Moist-air properties — humidity ratio, relative humidity, wet-bulb, dew point, enthalpy, and specific volume — from dry-bulb temperature and one humidity input.
W = 0.621945·Pw/(P − Pw); h = 1.006·T + W·(2501 + 1.86·T)
How it works
From the dry-bulb temperature and one humidity measure (relative humidity, wet-bulb, or dew point), the tool finds the water-vapour partial pressure, then the humidity ratio and every other moist-air property using the ASHRAE ideal-gas psychrometric relations.
Worked example
Air at 25 °C dry-bulb and 50 % relative humidity at sea level. ≈ 9.9 g of water per kg of dry air, a 17.9 °C wet-bulb, a 13.9 °C dew point, and about 50.4 kJ/kg of enthalpy.
Inputs
Humidity input
Dry-bulb temperature (°C)
Relative humidity (if selected) (%)
Wet-bulb temp (if selected) (°C)
Dew-point temp (if selected) (°C)
Barometric pressure (kPa) — Drops with altitude (≈ 89.9 kPa at 1000 m).
Frequently asked questions
What is the humidity ratio?
The mass of water vapour per unit mass of dry air (kg/kg or g/kg). Unlike relative humidity it doesn't change when you heat or cool the air without adding or removing moisture, which makes it the natural variable for HVAC and drying calculations.
What is the difference between wet-bulb and dew point?
The dew point is the temperature at which the air becomes saturated on cooling (condensation starts); the wet-bulb is the temperature reached by evaporative cooling. They are equal only at saturation — otherwise the wet-bulb sits between the dew point and the dry-bulb.
Why does pressure (altitude) matter?
Humidity ratio and specific volume depend on the total pressure, which falls with altitude — about 89.9 kPa at 1000 m versus 101.3 kPa at sea level. Using sea-level pressure at altitude introduces a real error.
Assumptions
Air–water vapour mixture treated as ideal gases (ASHRAE Handbook of Fundamentals).
Saturation pressure over liquid water (Hyland–Wexler), valid 0–100 °C.
Humidity ratio and enthalpy are per kilogram of dry air, the psychrometric convention.
Hyland, R.W. & Wexler, A. (1983), ASHRAE Transactions 89(2A) — saturation pressure of water vapour.
Related Thermodynamics tools
Vapor Pressure (Antoine) — Saturation pressure of a pure component from Antoine coefficients. Pick a preset or enter your own A, B, C.
Ideal Gas Law — Moles, mass, and density of an ideal gas from P, V, and T.
Gas Compressor Power — Single-stage adiabatic compression power, head, and discharge temperature for an ideal gas.
Steam Tables (IAPWS-IF97) — Saturated and single-phase water/steam properties — saturation pressure/temperature, enthalpy, entropy, and specific volume — from the IAPWS-IF97 standard.