Dew Point Calculator

How to use this tool

"Dew point from temperature and humidity" is the everyday case. "Humidity from temperature and dew point" runs the other way, for a forecast that quotes a dew point. "Will this surface sweat" adds a surface temperature for a cold pipe, slab, duct or window and reports the margin and the humidity at which condensation starts. Leave the pressure at 1013.25 hPa unless you are well above sea level; it only affects the mixing ratio and specific humidity.

The input people most often get wrong is the humidity. Relative humidity belongs to a time and a place: the 7 am forecast is not your basement at 3 pm, and the middle of a room is not the cold corner behind the freezer. Measure where the problem is, when it happens.

What the calculation is doing

Everything starts from the saturation vapour pressure, the most vapour air at a given temperature can hold. The calculator uses the Magnus formula with the Alduchov and Eskridge (1996) constants, es = 6.1094 × exp(17.625 T ÷ (243.04 + T)) hPa with T in °C. It is an approximation, and a good one: from 0 to 50 °C it stays within 0.22% of the Goff Gratch reference equation and of Buck 1996, and its dew points agree with Buck to within 0.031 °C from 0 to 40 °C at 20% humidity or more.

The actual vapour pressure is e = es × RH ÷ 100, and the dew point is where the saturation curve equals e. The Magnus form inverts algebraically: γ = ln(RH ÷ 100) + 17.625 T ÷ (243.04 + T), then Td = 243.04 γ ÷ (17.625 − γ). From a dew point the same curve gives the humidity back, RH = 100 × es(Td) ÷ es(T). Absolute humidity is e ÷ (461.52 × T in kelvin) in g/m³, the mixing ratio is 621.97 × e ÷ (p − e) g/kg of dry air, and grains per pound is that times seven.

The wet bulb uses the Stull 2011 fit, valid from 5% to 99% humidity and −20 to 50 °C at sea level pressure with errors between −1 and +0.65 °C, except cold, dry air, where the tool withholds it. Below freezing the tool also reports the frost point, slightly above the dew point because ice has a lower saturation pressure than supercooled water.

Why the dew point, not the humidity, tells you what will drip

Relative humidity is a ratio, and the denominator moves with temperature. Outdoor winter air at 2 °C and 90% sounds damp; its dew point is 0.5 °C. Heat that air to 21 °C indoors without adding water and it reads 25.6%, because the saturation pressure at 21 °C is three and a half times that at 2 °C. The moisture never changed, and the dew point says so.

Run it the other way for a summer basement. Outdoor air at 28 °C and 65% has a dew point of 20.8 °C; let it reach a slab at 15 °C and the concrete sweats, whatever the meter in the middle of the room says. A dehumidifier lowers the dew point of the whole room; a fan blowing that air at the wall only speeds up the condensation.

Coating work uses the dew point as a hold point. Steel painting specifications (the City of Columbus Item 514 and the Caltrans primer specification among them) forbid painting steel less than 5 °F (3 °C) above the dew point, because a film of condensation you cannot see spoils adhesion. At 15 °C and 80% humidity the dew point is 11.6 °C, so steel at 14 °C is dry but only 2.4 °C above it. A wet floor or fresh plaster only dries while its surface is warmer than the dew point of the air over it.

Dew point by temperature and relative humidity

Cells are Magnus dew points in °C, shaded by National Weather Service band: blue below freezing, teal dry and comfortable to 55 °F (12.8 °C), amber becoming sticky, red oppressive from 65 °F (18.3 °C). Steps shrink across a row and grow down a column: the curve in the diagram at work.

A worked example you can check on paper

Take the tool's defaults, 24 °C and 60% relative humidity. Saturation vapour pressure: es = 6.1094 × exp(17.625 × 24 ÷ 267.04) = 29.78 hPa. Actual vapour pressure: e = 29.78 × 0.60 = 17.87 hPa. Magnus term: γ = ln(0.60) + 17.625 × 24 ÷ 267.04 = −0.5108 + 1.5840 = 1.0732. Dew point: Td = 243.04 × 1.0732 ÷ (17.625 − 1.0732) = 15.76 °C, or 60.4 °F. Absolute humidity: 1,787 Pa ÷ (461.52 × 297.15 K) = 0.01303 kg/m³ = 13.0 g/m³. Mixing ratio: 621.97 × 17.87 ÷ (1,013.25 − 17.87) = 11.17 g/kg, or 78 grains per pound. Wet bulb by Stull: 18.60 °C.

Now the surface check with the same air and the default 12 °C surface. Margin = 12 − 15.76 = −3.76 °C: the surface is below the dew point and sweats. It sits exactly at the dew point when the humidity is 100 × es(12) ÷ es(24) = 100 × 14.00 ÷ 29.78 = 47%; below that it stays dry. Painting would want the surface at 18.8 °C or warmer.

Mistakes that catch people out

Typing the humidity as a fraction. 0.6 means 0.6%, not 60%. The tool refuses it, but a spreadsheet with the same formula will quietly return a dew point near −40 °C.

Comparing rooms by relative humidity. A cool room at 60% can hold less water than a warm room at 45%. Compare dew points, or absolute humidity in g/m³, to see which way moisture will move.

