Psychrometric Calculator
Enter dry-bulb temperature and RH to get the moist-air properties coil and load calculations need: humidity ratio (g/kg), enthalpy (kJ/kg dry air) and dew point — altitude corrected.
Last updated: 2026-09-28
How the calculation works
- Saturation pressure comes from the ASHRAE correlation — the same relation behind psychrometric charts.
- The humidity ratio W converts partial vapor pressure into grams of water per kilogram of dry air, the unit HVAC mixing calculations use.
- Enthalpy combines sensible heat (1.006·T) with the latent content of the moisture (2501 kJ/kg at reference, adjusted by temperature).
- Altitude lowers barometric pressure, which raises W at the same RH — a real effect above ~1000 m that flatland charts miss.
Formula
p_ws = f(T) (ASHRAE saturation correlation) W = 0.621945 × p_w / (p − p_w) h = 1.006·T + W·(2501 + 1.86·T) kJ/kg dry air
| Symbol | Meaning | Unit |
|---|---|---|
W | Humidity ratio | kg/kg (shown as g/kg) |
h | Moist-air enthalpy | kJ/kg dry air |
p_ws | Saturation vapor pressure | kPa |
p | Barometric pressure at altitude | kPa |
Worked example
Interpreting the result
Enthalpy is the number cooling loads are computed from: the enthalpy difference between outdoor and indoor states, times the airflow, gives the total (sensible + latent) load the coil must handle. Two air states with the same temperature but different W have very different enthalpies — humidity is invisible in °C but dominates cooling cost in humid climates.
Assumptions
- Ideal gas behavior, standard atmosphere lapse for the altitude correction.
- ASHRAE correlation accuracy ~0.1% over 0-60 °C.
Limitations
- Wet-bulb and specific volume are not computed — use the full chart for complete state-point work.
- Correlation band is −20 to 60 °C; outside that, use dedicated psychrometric software.
Frequently asked questions
What is humidity ratio vs relative humidity?
RH is water vapor pressure relative to saturation at the current temperature — it changes when temperature changes. Humidity ratio (g/kg) is absolute moisture per kg of dry air — it only changes when moisture is added or removed, which is why coil and mixing calculations use W.
Why does altitude matter in psychrometrics?
Lower barometric pressure leaves less room for vapor: at the same RH, thinner air actually carries a higher humidity ratio per kg of dry air. Cooling towers, coils and evaporative coolers all behave differently above ~1000 m.