Resolves the full moist-air state from the dry-bulb temperature plus one second property: relative humidity, wet-bulb or dew-point. Returns humidity ratio, enthalpy, vapour pressure and degree of saturation.
Tender-stage indicative. Verify against ASHRAE / CIBSE psychrometric data for design.
For design guidance only. Always verify with a qualified engineer.
About this psychrometric calculator
This free psychrometric calculator resolves the complete state of moist air from the dry-bulb temperature plus one second property: relative humidity, wet-bulb temperature or dew-point. From that pair it returns the humidity ratio (in g/kg and kg/kg), specific enthalpy, partial and saturation vapour pressures, the degree of saturation and the remaining temperatures. It is aimed at UK building-services and HVAC engineers sizing cooling coils, sanity-checking air-handling conditions, or reading off a single point that would otherwise need a psychrometric chart. Treat the figures as tender-stage indicative and confirm critical design against ASHRAE or CIBSE data.
How the moist-air state is calculated
The method follows the ASHRAE Handbook — Fundamentals (2021), Chapter 1 (Psychrometrics) and CIBSE Guide C. Saturation vapour pressure uses the Alduchov–Eskridge (1996) improved Magnus form Pws = 610.94 × exp(17.625·T ÷ (T + 243.04)). The second property fixes the partial vapour pressure Pw: relative humidity gives Pw = (RH÷100) × Pws, dew-point gives Pw = Pws(Td), and wet-bulb uses the ASHRAE psychrometer energy balance. The humidity ratio is then W = 0.621945 × Pw ÷ (P − Pw), the specific enthalpy h = 1.006·T + W·(2501 + 1.86·T), and the degree of saturation μ = W ÷ Ws. Wet-bulb, where it is an output, is found by bisection on the same balance. The calculator returns identical numbers to the Ensign Toolbox engine.
Frequently asked questions
What inputs does it need?
Always the dry-bulb (air) temperature, plus exactly one of relative humidity, wet-bulb temperature or dew-point. You can also set the atmospheric pressure, the sea-level standard is 101325 Pa, lower it for altitude.
What is the difference between humidity ratio and relative humidity?
Relative humidity is the partial vapour pressure as a percentage of the saturation pressure at that temperature. Humidity ratio is the actual mass of water vapour per kilogram of dry air (g/kg or kg/kg). The quantity you need for coil loads and mixing calculations.
How accurate is it?
Saturation pressure is within about ±0.3% of Hyland–Wexler across the HVAC range, so the derived properties are well inside chart-reading accuracy. It is intended for indicative sizing. Verify safety-critical or contractual figures against full ASHRAE or CIBSE psychrometric data.