Humidity Ratio, Explained

Of all the numbers this calculator reports, humidity ratio is the one HVAC engineers actually build load calculations around — not relative humidity, despite RH being the number most people are used to reading off a thermostat.

The formula

Humidity ratio W is the mass of water vapor carried per unit mass of dry air (not per unit mass of the moist air mixture):

W = 0.622 × e / (P − e)

where e is vapor pressure and P is total barometric pressure, both in kPa. The constant 0.622 is the ratio of water vapor's molar mass (18.02 g/mol) to dry air's effective molar mass (28.97 g/mol) — it converts a pressure ratio into a mass ratio.

Worked example and unit conversion

At T_db = 25°C, 50% RH, sea level: e = 1.584 kPa, P = 101.325 kPa. W = 0.622 × 1.584 / (101.325 − 1.584) = 0.9882 / 99.741 ≈ 0.00988 kg/kg — or 9.88 g per kg of dry air.

US HVAC references often use grains per pound of dry air instead (1 lb = 7,000 grains). Since W is a dimensionless mass ratio, converting is just a unit-factor multiplication: grains/lb = W(kg/kg) × 7,000. For the example above: 0.00988 × 7,000 ≈ 69 grains/lb — a figure you'll see quoted directly on manufacturer psychrometric data and older US-market HVAC textbooks.

Why humidity ratio, not RH, drives load calculations

Relative humidity is temperature-dependent — the same parcel of air reads a different RH at a different temperature even with zero moisture added or removed, which is exactly why heating cold winter air makes a room feel dry (the humidity ratio hasn't changed, but RH drops sharply as temperature rises). Humidity ratio stays constant through any process that only adds or removes sensible heat (no moisture added or removed), which makes it the correct basis for dehumidifier sizing, duct and coil moisture balances, and total system latent-load calculations — all of which need a number that doesn't shift just because the air got warmer or cooler.

Reading it off the chart

On the psychrometric chart this calculator draws, humidity ratio is exactly what the y-axis represents — your calculated point's height above the x-axis is its humidity ratio, read directly, independent of where it sits left-to-right (temperature).

FAQ

Why isn't humidity ratio expressed per unit of total (moist) air mass instead?

Because dry air mass stays constant through heating, cooling, or moisture addition/removal, while total moist-air mass changes slightly every time water is added or removed — using dry air as the fixed reference basis makes W a stable, comparable quantity across a whole air-handling process.

Is grains per pound still commonly used?

Yes, especially in older US HVAC literature, some manufacturer psychrometric software, and dehumidifier/desiccant equipment spec sheets — this calculator reports kg/kg (shown as g/kg) but the conversion above is exact and simple.

Does humidity ratio change with altitude at the same RH?

Yes, non-obviously — see the altitude page for why the same percent RH corresponds to a higher humidity ratio at lower barometric pressure.

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