bulb and dry-bulb: the two-thermometer method
Before electronic sensors, humidity was measured with two thermometers. The difference between their readings still explains the physics.
A psychrometer carries two thermometers side by side. The dry-bulb thermometer reads the ordinary air temperature. The wet-bulb thermometer has its bulb wrapped in a wet wick, and air is moved past it — by whirling the instrument, or by a fan — so that water evaporates from the wick and cools the bulb. The wet-bulb reading is always at or below the dry-bulb reading, and the gap between them, the wet-bulb depression, measures how much evaporation the air can still support. In dry air, evaporation is vigorous, the wet bulb cools a long way, and the depression is large. In humid air near saturation, evaporation is sluggish, the wet bulb barely cools, and the depression is small. At saturation the two readings coincide, because no net evaporation is possible.
The wet-bulb temperature always sits between the dew point and the dry-bulb temperature: below the air temperature, because evaporation cools it, and above the dew point, because the air at the wet bulb is saturated at the wet-bulb temperature, which is warmer than the temperature at which the undisturbed air would saturate. Given the dry-bulb and wet-bulb readings together with the air pressure, the dew point and the relative humidity can both be computed from psychrometric relations, which is how humidity was determined for a century before capacitive sensors took over. The wet bulb also has a life of its own in heat-stress work, because it limits what evaporative cooling — sweating, swamp coolers, cooling towers — can achieve, and the companion guide compares dew point, wet-bulb, and frost point in full, including the frost-point variant used below freezing.
Frequently asked questions
What is the dew point?
The dew point is the temperature the air would have to cool to, with its moisture content unchanged, before it becomes saturated and water starts to condense out of it — as dew on grass, fog in the air, or droplets on a cold surface. It is always at or below the air temperature: the closer it sits to the air temperature, the more saturated and clammy the air is.
How is the dew point calculated?
This calculator uses the Magnus formula, the approximation weather services use worldwide: alpha equals ln(RH divided by 100) plus (17.27 times T) divided by (237.7 plus T), and the dew point is 237.7 times alpha divided by (17.27 minus alpha), with T in degrees Celsius. Worked by hand for 30 °C at 50 percent humidity, alpha is 1.242 and the dew point is 18.4 °C. Fahrenheit inputs are converted to Celsius, calculated, and converted back.
What is the difference between dew point and relative humidity?
Relative humidity compares the water in the air with the most that air could hold at its current temperature, so the same relative humidity means very different amounts of water on a cold day and a hot day. The dew point already folds the temperature in, so it tracks the actual moisture content: a dew point of 20 °C feels muggy whatever the air temperature is, while 50 percent relative humidity can feel dry or sticky depending on the temperature. That is why comfort guides quote the dew point, not the humidity.
What dew point feels comfortable?
On the scale weather services commonly use, a dew point below 10 °C (50 °F) feels dry, 10 to 16 °C feels comfortable, 16 to 19 °C feels sticky, 19 to 22 °C feels muggy, and 22 °C and over feels oppressive. The bands describe the dew point itself, not the air temperature — a mild 24 °C afternoon with a 21 °C dew point feels far more humid than a hot 35 °C afternoon with a dew point of 12 °C.
When does condensation form on windows and pipes?
Condensation forms on any surface whose temperature is at or below the dew point of the air touching it. Work out the dew point from the room’s temperature and humidity, then compare it with the surface: a single-glazed window on a cold night, a cold-water pipe in a humid room, or a drinks can from the fridge all sit below the line easily. Warming the surface (better glazing, pipe insulation) or drying the air (ventilation, a dehumidifier) both move you back above it.
Can the dew point be higher than the air temperature?
No. The dew point can equal the air temperature — at 100 percent relative humidity the two meet exactly, and the air is saturated — but it cannot pass it. If saturated air cools below its dew point, the excess water condenses out as dew, fog or cloud, and the dew point falls along with the temperature. A reading that seems to show a dew point above the air temperature is a sensor error, or two readings taken at different times or places.
What is the difference between dew point and frost point?
The dew point is the temperature at which water vapour condenses as liquid dew; the frost point is the temperature at which it deposits as frost. Below freezing the two differ because saturation over ice is lower than saturation over supercooled water, so for the same air the frost point sits slightly above the dew point, by roughly a degree in typical winter conditions. This calculator flags the frost case rather than treating the two as identical.
How do I express the dew point in Kelvin?
Add 273.15 to the dew point in Celsius. The default case, a dew point of 18.4°C, is 291.6 K when worked from the unrounded value. The Advanced options panel shows the Kelvin figure live for your numbers, worked from the unrounded Magnus result so it can differ by 0.1 from adding 273.15 to the rounded display.
Can I estimate cloud base from the dew point?
Yes, approximately: cloud base height in metres is roughly 125 times the spread between air temperature and dew point in Celsius. A spread of 11.6°C, as in the default case, gives a cloud base near 1,450 metres, while a spread of 1.8°C gives a cloud base near 225 metres, which is low cloud or fog. Real cloud bases depend on the full temperature profile, so treat the figure as an approximation.
Does altitude change the dew point?
No pressure correction is needed for the dew point itself: it is a vapour-pressure temperature, and a parcel keeps its dew point when total pressure changes, as long as no water is added or removed. What altitude changes is saturation and the fate of rising air: a lifted parcel cools about 9.8°C per kilometre while dry, then more slowly once saturated, which is why the cloud-base estimate uses the spread rather than the pressure.
What is wet-bulb temperature, and how is it measured?
The wet-bulb temperature is the reading of a thermometer whose bulb is kept wet while air moves past it, so evaporation cools it. It is measured with a psychrometer: two thermometers, one dry and one with a wet wick, whirled or fanned in the air. The gap between the two readings, the wet-bulb depression, is large in dry air and small in humid air, and vanishes at saturation.
Why is the wet-bulb temperature always between the dew point and the air temperature?
Evaporation cools the wet bulb below the air temperature, but the air at the wet bulb is saturated at the wet-bulb temperature, which is warmer than the temperature at which the undisturbed air would saturate. So the wet bulb sits below the dry bulb and above the dew point, coinciding with both only at saturation, when no evaporation is possible. It limits evaporative cooling; the dew point governs condensation.
What is the frost point, and when does it replace the dew point?
Below freezing, air in contact with ice saturates with respect to ice rather than liquid water, and the corresponding temperature is the frost point, computed with different Magnus coefficients, 22.587 and 273.86. The difference from the dew point is small, a degree or two in the usual range, but it matters for frost forecasting and cold-climate building work. This calculator applies the frost-point variant below freezing.
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