Dew Point Calculator
Find dew point from temperature and relative humidity, plus comfort level.
About Dew Point Calculator
The dew point is the temperature to which air must be cooled (at constant pressure and humidity) for water vapour to condense into liquid water. It is a direct measure of the absolute moisture content of the air — unlike relative humidity, which changes with temperature even when the actual moisture content stays the same. The Magnus-Tetens formula gives a fast and accurate approximation using only temperature and relative humidity. Dew points above 18°C feel muggy to most people; above 21°C is oppressive; above 24°C is dangerous. A dew point near the air temperature (small temperature–dew point spread) signals near-saturation and fog risk.
Formula
Magnus-Tetens dew point
γ = ln(RH/100) + (a × T) / (b + T); dew point = b × γ / (a − γ)
- T
- Air temperature in °C
- RH
- Relative humidity as a percentage
- a
- Magnus constant 17.625
- b
- Magnus constant 243.04 °C
- Td
- Dew point temperature in °C
- γ
- Intermediate Magnus-Tetens term
Worked example
With the default values below, Dew Point Calculator returns Dew Point (°C) of 18.4. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Unit Systemone of Celsius (°C), Fahrenheit (°F)
- Celsius (°C)
- Air Temperature (°C)accepts -40–60
- 30
- Air Temperature (°F)accepts -40–140
- 86
- Relative Humidity (%)accepts 1–100 %
- 50 %
Results
- Dew Point (°C)Temperature at which the air becomes saturated and condensation forms.
- 18.4
- Dew Point (°F)
- 65.1
- Temperature–Dew Point Spread (°C)A spread below 2.5°C indicates fog or heavy condensation risk.
- 11.6
- Comfort Level
- Very humid — uncomfortable, oppressive.
Step by step
- 1Inputs= T = 30°C (86°F), RH = 50%
- 2Compute γ = ln(RH/100) + a·T/(b+T)= 1.243382
- 3Dew point = b·γ / (a − γ)= 18.4°C (65.1°F)
- 4Temperature–dew point spread= 11.6°C (< 2.5°C indicates near-saturation / fog risk)
- 5Comfort level= Very humid — uncomfortable, oppressive.
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
Why is dew point a better measure of humidity comfort than relative humidity?
Relative humidity depends on temperature: the same amount of water vapour gives 100% RH at the dew point but only 40% RH at a much warmer temperature. On a hot day, 50% RH can feel very muggy, while 50% RH on a cold day is comfortable. Dew point is invariant to temperature and directly represents how much moisture is in the air, making it a more stable comfort metric.
At what dew point does it start to feel uncomfortable?
Dew points below 10°C feel dry and comfortable. From 13–16°C is noticeably humid. Above 18°C most people begin to feel oppressed; above 21°C is genuinely uncomfortable; above 24°C is considered dangerously tropical. The exact threshold varies with acclimatisation.
What does a small temperature–dew point spread mean?
A spread less than 2.5°C (4.5°F) means the air is very close to saturation. This is the condition for fog formation, heavy morning dew, and condensation on cool surfaces. A spread of 0 means the air is exactly at saturation (relative humidity = 100%).
Can the dew point ever exceed the air temperature?
No — by definition the dew point cannot be higher than the air temperature. If you observe the dew point ≥ temperature in a weather report, it means relative humidity ≈ 100% and the sensor has a small calibration error.