Specific Humidity & Mixing Ratio Calculator

How to use this calculator

Enter your air temperature and relative humidity (and, optionally, your elevation for an accurate station pressure) and press Calculate. In the psychrometric state-point block the results include the Mixing Ratio in grams per kilogram and Specific Humidity expressed as grains per pound (GPP) — the two numbers HVAC and weather professionals actually design around.

Specific humidity vs. mixing ratio vs. relative humidity

These three terms all describe moisture but answer different questions. Relative humidity is a percentage of saturation and swings with temperature. Mixing ratio (humidity ratio) is the mass of vapor per kilogram of dry air, and specific humidity is the mass of vapor per kilogram of total air. The last two barely move when you heat or cool the air, which is exactly why engineers prefer them for load calculations. For a deeper comparison, read Specific Humidity vs Relative Humidity.

The formula

The humidity ratio is derived from the actual vapor pressure and the total air pressure:

e = eₛ × (RH / 100)          actual vapor pressure
w = 621.98 × e / (p − e)      mixing ratio (g/kg dry air)
q = w / (1 + w/1000)          specific humidity (g/kg moist air)
GPP = w × 7                   grains per pound of dry air

eₛ = saturation vapor pressure (hPa)
p  = total / station pressure (hPa)

The station pressure p falls with altitude, so entering your elevation makes the mixing ratio and GPP more accurate — see Does Altitude Affect Humidity?

Worked example

At 75°F (23.9°C), 50% relative humidity, at sea level: the saturation vapor pressure is 29.60 hPa, so the actual vapor pressure is 14.80 hPa. The mixing ratio works out to 9.22 g/kg, which is 64.5 grains per pound — a comfortable indoor target. Push the same room to 70% RH and the moisture load climbs to 12.98 g/kg (90.9 GPP): the extra ~26 grains per pound of air is exactly what your dehumidifier has to remove.

Where these numbers come from

Every value is produced by the same psychrometric engine that powers the main relative humidity calculator, using the Magnus-Tetens saturation formula and the ASHRAE humidity-ratio relation. To see the vapor-pressure step on its own, use the vapor pressure calculator; to convert the same conditions into a temperature, try the dew point calculator.

Frequently Asked Questions

What is specific humidity?

Specific humidity is the mass of water vapor per unit mass of total (moist) air — grams of vapor per kilogram of air, for example. Because it is a ratio of masses, it does not change when air is heated or cooled, which makes it ideal for tracking a parcel of air.

What is the difference between specific humidity and mixing ratio?

The mixing ratio (also called the humidity ratio) is the mass of water vapor per unit mass of dry air, while specific humidity is per unit mass of total air (dry air plus vapor). The two are numerically very close at normal temperatures — usually within about 2% — because water vapor is a small fraction of the air.

What are grains per pound (GPP)?

Grains per pound is the imperial version of the humidity ratio used throughout North American HVAC. One pound equals 7,000 grains, so GPP expresses grains of moisture per pound of dry air. It is the number technicians read off a psychrometric chart to size dehumidification and check ventilation.

How do you calculate mixing ratio from relative humidity?

First find the actual vapor pressure e = eₛ × RH/100, then apply the humidity ratio formula w = 621.98 × e / (p − e), where p is the total (station) pressure in the same units and w is in grams per kilogram of dry air. This calculator does it automatically once you enter temperature and humidity.

Why does specific humidity matter for HVAC?

Cooling and dehumidifying air removes moisture mass, and that latent load is driven by the difference in humidity ratio (grains per pound) between incoming and target air. Sizing equipment on relative humidity alone is misleading because RH shifts with temperature; specific humidity and mixing ratio stay fixed until moisture is actually added or removed.