Absolute Humidity Calculator: g/m³ and Grains per Pound

Relative humidity tells you how full the air is compared with what it could hold. Absolute humidity tells you how much water is actually in it. This calculator turns a temperature and relative humidity into grams of water vapor per cubic meter (g/m³), plus the humidity ratio in grains per pound of dry air and grams per kilogram, the units on a psychrometric chart.

How the calculator works

1. Saturation vapor pressure. The calculator uses the Magnus formula with the Alduchov and Eskridge (1996) coefficients, the same one behind our dew point calculator, with T in °C:

es = 610.94 × exp(17.625·T ÷ (243.04 + T)) Pa

2. Actual vapor pressure: e = RH ÷ 100 × es.

3. Absolute humidity, from the ideal gas law for water vapor:

AH (g/m³) = e ÷ (461.52 × (T + 273.15)) × 1000

461.52 J/(kg·K) is the gas constant of water vapor: NIST’s molar gas constant (8.314462618) divided by the molar mass of water (18.0153 g/mol).

4. Humidity ratio, the mass of water vapor per mass of dry air (Stull, Practical Meteorology, eq. 4.4):

W = 0.622 × e ÷ (P − e), with P = 101,325 Pa (standard sea-level pressure)

Multiply W by 1000 for grams per kilogram, or by 7,000 for grains per pound (NIST: 1 pound = 0.45359237 kg and 1 grain = 64.79891 mg, exactly 1/7000 of a pound). °F inputs are converted to °C first.

Accuracy check: between 0 °C and 50 °C, the Magnus saturation vapor pressures used here are within 0.3% of NIST’s saturation table for water (for example 2.333 kPa against NIST’s 2.339 kPa at 20 °C), and absolute humidity scales directly with that number.

Worked examples

Air	Relative humidity	Absolute humidity	Grains per pound	g/kg
70 °F	30%	5.52 g/m³	32.5	4.64
70 °F	50%	9.20 g/m³	54.4	7.77
70 °F	100%	18.40 g/m³	110.1	15.73
75 °F	50%	10.79 g/m³	64.5	9.21
30 °F	70%	3.14 g/m³	17.0	2.43
90 °F	60%	20.47 g/m³	127.6	18.23
20 °C (68 °F)	50%	8.62 g/m³	50.7	7.25

So a 70 °F room at 50% holds about 9.2 grams of water in every cubic meter of air, roughly 0.26 g per cubic foot, or 4.02 grains per cubic foot.

Why warm air holds more water

The amount of water vapor air can hold before it saturates rises steeply with temperature:

Air temperature	Saturated (100% RH)	Grains per pound at 100% RH
30 °F	4.49 g/m³	24.3
50 °F	9.38 g/m³	53.3
70 °F	18.40 g/m³	110.1
90 °F	34.12 g/m³	216.9

Every 20 °F roughly doubles the capacity. That’s why the same water content can be “humid” on a cold day and “dry” indoors:

• Winter: outdoor air at 30 °F and 70% carries about 17 grains per pound. Heated to 70 °F, where saturated air holds 110 grains per pound, that same air drops to about 16% relative humidity, well below the EPA’s 30–50% indoor range. The relative humidity calculator shows this step by step.
• Summer: air at 90 °F and 60% carries about 128 grains per pound, more than 70 °F air can hold at all. Cool it to 70 °F and water has to condense out, which is what happens on an air conditioner’s coil and on cold drink glasses. If your home stays damp, see how to reduce humidity in the house.

Using absolute humidity at home

• Sizing a humidifier. Raising a 70 °F room from 30% to 45% relative humidity means adding about 2.8 g of water to every cubic meter of air, over and over as outdoor air leaks in. The humidifier size calculator turns that into gallons per day for your room.
• Finding the right level. Our guide to the ideal indoor humidity covers the EPA’s 30–50% range and why it shifts in cold weather.
• Wet windows. Water condenses when air cools to its dew point. If your windows stream in winter, read about condensation on windows.
• Heat and evaporation. For how humid air feels and how far evaporation can cool it, try the heat index calculator and the wet bulb calculator.

Limitations

The Magnus formula is accurate between −40 °C and 50 °C (−40 °F to 122 °F); outside that range the calculator shows no result. Below freezing it gives humidity relative to liquid water, not ice. The grams-per-cubic-meter result doesn’t depend on air pressure, but grains per pound and grams per kilogram assume sea-level pressure: at altitude the same vapor pressure gives a slightly higher humidity ratio.