What the Dew Point Calculator does
This calculator works out the dew point from the air temperature and relative humidity - the temperature at which moisture in the air starts to condense - and tells you whether a cold surface such as a window, an outside wall or a water pipe will get wet. It also works backwards, giving the humidity from a dew point, or the air temperature from both.
It uses the Magnus formula with the coefficients published by Alduchov and Eskridge in 1996, accurate to a fraction of a degree across everyday conditions, and shows how comfortable the air will feel using the US National Weather Service dew point bands. Celsius and Fahrenheit both work.
How to use it
- Choose what to calculate - dew point, humidity or air temperature - and the units.
- Enter the air temperature and the relative humidity from a thermometer and hygrometer, a weather report or a smart thermostat.
- To check for condensation, enter the temperature of the surface you are worried about: the inside of a window, a cold corner of a wall, a cold-water pipe or the inside of a tent. An infrared thermometer gives the best reading.
- Read the dew point in the big panel, the comfort band and the amount of water in the air, then the surface verdict: condensation, high risk, mould risk or low risk. The table shows how the dew point moves with humidity at the same temperature.
Reading the results
The dew point is a direct measure of how much water vapour the air holds, which relative humidity is not. Relative humidity depends on temperature: the same air that is 50% at 20 C becomes about 100% if it cools to 9.3 C. That is why cold surfaces in a warm room get wet.
A surface at or below the dew point collects condensation. Within about 3 C of it there is a high risk, because surface temperatures vary from place to place and over the night. Above that, the air touching the surface can still be humid enough for mould: the surface-humidity criterion used for building design is 80%, sustained over time.
For comfort in warm weather, look at the dew point rather than the humidity. A dew point up to 55 F (12.8 C) feels dry and comfortable, 55-65 F (12.8-18.3 C) sticky, and above 65 F (18.3 C) oppressive, whatever the temperature.
Worked example: a bedroom window on a winter night
The room is 21 C at 55% relative humidity. gamma = ln(0.55) + 17.625 x 21 / (243.04 + 21) = -0.598 + 1.402 = 0.804, and the dew point is 243.04 x 0.804 / (17.625 - 0.804) = 11.6 C.
The inside of single-glazed glass measures 8 C - 3.6 C below the dew point, so it streams with water. Bringing the room down to 45% humidity lowers the dew point to 8.6 C, still marginal; double glazing that keeps the inner pane at 15 C would stay dry, with the air against it at about 80% humidity.
Formulas and scoring rules
- Saturation vapour pressure (Magnus)
e_s(T) = 6.1094 x exp(17.625 x T / (243.04 + T)) [hPa, T in C]Coefficients a = 17.625, b = 243.04 C, C = 6.1094 hPa from Alduchov and Eskridge (1996), stated for -40 to 50 C over liquid water.- Dew point
gamma = ln(RH / 100) + a T / (b + T); Td = b x gamma / (a - gamma)- Relative humidity from dew point
RH = 100 x exp(a Td / (b + Td) - a T / (b + T))The same formula gives the humidity of the air right against a cold surface, using the surface temperature for T.- Air temperature from dew point and humidity
beta = a Td / (b + Td) - ln(RH / 100); T = b x beta / (a - beta)- Absolute humidity
rho_v = 216.7 x e / (T + 273.15) [g/m3, e in hPa]Ideal gas law for water vapour, R_v = 461.5 J/(kg K).
Limitations: what the result does not prove
- It is an approximation over liquid water. Below 0 C frost forms at the frost point, which is slightly higher than the dew point over water; outside -40 to 50 C the error grows, and the page flags it.
- Results are only as good as your readings. Cheap hygrometers are often 5% out, which moves the dew point by about a degree.
- A single surface reading is a snapshot. Corners, window frames and the bottom of glazing are usually colder than the middle of a pane, and night-time temperatures are lower than daytime ones.
- It is not a building survey. Persistent damp can come from leaks, rising damp or poor ventilation, which need a qualified person to diagnose.
Privacy: where your data goes
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Standards and sources
- Alduchov O.A. and Eskridge R.E. (1996) Improved Magnus form approximation of saturation vapor pressure, Journal of Applied Meteorology 35, 601-609
- US National Weather Service - Dew point vs humidity
- ISO 13788 - Hygrothermal performance of building components: internal surface temperature to avoid critical surface humidity
Frequently asked questions
How do you calculate the dew point from temperature and humidity?
Use the Magnus formula: gamma = ln(RH / 100) + 17.625 T / (243.04 + T), then dew point = 243.04 x gamma / (17.625 - gamma), with T in degrees Celsius. At 20 C and 50% humidity it gives 9.3 C.
What is a comfortable dew point?
In summer, a dew point of 55 F (12.8 C) or lower feels dry and comfortable; 55-65 F (12.8-18.3 C) feels sticky, with muggy evenings; and 65 F (18.3 C) or more feels oppressive. Indoors in winter a low dew point means dry air rather than discomfort.
What is the difference between dew point and relative humidity?
The dew point measures how much water vapour is in the air. Relative humidity says how close that air is to saturation at its current temperature, so it rises when the air cools even though the water content is unchanged.
Why do my windows get condensation on the inside?
Because the glass is colder than the dew point of the room air. Enter the room temperature, humidity and the glass temperature to see by how much; ventilating, lowering humidity or warmer glazing all fix it.
Can mould grow without visible condensation?
Yes. Building standards treat a surface relative humidity of 80% sustained over time as enough for mould. A cold wall a few degrees above the dew point can stay that damp without ever looking wet, and the calculator flags that case separately.
What dew point causes condensation on cold water pipes?
Any pipe colder than the dew point of the surrounding air sweats. A cold-water pipe at 15 C in a 24 C room at 65% humidity, where the dew point is 17 C, will drip; insulating the pipe keeps its outer surface above the dew point.
Does the calculator work in Fahrenheit?
Yes. Choose degrees F and every input and result switches; numbers already typed are converted for you. The formula runs in Celsius internally, so the answers are identical either way.
Last reviewed by the A2Z.Tools team against the sources listed above.
