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Dew Point Calculator
Enter the air temperature and relative humidity to get the dew point — the temperature at which the air becomes saturated and water starts to condense. You also get a plain-English read on how humid it feels, the absolute humidity, and, if you enter the temperature of a window, pipe or wall, whether it will sweat.
- Dew point in Celsius
- 12.8 °C
- How it feels
- Slightly humid
- Condensation on the surface
- —
- Air temperature above the dew point
- 19.9 °
- Absolute humidity
- 10.8 g/m³
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Formula
How to use it
- Enter the air temperature and choose °F or °C.
- Enter the relative humidity from a hygrometer, thermostat or weather report.
- Read the dew point and how it feels.
- Optionally enter a surface temperature to see if condensation will form on it.
Worked examples
75 °F at 50% humidity: γ = ln(0.5) + 17.625 × 23.89 ÷ (243.04 + 23.89) = 0.884, dew point 243.04 × 0.884 ÷ (17.625 − 0.884) = 12.8 °C
- Dew point
- 55.1 °F
- Dew point in Celsius
- 12.8 °C
- How it feels
- Slightly humid
- Air temperature above the dew point
- 19.9 °
- Absolute humidity
- 10.8 g/m³
A 70 °F room at 50% humidity with a 50 °F window pane: the glass is just below the 50.5 °F dew point, so it fogs
- Dew point
- 50.5 °F
- Condensation on the surface
- Yes — 0.5 °F below the dew point
20 °C at 60% humidity
- Dew point in Celsius
- 12 °C
- Dew point
- 53.6 °F
- How it feels
- Comfortable
- Air temperature above the dew point
- 8 °
A tropical 30 °C at 70% humidity, and a 26 °C wall
- Dew point in Celsius
- 23.9 °C
- Dew point
- 75.1 °F
- How it feels
- Miserable
- Condensation on the surface
- No — 2.1 °C above the dew point
- Absolute humidity
- 21.2 g/m³
Why dew point beats relative humidity for comfort
Relative humidity depends on temperature: the same air reads 90% humid at dawn and 45% in the afternoon without any moisture being added. The dew point stays put, so it tells you how much water is actually in the air and how sticky it will feel.
A rule of thumb many forecasters use: below 55 °F feels dry and comfortable, 55–65 °F starts to feel sticky, 65–70 °F is muggy, and above 70 °F is oppressive. A 75 °F dew point is tropical, and sweat barely evaporates.
Condensation, mold and windows
Any surface colder than the dew point collects water: single-pane windows in winter, cold-water pipes, uninsulated basement walls and air-conditioning ducts in summer. Raising the surface temperature with insulation, or lowering the indoor dew point with ventilation or a dehumidifier, stops it.
The US EPA recommends keeping indoor relative humidity below 60%, ideally between 30% and 50%, to limit mold and dust mites. At 70 °F, 50% humidity is a dew point of about 50.5 °F, so a window pane colder than that will fog.
About the formula
This uses the Magnus formula with constants published by Alduchov and Eskridge (1996), a standard approximation for weather temperatures from about −40 to 50 °C. It assumes normal air pressure over liquid water; for frost points well below freezing, or industrial compressed-air systems, use a specialised psychrometric tool.
Questions people ask
What is the dew point at 75 °F and 50% humidity?
About 55.1 °F (12.8 °C) — pleasant, only slightly humid.
What is a comfortable dew point?
For most people, 50–60 °F (10–16 °C). Below about 40 °F indoors the air feels dry; above 65 °F it feels muggy.
Can the dew point be higher than the temperature?
No. At 100% relative humidity the dew point equals the air temperature; below that, it is always lower. If air cools to its dew point, fog, dew or condensation forms.
What humidity will make my windows fog?
Whenever the glass is colder than the room’s dew point. In a 70 °F room, 50% humidity fogs glass below about 50.5 °F; dropping to 35% humidity lowers that to about 41 °F.