Find every property of moist air from the dry bulb and one more value: relative humidity, wet bulb, dew point, humidity ratio or enthalpy. The calculator uses the ASHRAE Fundamentals equations, corrects for altitude, shows results in SI and IP side by side, and plots the point on a psychrometric chart.
| Property | SI | IP |
|---|
Psychrometrics describes moist air: how much water vapour it holds and how much energy it carries. Two independent properties plus the barometric pressure fix the state of the air completely, and from them every other property follows. Enter the dry bulb and one more value, and the calculator gives relative humidity, humidity ratio, dew point, wet bulb, enthalpy, specific volume, density and vapour pressure.
It uses the equations of the ASHRAE Handbook – Fundamentals, including the Hyland-Wexler saturation pressure over water and over ice, and it solves the wet bulb exactly rather than with an empirical shortcut. Results were checked against PsychroLib, the reference implementation of the ASHRAE equations, and agree within 0.001 °C.
| Property | Meaning | Equation (SI) |
|---|---|---|
| Humidity ratio W | kg of water vapour per kg of dry air | W = 0.621945 pw ÷ (p − pw) |
| Relative humidity | Vapour pressure as a percentage of the saturation pressure | RH = pw ÷ pws(t) × 100 |
| Enthalpy h | Heat content per kg of dry air, from 0 °C | h = 1.006 t + W (2501 + 1.86 t) kJ/kg |
| Specific volume v | Volume of moist air per kg of dry air | v = 0.287042 (t + 273.15)(1 + 1.607858 W) ÷ p |
| Dew point | Temperature at which the vapour starts to condense | pws(td) = pw |
| Wet bulb | Temperature reached by evaporating water into the air | Solved from the ASHRAE psychrometric equation |
Air at 25 °C and 50% RH at sea level (101.325 kPa):
The same 25 °C and 50% RH at 1,500 m altitude (84.6 kPa) holds 11.88 g/kg, has a wet bulb of 17.5 °C and a specific volume of 1.031 m³/kg. Ignoring altitude would put the humidity ratio 17% low, which is why the calculator always asks for the site altitude or pressure.
Both are below the dry bulb unless the air is saturated, but they mean different things. The dew point depends only on the moisture content: cool the air below it and water condenses, which is what happens on a cooling coil or a cold pipe. The wet bulb is the temperature of a wetted thermometer in moving air; it is always between the dew point and the dry bulb, and it sets the performance of cooling towers and evaporative coolers. Use the Dew Point Calculator for condensation checks on surfaces and pipes.
For the processes behind these numbers, read psychrometric processes in HVAC, and for a printable chart download the psychrometric chart PDF.
Find the saturation vapour pressure at the dry bulb, multiply by RH to get the vapour pressure pw, then W = 0.621945 × pw ÷ (p − pw), where p is the barometric pressure. At 25 °C and 50% RH at sea level this gives 9.88 g/kg.
In SI units h = 1.006 t + W (2501 + 1.86 t) kJ per kg of dry air, with t in °C and W in kg/kg. In IP units h = 0.240 t + W (1061 + 0.444 t) Btu/lb with t in °F.
Yes. Lower barometric pressure means the same dry bulb and RH hold more moisture per kg of dry air, the air is less dense and the wet bulb changes. At 1,500 m the humidity ratio at 25 °C and 50% RH is about 17% higher than at sea level.
Dew point is the temperature at which moisture starts to condense and depends only on the moisture content. Wet bulb is the temperature of a wetted thermometer in moving air and always lies between the dew point and the dry bulb.
Many comfort cooling designs use about 23 to 24 °C with 50% RH, which is a dew point of roughly 12 to 13 °C. ASHRAE 55 defines the acceptable range through operative temperature, humidity ratio, air speed, clothing and activity rather than a single number.
Found a number that looks wrong, or a case the tool does not cover? Say so here — corrections are welcome and get answered.
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