Dew Point Calculator
Calculate dew point from temperature and humidity using the Magnus formula, with what each dew point actually feels like.
How to use this calculator
- 1Enter air temperature and relative humidity — both are on any weather report.
- 2Use the dew point, not the humidity percentage, to judge how the air will actually feel.
- 3A spread under about 3 °C means fog or dew is likely overnight.
How the calculation works
γ = ln(RH/100) + (a·T)/(b + T)
Td = (b·γ)/(a − γ) with a = 17.62, b = 243.12- Td
- Dew point in °C
- T
- Air temperature in °C
- RH
- Relative humidity as a percentage
- a, b
- Magnus coefficients, Sonntag 1990 values
The Magnus-Tetens approximation is accurate to about ±0.35 °C between −45 °C and +60 °C, which comfortably covers meteorological use.
Relative humidity is the ratio of actual vapour pressure to saturation vapour pressure, which is why dew point and relative humidity are two views of the same underlying quantity.
Dew point can never exceed the air temperature. At 100% humidity they are equal.
Worked example
25 °C at 65% humidity
- 1.γ = ln(0.65) + (17.62 × 25) / (243.12 + 25) = −0.4308 + 1.6428 = 1.2120.
- 2.Td = (243.12 × 1.2120) / (17.62 − 1.2120) = 294.66 / 16.408 = 17.96 °C.
- 3.A dew point of about 18 °C is on the boundary of "slightly humid" — noticeable but not oppressive.
- 4.The 7 °C spread means condensation is unlikely unless it cools substantially overnight.
Result: About 18.0 °C — slightly humid
What dew point actually is
Dew point is the temperature air would have to cool to, at its current moisture content and pressure, before it becomes fully saturated and water starts condensing out of it — as dew on grass, fog in the air, or droplets on a cold glass. It is a direct measure of how much water vapour the air is actually carrying, unlike relative humidity, which measures that same moisture only as a percentage of what the air could hold at its current temperature.
Because it is tied to absolute moisture content rather than a temperature-dependent ratio, dew point can never exceed the actual air temperature — the two become equal only at the point of saturation, which is exactly the condition that produces dew, frost or fog in the first place.
Why dew point is the more useful number, day to day
Relative humidity moves for two completely different reasons that its own number cannot distinguish between: the air can gain or lose moisture, or it can simply warm up or cool down while carrying exactly the same amount of water it started with. A reading of 90% humidity at dawn and 40% by mid-afternoon can describe air that never gained or lost a single gram of moisture — only warmed up, which on its own is enough to push the percentage down sharply.
Dew point does not have that blind spot. It stays essentially flat through a normal day’s heating and cooling because it tracks the water content of the air directly rather than a ratio, which is why meteorologists treat it as the more reliable comfort figure — a dew point in the high teens Celsius reads as noticeably humid whether it is measured at a cool morning or a hot afternoon, while the relative-humidity percentage for that same air would tell two very different stories.
From an 1844 laboratory formula to a same-day forecasting shortcut
The relationship between temperature, vapour pressure and saturation was first worked out experimentally by the German physicist Gustav Magnus in 1844, and refined by later researchers — notably Otto Tetens in 1930 — into the compact exponential approximation still known as the Magnus, or Magnus–Tetens, formula. This calculator uses the coefficient set published by Dietrich Sonntag in 1990, a refinement accurate to a few tenths of a degree across the temperature range most weather applications need, without requiring the far more demanding exact thermodynamic equations meteorologists reserve for research work.
Where dew point gets used in practice
Beyond a line on a weather report, dew point is a working number in several fields that depend on knowing exactly when moisture will condense.
- Fog and frost forecasting — overnight temperature has to fall only as far as the dew point for fog to form, or below freezing at a high dew point for frost — which is why forecasters watch the gap between the two, not just the low temperature alone.
- Aviation — pilots use the spread between air temperature and dew point to estimate cloud base height before takeoff — a rule of thumb taught in flight training multiplies the Celsius spread by about 400 to estimate the cloud base in feet, since temperature and dew point converge at a fairly predictable rate with altitude.
- Agriculture — growers watch dew point to time irrigation and anticipate frost risk on clear, calm nights, when temperatures near the ground can fall well below the forecast low.
- Building design and condensation control — any surface cooler than the dew point of the air touching it — a cold window, an underinsulated wall cavity, an air-conditioning duct — will collect condensation, which over time is a common cause of mould if it is not accounted for in design.
What this assumes, and where it stops
Assumptions
- Standard atmospheric pressure at sea level.
- Air is well mixed and measurements are taken in shade.
Limitations
- The Magnus approximation carries about ±0.35 °C of error across normal meteorological ranges, and more at extremes.
- Absolute humidity is calculated with a standard approximation and assumes sea-level pressure.
- Says nothing about heat stress on its own — pair it with the heat index for that.
Common questions
Why is dew point better than relative humidity?
Because relative humidity changes as the air warms and cools even when the actual water content is unchanged. The same air can read 90% at dawn and 50% by midday. Dew point is an absolute measure of moisture content, so it tells you directly how the air will feel — above 18 °C is muggy regardless of what the humidity percentage says.
What causes fog?
Air cooling to its dew point. When the temperature and dew point converge — a spread under about 2–3 °C — water vapour condenses into suspended droplets. This is why fog forms overnight after a humid day, and why it usually burns off as the morning sun raises the temperature back above the dew point.
Sources
- Dew point and humidity — US National Weather Service
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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