Dew Point Calculator
Enter the current air temperature and relative humidity. The calculator returns the dew point — the temperature at which the air would have to cool for water to start condensing — plus the temperature–dew-point spread and a perceived-comfort band.
Dew point
9.3 °C
- Spread (air − dew point)
- 10.7 °C
- Dew point in other unit
- 48.7 °F
- Air temperature
- 20.0 °C
- Relative humidity
- 50%
- Comfort band
- Very Dry — Very dry. Static, dry skin, and dry sinuses likely; comfortable to most.
The Magnus formula (Alduchov & Eskridge, 1996) gives Td from air temperature and relative humidity: α = ln(RH/100) + (17.625·T)/(243.04 + T); Td = (243.04·α)/(17.625 − α), with T and Td in °C. It is accurate to ±0.4 °C for 0% < RH ≤ 100% and −40 °C ≤ T ≤ 50 °C.
How to use this calculator
Choose your unit system (°C or °F — humidity is always a percentage). Enter the actual air temperature and the relative humidity. The calculator returns the dew point in both unit systems, the spread between air temperature and dew point, and a perceived-comfort band. The Magnus formula it uses is accurate to about ±0.4 °C for 0% < RH ≤ 100% and −40 °C to 50 °C — the practical range for surface weather, indoor air, brewing, and greenhouse work.
How the calculation works
Air can only hold so much water vapour at a given temperature; cool air can hold less. The dew point is the temperature to which a parcel of air, at constant pressure and water-vapour content, would have to be cooled for saturation to occur — once you cross it, water condenses out as dew, fog, or droplets on a cold drink. The Magnus formula expresses that relationship as α = ln(RH/100) + (a·T)/(b + T) and Td = (b·α)/(a − α), with coefficients a = 17.625 and b = 243.04 °C from Alduchov & Eskridge (1996). T and Td are in degrees Celsius and RH is relative humidity in percent. At 100% relative humidity the formula collapses to Td = T (the air is already saturated, so it would not need to cool at all to condense). As humidity drops, Td falls below T; the gap between them — the "spread" — is a good rule-of-thumb proxy for how dry the air feels.
Worked example
Take an air temperature of 20 °C and 50% relative humidity. α = ln(0.5) + (17.625·20)/(243.04 + 20) = −0.6931 + 1.3401 = 0.6470. Td = (243.04·0.6470)/(17.625 − 0.6470) = 157.25/16.978 ≈ 9.26 °C. So the dew point is about 9 °C, a spread of roughly 11 °C — comfortably dry indoor air. Cool the air below 9 °C without changing its moisture content (for example, against a cold window pane on a winter morning) and the surplus water vapour will condense as dew or fog.
Frequently asked questions
What is dew point in plain English?
The dew point is the temperature your air would have to cool to before it could no longer hold all of its current water vapour — so some of that vapour would condense out as dew, fog, or droplets on a cold glass. Unlike relative humidity, which depends on both moisture content and temperature, dew point is a near-direct measure of how much water is in the air. A dew point of 5 °C is dry no matter what the thermometer reads; a dew point of 22 °C is muggy no matter what the thermometer reads.
Why is dew point a better humidity measure than relative humidity?
Relative humidity is a ratio — how full the air is, as a percentage of its capacity at the current temperature. That capacity changes sharply with temperature: 70% relative humidity at 5 °C is genuinely dry, while 70% at 30 °C is oppressively humid. Dew point, by contrast, is an absolute measure of moisture content and does not change as the air heats or cools (unless the moisture itself changes). Meteorologists, brewers, HVAC engineers, and pilots all default to dew point for this reason.
What dew points feel comfortable?
Most people perceive dew points up to about 13 °C (55 °F) as comfortable, 13–16 °C (55–60 °F) as slightly humid, 16–18 °C (60–65 °F) as sticky, 18–21 °C (65–70 °F) as oppressive, and above 21 °C (70 °F) as tropical and uncomfortable for sustained exertion. These are perception bands for healthy adults at typical activity levels — outdoor labour in direct sun shifts each band one step warmer.
Can the dew point be higher than the air temperature?
No — physically, the dew point is bounded above by the air temperature, because that is the definition: it is the temperature to which the current air would have to cool for saturation. If a measurement instrument reports Td > T, the air is supersaturated (which is unstable and rare at the surface) or the instrument needs calibration. The formula returns Td = T exactly when RH = 100%.
How is dew point different from wet-bulb temperature?
Both are humidity measures, but they answer different physical questions. Dew point asks: if I cool this air at constant moisture content, when does it saturate? Wet-bulb temperature asks: if I evaporate water into this air at constant pressure, what temperature can the cooling reach? Wet-bulb is always between the dew point and the air temperature, and is what controls evaporative cooling, sweat efficiency, and outdoor heat-stress safety (the WBGT index used by OSHA and athletic associations).
How accurate is the Magnus formula used here?
The Alduchov & Eskridge (1996) coefficients used in this calculator are accurate to within ±0.4 °C for 0% < RH ≤ 100% and −40 °C to 50 °C — the range that covers essentially all surface weather, indoor air, brewery and greenhouse conditions, and HVAC duty. Outside that band the formula is an extrapolation; for cryogenic or supercritical applications you would use a Wagner-type equation of state instead.
Why does my breath fog up in cold weather?
Your exhaled breath is roughly 35 °C and nearly 100% relative humidity. When that warm, humid air mixes with cold ambient air, the mixture briefly sits below its own dew point — so the surplus water vapour condenses into a visible cloud of tiny droplets. The colder the surrounding air, the further below the dew point the mixture falls, and the more dramatic the fog.