How do I calculate the dew point?

From the room temperature and humidity, find the amount of water vapor in the air (vapor pressure), then work backward to the temperature at which that amount saturates the air. With the Magnus formula, let γ = ln(humidity/100) + 17.62 × room temperature ÷ (243.12 + room temperature); the dew point = 243.12 × γ ÷ (17.62 − γ). At 20°C and 50% humidity the dew point is 9.3°C, and at 60% it is 12.0°C; water droplets form on any surface colder than that.

What humidity avoids window condensation?

It depends on the window type and the outdoor temperature. With a room at 20°C and an outdoor temperature of 0°C, there is no condensation up to 42% for single glazing, 66% for double glazing (12 mm air gap), 81% for Low-E double glazing (argon) and 39% for an aluminum sash frame. At an outdoor temperature of −5°C, this drops to 33% for single glazing and 60% for double glazing.

At what outdoor temperature does single glazing start to fog up?

In a room at 20°C and 50% humidity, the center of single glazing starts to condense when the outdoor temperature falls below about 3.7°C. An aluminum sash frame starts at about 5.0°C, so it gets wet before the glass. Double glazing (12 mm air gap) does not condense until about −13.7°C.

Why does closing the curtains make condensation worse?

Warm room air barely reaches the space between the curtain and window, so the glass gets even colder, while humid air still leaks in through the gaps. For single glazing at a room temperature of 20°C and an outdoor temperature of 0°C, the surface temperature at the center is about 6.8°C, but behind a closed curtain it drops to about 3.7°C, and the maximum non-condensing humidity falls from 42% to 34%.

Why do closets get condensation and mold?

When heated room air enters a closet that is cold because it sits against an exterior wall, the amount of water vapor stays the same while the temperature drops, so the humidity rises. Air at 20°C and 50% humidity entering a 12°C closet reaches about 83% humidity, and at 60% it reaches 100% and condenses. Raise the bedding off the wall with a slatted rack, leave the sliding doors slightly open, and lower the room humidity.

What is the difference between absolute and relative humidity?

Absolute humidity is the amount of water vapor in 1 m³ of air (g/m³); relative humidity is that amount as a percentage (%) of the maximum the air can hold at that temperature (the saturated vapor content). The saturated vapor content at 20°C is about 17.3 g/m³, so the absolute humidity at 50% is about 8.6 g/m³. When the same air gets colder, its saturated vapor content falls, so relative humidity rises.

Why do rooms get dry in winter?

Cold outdoor air holds little water vapor, so when it is warmed, its relative humidity drops sharply. Air at 5°C and 70% humidity warmed to 20°C is about 26% humidity, and air at 0°C and 60% becomes about 16%. Each time you ventilate, dry air comes in, so humidifying becomes necessary.

How many mL/h of humidifier output does an 8-tatami room need?

To keep 20°C and 50% humidity with an outdoor temperature of 5°C, 60% humidity and a 2.4 m ceiling, you need about 71 mL/h in an airtight house (0.5 air changes per hour), about 141 mL/h in an ordinary detached house (1 per hour) and about 212 mL/h in a drafty wooden house (1.5 per hour). Considering moisture absorbed by walls and furniture, a humidifier with about 1.5 times that capacity is a safe choice.

Condensation & Dew Point Calculator | Humidity and Outdoor Temperature at Which Windows and Sashes Fog Up, Closet Humidity, Humidifier Output

The Condensation & Dew Point Calculator is a free tool that predicts winter condensation on windows, in closets and in north-facing rooms with numbers. Enter room temperature and humidity to get the dew point (the temperature at which condensation starts), absolute humidity (g/m³), vapor pressure and mixing ratio. For window condensation, it calculates the indoor surface temperature from the thermal transmittance (U value) of single, double, Low-E and triple glazing and of aluminum, aluminum-resin composite and resin sashes, then shows whether the window fogs up at the current outdoor temperature, how far to lower the humidity to stop it, and below what outdoor temperature condensation begins. You can also pick the inside of a closed curtain or the bottom edge of the window. It also shows the humidity and mold risk when heated room air enters a closet, behind furniture or a north-facing room, the humidity when cold winter outdoor air is warmed, and the humidifier output (mL/h) required for your room size (in tatami mats) and ventilation rate. Quick-reference tables for dew point, non-condensing humidity and saturated vapor content are included.

Dew point and absolute humidity

Enter the readings from your thermometer and hygrometer to get the temperature that air must cool to before it condenses (the dew point) and the amount of water vapor in 1 m³ of air (absolute humidity).

Room temperature (air temperature)

℃

Humidity (relative humidity)

Dew point

9.3℃

Water droplets form on surfaces at 9.3℃ or below

Absolute humidity (by volume)

8.6 g/m³

50% of the saturated vapor content at this temperature, 17.2 g/m³

How many more degrees until condensation?

