What each property actually tells you
Humidity ratio (W)
Kilograms of water vapour per kilogram of dry air. Not per kilogram of moist air — that distinction trips people up. It is the property that stays constant when you heat or cool air without adding or removing moisture, which is why it is the vertical axis of a psychrometric chart.
Enthalpy (h)
Total heat content per kilogram of dry air, sensible plus latent. The difference in enthalpy between two states, multiplied by the mass flow of dry air, is the coil load. This is the number an HVAC engineer actually sizes equipment from.
Dew point
Cool the air below this and water condenses. It depends only on the vapour pressure — not on the total pressure — so it is unchanged by altitude. That is a useful sanity check: if a calculator's dew point moves when you change the elevation, it is wrong.
Wet bulb
The temperature a wet thermometer reaches in moving air — the adiabatic saturation temperature. There is no closed-form expression for it; it must be solved iteratively. Calculators that give you a one-line "wet bulb formula" are giving you an approximation, usually the Stull correlation, which is good to about ±0.3 °C over a limited range.
The altitude mistake
Take air at 25 °C and 50 % RH.
- At sea level (101.325 kPa): W = 9.88 g/kg.
- In Denver (≈83.4 kPa): W = 12.05 g/kg — 22 % more water per kilogram of dry air.
- Dew point in both cases: 13.86 °C. Unchanged.
Same temperature, same relative humidity, same dew point — but a fifth more moisture to condense out. Size a dehumidification coil from a sea-level chart at altitude and you will undersize it. This calculator applies the ASHRAE altitude relation to the barometric pressure, so the numbers are right where you actually are.
Psychrometric Calculator — frequently asked
The humidity ratio W is the mass of water vapour per unit mass of dry air, W = 0.621945 × pw / (p − pw), where pw is the partial pressure of water vapour and p is the total barometric pressure. It is expressed in kg/kg or g/kg of dry air — note that the denominator is dry air, not moist air.
h = 1.006·t + W(2501 + 1.86·t), in kJ per kg of dry air, with t in °C and W in kg/kg. The first term is the sensible heat of the dry air and the second is the latent plus sensible heat carried by the water vapour. The enthalpy difference across a coil, times the dry-air mass flow, is the coil load.
No. The dew point depends only on the water-vapour partial pressure, so it is unaffected by the total barometric pressure. But the humidity ratio does change — at the same temperature and relative humidity, air at altitude holds significantly more water per kilogram of dry air. At 25 °C and 50% RH the humidity ratio is 9.88 g/kg at sea level and 12.05 g/kg in Denver.
The dew point is the temperature at which the air becomes saturated if you cool it without changing its moisture content. The wet bulb is the temperature reached by evaporative cooling — it is always between the dew point and the dry bulb. They are equal only when the air is saturated (100% RH).
Not a closed-form one. The wet bulb is defined by the adiabatic saturation relation, which must be solved iteratively. Published one-line formulas (such as the Stull correlation) are approximations accurate to roughly ±0.3 °C over a limited range. This calculator solves the relation properly by bisection.
Dry air at 20 °C and 101.325 kPa has a density of about 1.204 kg/m³. Moist air is slightly less dense than dry air at the same temperature and pressure, because a water molecule (18 g/mol) is lighter than the average air molecule (29 g/mol) it displaces — which surprises most people.
Most likely one of two reasons. Either it uses the Magnus approximation for saturation pressure instead of the ASHRAE formulation, or it assumes sea-level barometric pressure. Both are acceptable near 20 °C at sea level and increasingly wrong away from it.