Mixing up the wet bulb and the dew point. The dew point decides condensation; the wet bulb governs evaporative cooling, drying rates and heat stress. A fan on a sweating pipe changes nothing, because the pipe is still below the dew point.

Trusting the second decimal from a cheap sensor. Saturation pressure climbs about 6% per degree near room temperature, so a hygrometer 5 points out, which is common, moves the dew point about 1.3 °C at 24 °C. The formula is good to hundredths of a degree; the reading is not. Check it against a second instrument or the salt test before a costly call.

Popular calculations

Each address opens the calculator with those readings filled in; change anything once it is open.

Dew point from Celsius readings

Dew point from Fahrenheit readings

Humidity from a dew point

Will the surface sweat

How this was checked

The saturation curve was compared with the CRC Handbook and NIST Chemistry WebBook tables every 5 °C from 0 to 50 °C (Magnus within 0.23% and 0.26%), and with Goff Gratch 1946, Buck 1996 and the WMO 2008 form coded separately in a test script (within 0.22% from 0 to 50 °C, 0.46% near −30 °C). Dew points were checked at 1,420 combinations of temperature (−20 to 50 °C) and humidity (5% to 100%) against the Buck and Goff Gratch curves inverted numerically, largest disagreement 0.063 °C at −20 °C and 5%, and 20,000 random round trips from humidity to dew point and back returned the starting figures to thirteen decimals.

Published readings were reproduced too: the University of Nebraska extension guide reads a 59 °F dew point and 67 °F wet bulb at 80 °F and 50% (engine: 59.7 °F and 67.0 °F), and the National Weather Service heat index page puts 80 °F with a 60 °F dew point at 50% humidity (engine: 50.5%). The Stull wet bulb matches the NCL documentation example of 13.699 °C at 20 °C and 50% to five decimals, and across 3,280 points from 0 to 40 °C and 20% to 99% it sits within 0.72 °C of the psychrometric equation (mean 0.28 °C), inside the accuracy Stull claims. Every address above was opened and its title and answer checked.

Common questions

What is the dew point?

The temperature the air must be cooled to, with no change in its moisture, to be saturated. Cool it further and vapour condenses as dew, fog or a wet film on anything that cold. It depends only on how much vapour the air holds: air at 24 °C and 60% has a dew point of 15.8 °C whether the room is later heated or cooled, while the humidity reading changes with every degree.

What is a comfortable dew point?

The National Weather Service summer scale calls 55 °F (12.8 °C) or less dry and comfortable, 55 to 65 °F becoming sticky with muggy evenings, and 65 °F (18.3 °C) or more oppressive. Indoors the target is lower: the EPA advises keeping relative humidity below 60%, ideally 30% to 50%, and at 21 °C that 50% ceiling is a dew point of 10.2 °C.

How do I calculate the dew point from temperature and humidity?

The exact route is the Magnus inversion shown in the worked example, Td = 243.04 γ ÷ (17.625 − γ) with γ = ln(RH ÷ 100) + 17.625 T ÷ (243.04 + T); for 24 °C and 60% it gives 15.76 °C. Above 50% humidity a quick estimate is one degree Celsius for every 5 points of humidity below 100: 24 − 40 ÷ 5 = 16 °C, good to about 2 °C in that range.

Can the dew point be higher than the air temperature?

No. At 100% relative humidity the dew point equals the air temperature, and air holding more vapour than that sheds the excess as fog or condensation. A reported dew point above the temperature is a measurement error, usually a wet or aged humidity sensor.

Why do my windows sweat, and at what humidity does it start?

Condensation starts on any surface colder than the dew point of the air touching it, and glass is usually the coldest surface in a room. Air at 21 °C and 40% has a dew point of 6.9 °C; on a cold night single glazing, or the aluminium edge of a sealed unit, is often below that. Lower the humidity until the dew point is below the glass (30% gives 2.8 °C), warm the glass with better glazing or air movement, or both.

How is the wet bulb different from the dew point?

The dew point is where cooling alone saturates the air, and it decides whether a surface gets wet. The wet bulb is how cold evaporation can make a wet surface in that air, which matters for evaporative coolers, drying and heat stress; it sits between the dew point and the air temperature. At 30 °C and 80% the two are 26.2 °C and 27.1 °C; at 30 °C and 20% they are 4.6 °C and 15.9 °C. Dry air opens a wide gap; saturated air closes it.

Related Calculators and Reference

• Humidex Calculator: the Canadian comfort index, built on the dew point.
• Heat Index Calculator: the US felt temperature from the same temperature and humidity.
• Bathroom Fan CFM Calculator: the exhaust rate that keeps the dew point of a shower room below its mirror and walls.
• Insulation Calculator: insulation lifts a cold surface out of the sweating zone.
• Temperature Converter: for readings that arrive in kelvin or Rankine.
• Dehumidifier Size Calculator: size a dehumidifier once you know a space is damp enough to sweat.
• Density Altitude Calculator: how the dew point changes air density.
• National Weather Service, Dew Point vs Humidity: the comfort bands used here and why the dew point is the better measure.