Dew Point Calculator

The dew point is the temperature at which air, cooled, starts to drop water

The dew point is the temperature to which air must be cooled, with its water vapor content unchanged, for the relative humidity to reach 100%. Cool air any further and the vapor has nowhere to go but into liquid: dew on grass, droplets on a glass, fog in the valley. Take a room at 72°F and 50% relative humidity. Its dew point is 52.3°F, so any window pane, wall corner or pipe colder than 52.3°F in that room will collect water.

This calculator takes any two of air temperature, relative humidity and dew point and solves for the third. Switch the same room to solve for relative humidity and the 50% comes back (typing the rounded 52.3°F gives 49.9%), or find out that air with a 50°F dew point warms to 76.1°F before its relative humidity drops to 40%. Add a surface temperature and it tells you whether that surface will get wet or frosted. The saturation curve chart and the humidity cells show what happens as the air cools.

Why the dew point beats relative humidity, and when to use it

Relative humidity is tied to temperature, so the same air gives a different number every time it warms or cools. Air at 86°F and 40% and air at 68°F and 70% differ by 30 points of relative humidity, yet their dew points are 58.9°F and 57.8°F. They hold about the same amount of water vapor. The dew point expresses the amount of vapor as a temperature, which makes several everyday questions easy to answer.

• Window and wall condensation: when the surface of a window pane or a cold exterior wall corner drops below the dew point of the room air, water collects there, and damp that lasts invites mold. Enter an infrared thermometer reading to get a yes or no.
• How muggy it feels: the National Weather Service office in La Crosse says that in summer a dew point of 55°F (12.8°C) or less feels dry and comfortable, and 65°F (18.3°C) or more gets oppressive. Stickiness follows the dew point better than the humidity percentage.
• Sweating glasses and pipes: a cold drink or a cold-water pipe gets wet because its surface is colder than the dew point. At 86°F and 65%, the dew point is 72.9°F, so almost anything cold from the fridge will sweat.
• Painting and coatings: a film of moisture too thin to see can ruin adhesion. The coating-inspection equipment maker Elcometer notes that a typical coating specification asks for a surface at least 5°F (3°C) above the dew point and relative humidity no higher than 85%.
• Fog: the FAA's Aviation Weather Handbook says fog seldom forms when the gap between temperature and dew point is more than 4°F (2°C). If the overnight low is headed for the dew point, expect fog.

How to use the calculator

1. Under "Solve for", pick dew point, relative humidity or air temperature. The field for that value disappears and two remain.
2. Fill in the two fields. You can switch between °F and °C at any time, and the values you typed are kept as they are.
3. If a surface worries you, enter its temperature in the surface field. Leave it empty to skip the surface check.
4. Read the big number and the line below it, then check where the dew point lands on the summer-feel scale.
5. Tap or drag on the saturation curve chart to set the air to that point. The arrow shows the path the air takes as it cools, and the droplet at its tip marks the dew point.
6. In the humidity cells, watch the water level rise as the air cools and see how much water forms once you go past the dew point. "Copy link" sends the same inputs to someone else.

How the dew point is calculated

The dew point comes from running a saturation vapor pressure formula backwards. The calculator uses the formulas published in the WMO guide to instruments and observations. Over water, ew(t) = 6.112 × exp(17.62t ÷ (243.12 + t)) hPa, which this calculator applies from −45 to 60°C (−49 to 140°F). For the frost point it uses the formula over ice, ei(t) = 6.112 × exp(22.46t ÷ (272.62 + t)) hPa. The formulas work in °C, so a °F input is converted first and the result is converted back.

1. Find the saturation vapor pressure ew(T) at the air temperature. At 72°F (22.2°C) it is 26.7 hPa.
2. Multiply by the relative humidity to get the actual vapor pressure e. At 50%, e = 0.5 × 26.7 = 13.4 hPa.
3. Find the temperature Td where ew(Td) = e. With L = ln(e ÷ 6.112), Td = 243.12L ÷ (17.62 − L). For 13.4 hPa that gives 11.3°C, which is 52.3°F.
4. If you know the dew point instead, relative humidity is ew(Td) ÷ ew(T) × 100, and the air temperature is the one where ew(T) equals e ÷ (relative humidity ÷ 100).
5. If the dew point is below freezing, the same e is run through the ice formula to get the frost point, Tf = 272.62L ÷ (22.46 − L).

