The calculation, without hidden assumptions
Dew point is the temperature to which air must cool, at roughly constant pressure and water-vapor content, for saturation to occur. This calculator uses the Alduchov–Eskridge form of the Magnus approximation with a = 17.625 and b = 243.04 °C. Unlike a one-way lookup, the same relationship can solve dew point from air temperature and relative humidity, relative humidity from air temperature and dew point, or air temperature from dew point and relative humidity.
How to use this calculator
Choose whether the unknown is dew point, relative humidity or air temperature.
Choose Celsius or Fahrenheit, then enter the two known weather values. Relative humidity must be above 0% and no more than 100%.
Read the solved value together with dew-point depression, actual vapor pressure and saturation vapor pressure.
Open the working to verify the normalized values and substituted Magnus arithmetic, then compare the humidity scenario table if you are testing condensation sensitivity.
Where people use it
- •HVAC and building checks where surface temperature may approach the air dew point and condensation becomes possible
- •Weather and agriculture checks comparing humidity conditions at a known air temperature
- •Fog, cloud or condensation screening using the gap between air temperature and dew point
- •Reverse psychrometric checks when air temperature and dew point are known but relative humidity is missing
Worked example — 25 °C and 60% RH
For T = 25 °C and RH = 60%, γ ≈ 1.1928. Substituting that value into the Magnus equation gives a dew point of about 16.7 °C (62.1 °F). The dew-point depression is about 8.3 °C. Raising RH while holding the same air temperature moves the dew point closer to 25 °C.
What the result does not assume
- •The Magnus relationship is an approximation, not a complete moist-air property model. The Alduchov–Eskridge constants used here are commonly cited for ordinary meteorological temperatures, with strongest accuracy roughly across −40 °C to 50 °C.
- •At 100% relative humidity, air temperature and dew point coincide. A dew point above the entered air temperature is physically inconsistent for the same parcel and is rejected in reverse-RH mode.
- •Below freezing, dew point defined over liquid water is not identical to frost point defined over ice. Use an ice-aware psychrometric method when frost deposition is the actual decision.
- •Condensation on a real surface depends on the surface temperature, local air film, pressure and moisture transport. Compare the calculated dew point with measured or modeled surface temperature rather than treating dew point alone as a building diagnosis.
Frequently asked questions
Can this calculator solve relative humidity from dew point?+
Yes. Choose Relative humidity as the unknown, then enter air temperature and dew point. The calculator rearranges the same Magnus relationship and rejects a dew point above the air temperature.
What happens at 100% relative humidity?+
At saturation, dew point equals air temperature. The dew-point depression becomes zero.
Can dew point be higher than air temperature?+
Not for an equilibrium parcel described by these inputs. A higher dew point would imply supersaturation or inconsistent measurements, so the reverse calculation treats that state as invalid.
Is dew point below 0 °C the same as frost point?+
No. Dew point is normally expressed relative to saturation over liquid water; frost point uses saturation over ice. The difference matters below freezing, especially when frost formation is the decision.
Semantic next steps
Continue the calculation
These links move to a different input, formula or project stage rather than a keyword variation of this page.
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