10.7℃

Condensation starts on surfaces 10.7℃ colder than the room

Vapor pressure

11.7 hPa

Mixing ratio (absolute humidity by weight)

7.2 g/kg

Per 1 kg of dry air (1 atm)

Water in the air of a 6-tatami room

About 201 g

With a 2.4 m ceiling, about 23 m³ (≈ mL)

Dew point as humidity changes at a room temperature of 20.0℃

Dew point by humidity
Humidity	Dew point	Absolute humidity	Temperature gap at which condensation starts
30%	1.9℃	5.2 g/m³	Room temp. −18.1℃
40%	6.0℃	6.9 g/m³	Room temp. −14.0℃
50%	9.3℃	8.6 g/m³	Room temp. −10.7℃
60%	12.0℃	10.3 g/m³	Room temp. −8.0℃
70%	14.4℃	12.1 g/m³	Room temp. −5.6℃
80%	16.4℃	13.8 g/m³	Room temp. −3.6℃

All calculations run in your browser, and the numbers you enter are never sent to a server. Settings are saved in this browser.

ABOUT

About Condensation & Dew Point Calculator | Humidity and Outdoor Temperature at Which Windows and Sashes Fog Up, Closet Humidity, Humidifier Output

The Condensation & Dew Point Calculator takes room temperature and humidity and gives you the temperature at which condensation starts (the dew point) and the absolute humidity, then judges whether cold spots such as window glass, sashes and closets will fog up. For windows, it calculates the indoor surface temperature from the thermal transmittance (U value) of single, double (pair), Low-E and triple glazing and of aluminum, aluminum-resin composite and resin sashes, and shows whether condensation forms at the current outdoor temperature, the maximum humidity that avoids it, and the outdoor temperature below which it begins. With a room at 20°C and an outdoor temperature of 0°C, single glazing fogs up above 42% humidity, double glazing (12 mm air gap) above 66%, and an aluminum sash frame above 39%. It also covers closet and north-room humidity, the humidity when winter outdoor air is warmed, the humidifier output (mL/h) you need, and quick-reference tables for dew point, non-condensing humidity and saturated vapor content.

How to use Condensation & Dew Point Calculator | Humidity and Outdoor Temperature at Which Windows and Sashes Fog Up, Closet Humidity, Humidifier Output

1Measure the room temperature and humidity with a thermometer and hygrometer and enter them on the “Dew point & absolute humidity” tab (condensation starts on surfaces colder than the dew point)
2On the “Window condensation” tab, choose the lowest outdoor temperature of the night to early morning, the glass type and the sash type
3Check the glass and sash surface temperatures and whether they condense, the maximum humidity that avoids condensation, and the outdoor temperature at which condensation begins
4Change “Location to check” to the inside of a closed curtain or the bottom edge of the window to check the coldest spots too
5On the “Humidity in cold spots” tab, choose the temperature of a closet, north-facing room or the back of furniture, and see whether humidity exceeds 70% (mold guideline) or 100% (condensation)
6On the “Humidifier output” tab, enter the room size in tatami mats and your target humidity to see the mL/h a humidifier needs and whether the window will fog up at that humidity

Great for

Windows are soaked on winter mornings and you want to know what humidity to keep it under
Compare how much single glazing, double glazing and Low-E glass differ in condensation
Find out why only the aluminum sash frame gets wet, and from what temperature and humidity it condenses
Check how much condensation would drop after adding a secondary window (double window) or replacing the windows
Understand with humidity numbers why closets, north-facing rooms and the backs of furniture get moldy
Calculate the dew point and absolute humidity (g/m³) from temperature and humidity (science homework, painting work, warehouse management and so on)
See whether a humidifier’s output (mL/h) is enough for the room size, and whether over-humidifying will cause condensation
Explain why rooms get dry in winter using the humidity of outdoor air once it is warmed

