Absolute Humidity Calculator

How to use this calculator

Enter any valid pair of readings in the calculator above (for example air temperature and relative humidity), press Calculate, and look in the psychrometric state-point block for Absolute Humidity in g/m³. Alongside it you'll find the mixing ratio (g/kg) and specific humidity (grains per pound) for HVAC work.

What is absolute humidity?

Absolute humidity is the straightforward one: it is simply the mass of water vapor packed into a cubic meter of air. If you could weigh just the water floating in a 1 m³ box of air, that weight in grams is the absolute humidity. Because it counts molecules rather than comparing against a temperature-dependent maximum, it doesn't change when you heat or cool the air — only adding or removing moisture changes it.

Absolute vs relative humidity

Relative humidity tells you how full the air is (a percentage of its capacity at the current temperature); absolute humidity tells you how much water is actually there. This is why 90% relative humidity on a freezing morning can hold less real moisture than 40% on a hot afternoon — cold air's capacity is tiny. For the full comparison, read Relative vs Absolute vs Specific Humidity: The Complete Guide.

How it's calculated

The calculator finds the actual vapor pressure e from your temperature and relative humidity, then applies the standard relation:

AH = 216.7 × e / (T + 273.15)

AH = absolute humidity (g/m³)
e  = actual vapor pressure (hPa)
T  = air temperature (°C)

Worked example

At 30°C (86°F) and 60% relative humidity, the saturation vapor pressure is about 42.4 hPa, so the actual vapor pressure is roughly 25.5 hPa. Plugging in: AH = 216.7 × 25.5 / 303.15 ≈ 18.2 g/m³ — a genuinely muggy load of moisture. See How to Calculate Relative Humidity from Absolute Humidity to run the conversion the other way.

Frequently Asked Questions

What is absolute humidity?

Absolute humidity is the actual mass of water vapor in a given volume of air, expressed in grams per cubic meter (g/m³). Unlike relative humidity, it does not depend on temperature — it simply counts the water molecules present.

What is the difference between absolute and relative humidity?

Absolute humidity measures how much water vapor is in the air (g/m³). Relative humidity measures how full the air is compared to its maximum capacity at the current temperature (a percentage). Warm the air without adding moisture and the relative humidity falls, but the absolute humidity stays the same.

How do you calculate absolute humidity?

First find the actual vapor pressure from the temperature and relative humidity, then apply AH = 216.7 × e / (T + 273.15), where e is vapor pressure in hPa and T is temperature in °C. This calculator runs that chain automatically and shows the result in g/m³.

What are typical absolute humidity values?

Comfortable indoor air holds roughly 8–12 g/m³. A hot, muggy summer day can exceed 20 g/m³, while cold winter air often drops below 3 g/m³ even at high relative humidity, because cold air simply cannot hold much moisture.

What units is absolute humidity measured in?

The standard unit is grams of water vapor per cubic meter of air (g/m³). HVAC work often uses related measures instead — mixing ratio in grams per kilogram, or grains per pound — all of which this calculator also reports.