Dew Point Calculator
Dew point from air temperature and relative humidity (Magnus formula)
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Dew Point Calculator is built for dew point from air temperature and relative humidity (Magnus formula) — fast, free, and private. Enter Air Temperature (°C) and Relative Humidity (%) in any order — Dew Point recalculates on every edit. Dew Point puts the headline number first. Students and professionals reach for Dew Point when speed matters. Nothing is uploaded while Dew Point works, and the figures stay put. Dew Point sits in the Science section next to related tools and dew point calculator humidity temperature magnus formula condensation. The labels in Dew Point match the terms in its result. Dew Point stays bookmarked for next time.
How is dew point calculated?
Dew point is the temperature the air must cool to before water vapour begins to condense. It comes from two inputs — air temperature and relative humidity — run through the Magnus formula, not from a lookup table.
That is a muggy reading. A dew point above about 21 °C means the air holds so much moisture that sweat evaporates slowly, and any surface cooler than 23.9 °C will collect condensation.
How dew point is calculated (Magnus formula)
α = ln(RH ÷ 100) + (17.27 × T) ÷ (237.7 + T), then Td = (237.7 × α) ÷ (17.27 − α).
At 30 °C and 70% relative humidity: α = ln 0.7 + 518.1 ÷ 267.7 = −0.3567 + 1.9354 = 1.5787, so Td = 375.3 ÷ 15.691 = 23.9 °C.
| RH at 30 °C | Dew point |
|---|---|
| 40% | 14.9 °C |
| 60% | 21.4 °C |
| 70% | 23.9 °C |
| 80% | 26.2 °C |
| 90% | 28.2 °C |
At 100% RH the dew point equals the air temperature, which is why fog forms when the two meet. Dew point is an absolute measure of moisture: the same 23.9 °C dew point feels muggy at 30 °C but is unchanged if the air warms to 35 °C, while relative humidity falls.
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The two steps of the Magnus formula
α = ln(RH ÷ 100) + (17.27 × T) ÷ (237.7 + T).
Dew point = (237.7 × α) ÷ (17.27 − α).
At 30 °C and 70%: α = ln(0.7) + (17.27 × 30) ÷ 267.7 = −0.357 + 1.935 = 1.579.
Dew point = (237.7 × 1.579) ÷ (17.27 − 1.579) = 375.3 ÷ 15.69 = 23.9 °C.
Why it is a better comfort measure than humidity
Relative humidity changes with temperature even when the amount of water in the air does not, so 70% at 30 °C and 70% at 18 °C describe very different amounts of moisture. The dew point is an absolute measure: it is the temperature to which air must cool before it is saturated.
Rule of thumb: below 13 °C is dry and comfortable, 16–21 °C is comfortable to slightly sticky, above 21 °C is muggy, and above 24 °C is oppressive for most people.
Where the number is useful
- Condensation on windows and walls. If a surface is colder than the dew point, moisture forms on it — the mechanism behind window streaming and mould in cold corners.
- Drying and painting. Paint and plaster need surface temperatures above the dew point, or the finish blooms and never cures properly.
- Air conditioning. A coil has to be below the dew point to dehumidify, which is why the machine produces water as well as cold air.
- Camera and lens care. Bringing cold gear into a warm, humid room condenses on the glass; the dew point tells you how warm the room can be before that happens.
Limits
The Magnus approximation is accurate to a few tenths of a degree over normal indoor and outdoor ranges, and less so above about 50 °C or below freezing, where ice-phase behaviour complicates it. The result describes a parcel of air, not a surface — for whether your wall will be wet, compare the dew point with the wall’s temperature, not the room’s.
Worked example
Everything below is the page’s own arithmetic: the defaults above, run through the same formula the calculator uses, so the figures can be reproduced by hand.
- Air Temperature (°C): 30
- Relative Humidity (%): 70
The steps it follows:
- alpha = ln(RH/100) + (17.27×T)/(237.7+T) = 1.579
- Dew point = (237.7×alpha)/(17.27−alpha)
The result is 23.9 °C. The same run also reports: Magnus formula | Humidex comfort: below 13 °C dry, 16–21 °C comfortable, above 21 °C muggy. Change one input on the page and every number above recalculates from scratch — nothing is stored, so a second run with the same values returns the same figures.
Frequently Asked Questions
Why is the dew point 23.9 °C at 30 °C and 70% humidity?
The Magnus formula combines the logarithm of the humidity ratio with a temperature term. That gives α = 1.579, and α goes back into the second expression to produce 23.9 °C.
Is a 24 °C dew point uncomfortable?
Yes for most people. Above about 21 °C the air is muggy, and above 24 °C it is oppressive — sweat evaporates slowly, so the body struggles to cool itself even at moderate temperatures.
Why does my window fog when the room feels fine?
Because the glass is colder than the dew point. Condensation depends on surface temperature, not air temperature, which is why the coldest surface in the room — usually single glazing — collects the water.
Does dew point change if the temperature changes?
Not if the amount of water in the air stays the same. Warm the air and relative humidity falls while the dew point holds; that is why heating a room reduces condensation risk.
At what dew point does mould become a risk?
There is no single threshold, but sustained relative humidity above about 70% at wall surfaces supports mould growth. Keeping surfaces warmer than the dew point — through insulation, ventilation or heating — is what prevents it.
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