Good to know

Dew point and saturated vapor pressure are calculated with the Magnus formula (Alduchov & Eskridge coefficients). For surfaces below 0°C, the comparison uses the saturated vapor pressure over ice, so frost or freezing forms instead of condensation (that temperature is shown as the frost point).
Window surface temperature is calculated as “room temperature − (indoor surface heat transfer resistance ÷ total window thermal resistance) × (room temperature − outdoor temperature).” The total window thermal resistance is 1/U − 0.11 + the indoor resistance, treating the U value as one that includes the indoor 0.11 common in catalogs.
The glass U values are typical guides: single 6.0, double (12 mm air gap) 2.9, Low-E double (argon) 1.5, triple 0.9; sashes: aluminum 6.5, aluminum-resin composite 3.5, resin 2.0. If you know the actual product’s value, you can enter it under “Enter thermal transmittance (U value).”
Solar gain, warm air from heaters, drafts and the spacer at the glass edge are not considered. The bottom edge and corners of a window are colder than the center, so you can choose them under “Location to check.”
Mold tends to grow at 70% humidity or above and dust mites at 60% or above. Near cold spots and windows, humidity is higher than the room’s humidity.
Humidifier output is “ventilation rate × room volume × (target absolute humidity − outdoor absolute humidity),” and excludes moisture from breathing, cooking and indoor drying, and moisture absorbed by walls and furniture. When choosing a humidifier, allow about 1.5 times extra capacity.
All calculations run in your browser, and the numbers you enter are never sent to a server.

Questions this tool answers

Use Condensation & Dew Point Calculator | Humidity and Outdoor Temperature at Which Windows and Sashes Fog Up, Closet Humidity, Humidifier Output for searches like these:

I want to calculate the dew point from temperature and humidity
I want to know what humidity avoids window condensation
At what outdoor temperature do single and double glazing fog up
Temperature and humidity at which aluminum sashes condense
I want to know the causes of closet condensation and mold
I want to calculate absolute humidity in g/m³
I want to see a dew point chart and a saturated vapor content table
I want to calculate whether a humidifier’s mL/h output is enough for the room size

FAQ

How do I calculate the dew point?: From the room temperature and humidity, find the amount of water vapor in the air (vapor pressure), then work backward to the temperature at which that amount saturates the air. With the Magnus formula, let γ = ln(humidity/100) + 17.62 × room temperature ÷ (243.12 + room temperature); the dew point = 243.12 × γ ÷ (17.62 − γ). At 20°C and 50% humidity the dew point is 9.3°C, and at 60% it is 12.0°C; water droplets form on any surface colder than that.
What humidity avoids window condensation?: It depends on the window type and the outdoor temperature. With a room at 20°C and an outdoor temperature of 0°C, there is no condensation up to 42% for single glazing, 66% for double glazing (12 mm air gap), 81% for Low-E double glazing (argon) and 39% for an aluminum sash frame. At an outdoor temperature of −5°C, this drops to 33% for single glazing and 60% for double glazing.
At what outdoor temperature does single glazing start to fog up?: In a room at 20°C and 50% humidity, the center of single glazing starts to condense when the outdoor temperature falls below about 3.7°C. An aluminum sash frame starts at about 5.0°C, so it gets wet before the glass. Double glazing (12 mm air gap) does not condense until about −13.7°C.
Why does closing the curtains make condensation worse?: Warm room air barely reaches the space between the curtain and window, so the glass gets even colder, while humid air still leaks in through the gaps. For single glazing at a room temperature of 20°C and an outdoor temperature of 0°C, the surface temperature at the center is about 6.8°C, but behind a closed curtain it drops to about 3.7°C, and the maximum non-condensing humidity falls from 42% to 34%.
Why do closets get condensation and mold?: When heated room air enters a closet that is cold because it sits against an exterior wall, the amount of water vapor stays the same while the temperature drops, so the humidity rises. Air at 20°C and 50% humidity entering a 12°C closet reaches about 83% humidity, and at 60% it reaches 100% and condenses. Raise the bedding off the wall with a slatted rack, leave the sliding doors slightly open, and lower the room humidity.
What is the difference between absolute and relative humidity?: Absolute humidity is the amount of water vapor in 1 m³ of air (g/m³); relative humidity is that amount as a percentage (%) of the maximum the air can hold at that temperature (the saturated vapor content). The saturated vapor content at 20°C is about 17.3 g/m³, so the absolute humidity at 50% is about 8.6 g/m³. When the same air gets colder, its saturated vapor content falls, so relative humidity rises.
Why do rooms get dry in winter?: Cold outdoor air holds little water vapor, so when it is warmed, its relative humidity drops sharply. Air at 5°C and 70% humidity warmed to 20°C is about 26% humidity, and air at 0°C and 60% becomes about 16%. Each time you ventilate, dry air comes in, so humidifying becomes necessary.
How many mL/h of humidifier output does an 8-tatami room need?: To keep 20°C and 50% humidity with an outdoor temperature of 5°C, 60% humidity and a 2.4 m ceiling, you need about 71 mL/h in an airtight house (0.5 air changes per hour), about 141 mL/h in an ordinary detached house (1 per hour) and about 212 mL/h in a drafty wooden house (1.5 per hour). Considering moisture absorbed by walls and furniture, a humidifier with about 1.5 times that capacity is a safe choice.

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SOLO GUILD | サービス作るAIエンジニア

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