Dew Point & Humidity Calculator
Enter air temperature and one more property — relative humidity, wet-bulb or dew point — and get the full psychrometric state of the air.
Method reviewed October 22, 2026. Version 1.0.0. How it works
Result
- Dew point
- 55.1°F
- Relative humidity
- 50%
- Wet-bulb (thermodynamic)
- 62.6°F
At 75 °F and 50 % RH the dew point is 55.1 °F — surfaces colder than that will collect condensation.
- Humidity ratio
- 64.6gr/lb
- Enthalpy
- 28.11BTU/lb
- Specific volume
- 13.68ft³/lb
- Barometric pressure used
- 14.7psia
Show the working
- Barometric pressure at elevation (standard atmosphere)p = 101.325 × (1 − 2.25577×10⁻⁵ × z)^5.2559 = 101.3 kPa
- Humidity ratio W = 0.621945 · pw / (p − pw)0.00924 kg/kg = 64.6 gr/lb
- Dew point: temperature where saturation pressure equals pw (Hyland–Wexler)55.12 °F
- Enthalpy h = 0.240·t + W(1061 + 0.444·t)28.11 BTU/lb
Why dew point matters in HVAC
Dew point is the temperature at which air becomes saturated and water starts to condense. Any surface colder than the dew point — a supply duct in an attic, a window, a cold-water pipe — will collect condensation. Unlike relative humidity, dew point does not change when air warms or cools without gaining or losing moisture, so it is the better measure of how much water the air holds.
Method
Calculations follow the psychrometric equations in the ASHRAE Handbook—Fundamentals: Hyland–Wexler saturation pressure over water and ice, humidity ratio W = 0.621945 × pw ÷ (p − pw), and the thermodynamic wet-bulb relation, solved numerically. Barometric pressure comes from the standard atmosphere at your elevation, which matters for wet-bulb, humidity ratio and enthalpy.
Results are checked automatically against PsychroLib, an independent open-source implementation of the same ASHRAE equations.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Dry-bulb (air) temperature
- 75 °F
- Second known property
- Relative humidity
- Relative humidity
- 50 %
- Site elevation
- 0 ft
Result
- Dew point
- 55.1 °F
- Relative humidity
- 50 %
- Wet-bulb (thermodynamic)
- 62.6 °F
- Humidity ratio
- 64.6 gr/lb
- Enthalpy
- 28.11 BTU/lb
- Specific volume
- 13.68 ft³/lb
At 75 °F and 50 % RH the dew point is 55.1 °F — surfaces colder than that will collect condensation.
Assumptions
- ASHRAE Handbook—Fundamentals psychrometric equations (Hyland–Wexler saturation pressure over water and ice) at the standard-atmosphere pressure for the elevation.
- Wet-bulb is the thermodynamic (adiabatic-saturation) wet-bulb; a well-ventilated psychrometer reads very close to it.
Limitations
- Valid for normal atmospheric air between about −40 °F and 200 °F.
- Sensor accuracy dominates: a ±2 % RH sensor gives roughly ±0.6 °F uncertainty in dew point at room conditions.
Frequently asked questions
What is a comfortable dew point?
Many people find dew points around 50–60 °F comfortable; above about 65 °F air feels muggy. Indoors, keeping relative humidity below about 60 % helps limit mould and dust mites.
What is the difference between wet-bulb and dew point?
Wet-bulb is the temperature air cools to by evaporating water into it; dew point is the temperature at which its existing moisture would begin to condense. Wet-bulb always lies between dew point and dry-bulb, and all three are equal at 100 % relative humidity.
Why does elevation change the result?
At lower barometric pressure the same vapour pressure corresponds to a higher humidity ratio, and the wet-bulb relationship shifts. Dew point from temperature and RH barely changes; wet-bulb, humidity ratio and enthalpy do.
Sources
- PsychroLib — psychrometric functions after ASHRAE Handbook—Fundamentals (2017) ch. 1 — PsychroLib (open source, MIT licence). Humidity ratio W = 0.621945·pw/(p − pw), wet-bulb, enthalpy and standard-atmosphere equations as published by ASHRAE.
- Water vapor pressure formulations — UCAR / NCAR Earth Observing Laboratory. Hyland–Wexler saturation vapour pressure coefficients over water and ice.
- NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.
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