Td = 243.12 × ln(e ÷ 6.112) ÷ (17.62 − ln(e ÷ 6.112)), e = RH ÷ 100 × 6.112 × exp(17.62T ÷ (243.12 + T)) (T and Td in °C, e in hPa)

The detail cards use the formulas from Vaisala's instrument manual: absolute humidity a = 216.679 × e ÷ (T + 273.15) g/m³ and mixing ratio x = 621.9907 × e ÷ (p − e) g/kg. The mixing ratio needs a pressure p, so the calculator assumes standard sea-level pressure, 1013.25 hPa, and the vertical axis of the saturation chart is this mixing ratio. Wet-bulb temperature uses Stull's 2011 approximation and is left blank outside the range where that formula was checked (5–99% relative humidity, −4 to 122°F).

Dew point table by temperature and humidity

These dew points come from the same formula as the calculator (°F, one decimal). At a given humidity, every 10°F of warming raises the dew point by 8.6 to 9.9°F. At a given temperature, every 10 points of extra humidity raise it by 3.0 to 8.3°F, with the biggest steps at low humidity and in hot air.

What a dew point feels like in summer

Sweat dries according to how much water vapor is in the air, which is why summer stickiness tracks the dew point more closely than the humidity percentage. The National Weather Service in La Crosse describes these summer ranges, and the summer-feel scale in the calculator follows them, judged on the original °F value rounded to one decimal.

These ranges describe outdoor summer air. In winter a low dew point can mean overly dry heated rooms, so read indoor numbers alongside the targets in the indoor humidity calculator. For how hot it feels once temperature is included, use the heat index calculator.

When water or frost forms on a surface

Even when the room air as a whole stays above its dew point, the thin layer touching a cold surface cools to that surface's temperature. So water forms on any surface at or below the dew point. The calculator compares the surface temperature with the dew point as displayed (one decimal) and calls it "condensation" when the surface is equal or lower. A surface below freezing gets frost instead of droplets.

The table shows indoor dew points. A window pane or wall corner colder than the value in the table will sweat. In a 72°F living room at 50%, the dew point is 52.3°F, so a 50°F pane is 2.3°F below it and collects droplets. Drop the humidity to 40% and the dew point falls to 46.3°F, so the same pane stays dry.

Painters need to be careful well before droplets appear. Elcometer says a typical coating specification wants the surface at least 5°F (3°C) above the dew point, with relative humidity at or below 85%. So even when nothing condenses, the calculator tells you how far you are from that margin. The real limits come from the paint's data sheet and the job specification.

If you can't measure the surface directly, the condensation risk calculator estimates it from the outdoor temperature and the insulation of the window or wall.

Below freezing, look at the frost point

When the dew point is below freezing, water vapor attaches to a surface as ice crystals rather than droplets. The saturation vapor pressure over ice is lower than over water at the same temperature, so the frost point sits a little above the dew point, and frost forms there first on a surface below freezing. On a morning at 21°F and 85%, the dew point is 17.2°F and the frost point is 18.9°F. A car window that has cooled to 16°F is 2.9°F below the frost point, so it frosts over.

Relative humidity in this calculator is always measured against saturation over water, even below freezing. That is why frost can form while the humidity reads below 100%. On the chart, the dotted line left of 32°F marks saturation over ice, and the point where the arrow first meets it is the frost point.

Fog and cloud base estimates

The gap between air temperature and dew point shows how far the air is from saturating. The FAA's Aviation Weather Handbook (FAA-H-8083-28A, 18.1.1) says fog seldom forms when the temperature-dewpoint spread is greater than 2°C (4°F). The calculator flags a spread of 4°F or less as "fog and low cloud form easily". At dawn, 48°F and 97% give a spread of only 0.8°F.

Air that rises outdoors cools faster than its dew point drops, and a cloud forms at the height where the two meet. The same handbook estimates the lowest cloud layer as the spread in °C divided by 2.5, multiplied by 1,000 ft, and gives 18°C with a 9°C dew point as 3,600 ft above ground. The FAA's Pilot's Handbook of Aeronautical Knowledge (chapter 12) writes the same estimate as the °F spread ÷ 4.4 × 1,000 ft. The calculator's card uses the first form and rounds to the nearest 100 ft.

