About the Dew Point Calculator
The dew point is the temperature air must cool to before it becomes saturated and water starts to condense as dew, fog or condensation on a cold glass. This dew point calculator works it out from the air temperature and relative humidity using the Magnus formula, and it can also run the other way, finding relative humidity from a temperature and dew point.
Meteorologists prefer dew point to relative humidity for describing how humid it feels, because relative humidity changes through the day as temperature rises and falls while dew point stays fairly steady. A dew point in the 50s °F feels pleasant; above 65 °F most people find it muggy. The calculator adds a comfort rating so you can read the number at a glance.
It is also practical indoors: if any surface — a window, a cold-water pipe, a basement wall — is colder than the dew point, moisture will condense on it, which can lead to mould. Builders, HVAC technicians, painters and coating applicators (who often need surfaces at least 5 °F / 3 °C above dew point) use the same calculation. The formula is accurate to within about 0.4 °C for temperatures between −40 °C and 50 °C.
How to use the dew point calculator
- 1Choose whether to find the dew point or the relative humidity.
- 2Pick °F or °C and enter the air temperature.
- 3Enter the relative humidity, or the dew point if you are finding humidity.
- 4Read the dew point in both units, the comfort rating and the vapour pressure.
Formula and method
The Magnus formula approximates the saturation vapour pressure of water as e_s = 6.1094 × e^(aT/(b + T)) hPa. Relative humidity is the actual vapour pressure divided by e_s, so the dew point is the temperature at which the actual vapour pressure would equal the saturation value. Taking logs and solving gives the closed-form expression above.
The coefficients a = 17.625 and b = 243.04 °C come from Alduchov and Eskridge (1996) and keep errors under about 0.4 °C from −40 °C to 50 °C over liquid water. Temperatures in °F are converted to °C first. Below freezing the true frost point is slightly higher than the dew point shown, because ice has a lower vapour pressure than supercooled water.
- T
- Air temperature (°C)
- RH
- Relative humidity (%)
- T_d
- Dew point (°C)
- a, b
- Magnus coefficients (17.625 and 243.04 °C)
Worked examples
Summer afternoon: 77 °F and 60% humidity
77 °F is 25 °C. γ = ln(0.60) + 17.625 × 25 ÷ 268.04 = 1.1330, so T_d = 243.04 × 1.1330 ÷ (17.625 − 1.1330) = 16.7 °C, or about 62 °F — noticeably sticky.
Indoor air: 20 °C and 50% humidity
A comfortable room at 20 °C and 50% RH has a dew point of about 9.3 °C. A single-glazed window colder than that on a winter night will fog up.
Humidity from dew point: 90 °F with a 75 °F dew point
Converting to 32.2 °C and 23.9 °C and applying the reverse Magnus formula gives a relative humidity of about 61.5%. A 75 °F dew point is tropical air that makes outdoor exercise risky.
Frequently asked questions
What is a comfortable dew point?+
Most people find dew points of 55 °F (13 °C) or lower comfortable. Between 55 and 65 °F it starts to feel sticky, 65–70 °F is muggy, and above 70 °F (21 °C) the air feels oppressive because sweat evaporates slowly.
What is the difference between dew point and relative humidity?+
Dew point is an absolute measure of how much moisture is in the air. Relative humidity is how close the air is to saturation at its current temperature, so it rises at night and falls in the afternoon even when the moisture content stays the same.
Can the dew point be higher than the temperature?+
No. When the air cools to its dew point it is saturated (100% relative humidity), and any further cooling makes water condense out as dew, fog or cloud. The dew point can equal the temperature but not exceed it.
How do I prevent condensation on windows or walls?+
Keep surfaces warmer than the indoor dew point, or lower the dew point by ventilating, running a dehumidifier or reducing indoor humidity sources. Better insulation and double glazing raise inside surface temperatures.
Is there a quick rule of thumb for dew point?+
When relative humidity is above about 50%, the dew point in °C is roughly the temperature minus (100 − RH) ÷ 5. For 25 °C and 60% RH that gives 17 °C, close to the precise 16.7 °C.