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Calculation Machine

Humidity Ratio Calculator

Grains and pounds of moisture per pound of air.

Reviewed by James Whitfield · How we check our maths

Your values

ⓘ Live results

Results
Humidity ratio
64.5 grains/lb
In lb/lb
0.00921
Vapour pressure
0.2145 psia
Saturation pressure
0.4291 psia
Dew point (approx)
55.1 °F

Quick answer

Humidity ratio is the mass of water vapour per pound of dry air, usually expressed in grains per pound.

The formula

W = 0.622 x Pw / (P - Pw)

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Dry bulb temperature75 °F
  • Relative humidity50 %
  • Barometric pressure14.696 psia

Result

  • Humidity ratio64.5 grains/lb
  • In lb/lb0.00921
  • Vapour pressure0.2145 psia
  • Saturation pressure0.4291 psia
  • Dew point (approx)55.1 °F

How to use the Humidity Ratio Calculator

  1. 1Measure dry bulb temperature and relative humidity.
  2. 2Adjust barometric pressure if you are well above sea level.
  3. 3Use the grain difference across a coil to calculate latent load.

Frequently asked questions

Why grains?

Grains per pound gives convenient whole numbers — there are 7,000 grains in a pound.

Does RH alone tell me moisture?

No — 50% RH at 90°F holds nearly three times the moisture of 50% RH at 60°F.

About this calculator