The estimate is for outside air that rises and forms cloud. It means nothing for indoor air, and real cloud heights can differ from it.

Why cooling raises humidity and makes water

The most vapor air can hold, its saturation vapor pressure, drops quickly as the temperature falls. The amount of vapor stays the same while the limit shrinks, so relative humidity climbs, and the dew point is the temperature at which the limit equals the vapor actually present. On the chart, the "air now" point slides left until it touches the saturation curve, and in the humidity cells it is the cell that fills to 100%.

Cool the air past its dew point and the excess vapor turns to water. An air conditioner dehumidifies the same way. Take 80°F air at 70% (dew point 69.3°F, 15.4 g of vapor per kg of air) passing over a cooling coil at 54°F. Cooled to the coil's temperature, it can keep only 8.8 g per kg, so about 6.5 g of every kilogram drops out as water. The last humidity cell shows this amount of water per kilogram of dry air.

Common mistakes

• Judging condensation from the spread alone: whether water forms depends on the surface, not on the air. Room air at 72°F and 40% has a dew point of 46.3°F, 25.7°F below the air temperature, yet a 44°F window pane still gets wet. Enter the surface temperature to get a verdict.
• Comparing humidity percentages: 40% on a hot afternoon and 70% on a cool evening can hold almost the same amount of vapor. When you compare air from different times or places, compare dew points.
• A dew point above the air temperature: that can't happen. If your readings say so, the two sensors measured different spots or one of them is off. The calculator reports it as an error.
• Ignoring the frost point below freezing: a surface under 32°F frosts at the frost point, which is a little higher. For car windows or a freezer, compare against the frost point.
• Planning indoor condensation from the forecast: the forecast dew point is for outdoor air. Cooking, showers and drying laundry push the dew point indoors above the outdoor value, so use the temperature and humidity you measure inside.

Limits

• The saturation vapor pressure formula is an approximation for everyday temperatures, so the calculator does not handle temperatures outside −45 to 60°C (−49 to 140°F), and neither is a dew point that would fall below −45°C.
• The mixing ratio and the chart's vertical axis assume standard sea-level pressure (1013.25 hPa). At altitude, the mixing ratio for the same vapor pressure is a little higher. The dew point, relative humidity and absolute humidity do not depend on this assumption.
• Accuracy depends on the readings. At 72°F and 50%, a hygrometer that is off by 3 points of relative humidity moves the dew point between 50.7 and 53.9°F, about ±1.6°F.
• The summer-feel ranges are a general description from one National Weather Service office, and people differ. Wind, sun and clothing are not included.
• The painting margin (dew point + 5°F) is an example of a typical specification, and the cloud base is a rough estimate.

As of October 21, 2026. The formulas and thresholds were checked on that date against the original NCAR/EOL summary of the WMO formulas, the National Weather Service, the FAA, Vaisala and Elcometer.

Sources

• 01NCAR/EOL — Water Vapor Pressure FormulationsWMO (2008, CIMO guide) formulas: over water 6.112·exp(17.62t ÷ (243.12 + t)) hPa, over ice 6.112·exp(22.46t ÷ (272.62 + t)) hPa
• 02NWS La Crosse — Why use dewpoint vs humidity?Summer dew points: 55°F or less dry and comfortable, 55 to 65°F sticky, 65°F or more oppressive
• 03FAA — Aviation Weather Handbook (FAA-H-8083-28A)18.1.1 fog seldom forms when the spread is above 2°C (4°F); lowest cloud layer = spread ÷ 2.5 × 1,000 ft
• 04FAA — Pilot's Handbook of Aeronautical Knowledge, chapter 12Temperature and dew point converge about 4.4°F per 1,000 ft, used to find the cloud base (pp. 12-14)
• 05Vaisala — HMT120/TMT120 manual, humidity conversion formulasMixing ratio x = 621.9907 × Pw ÷ (p − Pw) g/kg, absolute humidity a = 216.679 × Pw ÷ (T + 273.15) g/m³
• 06Elcometer — Measuring temperature, relative humidity and dewpointTypical coating specification: surface at least 3°C (5°F) above the dew point, relative humidity no higher than 85%
• 07Stull (2011) — Wet-Bulb Temperature from Relative Humidity and Air TemperatureJournal of Applied Meteorology and Climatology 50, pp. 2267–2269, range of validity and error

Frequently asked questions

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