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Psychrometric Properties Calculator

Work out psychrometric properties instantly with clear inputs, formula shown and shareable results.

About Psychrometric Properties Calculator

Moist air properties all follow from saturation vapour pressure, here from the Magnus relation. Relative humidity scales it to actual vapour pressure, the humidity ratio comes from the 0.62198 mass ratio of water to dry air, and enthalpy combines the sensible term 0.24 x T with the latent term of the water vapour carried.

Formulas

Saturation pressure

Pws = 0.61094 x exp(17.625 Tc / (Tc + 243.04)) kPa

Humidity ratio and enthalpy

W = 0.62198 Pw / (P - Pw); h = 0.24 T + W (1061 + 0.444 T)

Worked example

With the default values below, Psychrometric Properties Calculator returns Humidity ratio of 64.5. Change any input above and every figure here is recalculated in your browser.

Inputs used

Dry bulb temperatureminimum -20 °F
75 °F
Relative humidityaccepts 1–100 %
50 %
Barometric pressureminimum 5 psia
14.696 psia

Results

Humidity ratio
64.5 grains/lb
Dew point
55.1 °F
Enthalpy
28.08 BTU/lb dry air
Saturation vapour pressure
0.4291 psia

Step by step

  1. 1Values used
  2. 2Saturation pressure
  3. 3Humidity ratio and enthalpy
  4. 4Humidity ratio= 64.5 grains/lb
  5. 5Dew point= 55.1 °F
  6. 6Enthalpy= 28.08 BTU/lb dry air
  7. 7Saturation vapour pressure= 0.4291 psia

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Psychrometric Properties figure worked out?

Pws = 0.61094 x exp(17.625 Tc / (Tc + 243.04)) kPa. Moist air properties all follow from saturation vapour pressure, here from the Magnus relation.

Why is humidity ratio shown in grains?

Seven thousand grains make a pound, and HVAC coil selection tables are written in grains per pound of dry air because the numbers are easier to compare than four-decimal fractions.

What values do I need to enter?

This calculator takes 3 inputs: Dry bulb temperature, Relative humidity, Barometric pressure. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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