Absolute Humidity Calculator — Convert Relative Humidity to Absolute Humidity

What is Absolute Humidity? – Absolute Humidity Definition

Absolute humidity (AH) measures the actual amount of water vapor present in a given volume of air. It is expressed as the mass of water vapor (m) per unit volume of air (V):

AH = m / V

In the metric system, absolute humidity is measured in grams per cubic meter (g/m³). In the US Imperial system, it is expressed in grains per cubic foot (gr/ft³).

Relative Humidity

Relative humidity (RH) is the amount of water vapor in the air expressed as a percentage of the maximum possible water vapor the air can hold at a given temperature. It is defined as the ratio of the actual vapor pressure (P_a) to the saturation vapor pressure (P_s) at the same temperature:

RH = (P_a / P_s) × 100%

Absolute Humidity vs Relative Humidity

While both quantities describe the moisture content in air, they represent it differently:

• Absolute humidity — tells you exactly how many grams of water vapor are in one cubic meter of air. It does not change with temperature (assuming the same parcel of air).
• Relative humidity — tells you how close the air is to saturation. It changes with temperature even if the absolute amount of water vapor stays constant, because warm air can hold more water vapor than cold air.

For example, air at 20 °C with 60% RH has an absolute humidity of about 10.4 g/m³, while the same absolute humidity of water vapor in cooler air at 10 °C would correspond to about 100% RH — meaning the air is saturated.

How to Calculate Absolute Humidity from Relative Humidity and Temperature

To convert relative humidity to absolute humidity, we use a two-step process:

1. Calculate saturation vapor pressure (P_s) using the Buck equation:
   P_s = 6.1078 × 10^{(7.5·T / (237.3 + T))} [hPa]
   where T is temperature in °C.
2. Calculate actual vapor pressure (P_a):
   P_a = P_s × RH / 100
3. Calculate absolute humidity (AH) using the ideal gas law for water vapor:
   AH = (P_a [Pa] × M_w) / (R × T_K) [g/m³]
   where M_w = 18.016 g/mol (molar mass of water), R = 8.314 J/(mol·K), T_K = T + 273.15 K.

Simplified: AH = (P_a [Pa] × 18.016) / (8.314 × (T + 273.15))

Unit Systems

Metric System

• Temperature: degrees Celsius (°C)
• Absolute Humidity: grams per cubic meter (g/m³)
• Vapor Pressure: hectopascals (hPa) or pascals (Pa)

US Imperial (American) System

• Temperature: degrees Fahrenheit (°F)
• Absolute Humidity: grains per cubic foot (gr/ft³)
• Conversion: 1 g/m³ = 0.437007 gr/ft³

How to Use the Absolute Humidity Calculator

1. Select your preferred unit system: Metric (°C, g/m³) or US Imperial (°F, gr/ft³).
2. Enter the air temperature (valid range: −40 °C to 60 °C, or −40 °F to 140 °F).
3. Enter the relative humidity as a percentage (0–100%).
4. Click Calculate.

The calculator will show you:

• Absolute humidity in g/m³ (metric) or gr/ft³ (imperial)
• Saturation vapor pressure (P_s)
• Actual vapor pressure (P_a)
• The applied formula

FAQs

What is a typical value of absolute humidity?

In comfortable indoor conditions (20 °C, 50% RH), absolute humidity is approximately 8.6 g/m³. In tropical climates (30 °C, 80% RH), it can exceed 24 g/m³. In cold dry conditions (−10 °C, 50% RH), it drops below 1 g/m³.

Why does absolute humidity matter?

Absolute humidity is important in HVAC engineering, meteorology, and industrial processes where the actual amount of water vapor affects material properties, human comfort, and equipment performance. Unlike relative humidity, it does not depend on temperature, making it more reliable for comparing moisture levels across different conditions.

What is the difference between absolute and specific humidity?

Absolute humidity is mass of water vapor per unit volume of moist air (g/m³). Specific humidity is the mass of water vapor per unit mass of moist air (g/kg). Both express the actual water content but use different reference quantities.

Can absolute humidity exceed saturation?

No — once relative humidity reaches 100%, the air is saturated. Any additional water vapor will condense into droplets (fog, dew, rain). This saturation point corresponds to the maximum possible absolute humidity at that temperature.