How to calculate absolute humidity from relative humidity?

First, the saturation vapor pressure is determined from air temperature and multiplied by the relative humidity value to obtain actual vapor pressure. The result is then divided by the specific gas volume of water vapor (461.5 J/(kg·K)) multiplied by the temperature in kelvins.

How do you calculate absolute humidity from vapor pressure?

Divide the actual vapor pressure by the specific volume of water vapor (461.5 J/(kg·K)) multiplied by the temperature in kelvins. The result is the mass of water vapor per cubic meter of air.

What is the unit of measurement for absolute humidity?

Since the absolute humidity is a mass concentration per unit volume, it is expressed in grams per cubic meter (g/m3) or kilograms per cubic meter (kg/m3).

What does a humidity of 40% mean?

Relative humidity indicates the saturation level of water vapour in the air. A relative humidity of 40% means that the air actually contains 40% of the maximum amount of water vapour it can hold at that temperature before condensation occurs.

What is the difference between absolute and relative humidity?

Absolute humidity is the actual amount of water vapour in a given volume of air and does not change when the air warms or cools. Relative humidity is a ratio. If you warm up the same volume of air, the relative humidity can decrease even if the absolute humidity stays the same.

Absolute Humidity Calculator

Your air

What do you know?

Relative humidity

Dew point

Vapor pressure

Absolute humidity

Air temperature

Relative humidity

Atmospheric pressure

hPa

Absolute humidity (g/m³)

g/m³

Absolute humidity

g/m³

Dew point: °C
Maximum it can hold: g/m³

Saturation vapor pressure: hPa
Actual vapor pressure: hPa

Mixing ratio (g/kg)
Specific humidity (g/kg)

Pleasant

— a dew point of about 13.9 °C is how this air feels to most people.

Charts & reference

Saturation vapor pressure and moisture capacity across temperature
Temp (°C)	Sat. vapor pressure (hPa)	Max AH (g/m³)	AH at your RH (g/m³)
0	6.1	4.8	2.4
5	8.7	6.8	3.4
10	12.3	9.4	4.7
15	17	12.8	6.4
20	23.4	17.3	8.6
25	31.7	23	11.5
30	42.5	30.4	15.2
35	56.3	39.6	19.8
40	73.9	51.2	25.6

Absolute humidity measures the actual amount of water present in air most directly by giving the mass of water vapour per cubic metre. This tool uses a standard Magnus-Tetens formula to convert water vapour pressure and allows you to convert any known data - relative humidity, dew point, water vapour pressure or measured absolute humidity - into all other moisture indicators.

What is Absolute Humidity?

Absolute humidity (AH) is the mass of water vapour contained within a given volume of air.

AH = \frac{m_{\text{water vapor}}}{V_{\text{air}}}

It is usually expressed in grams per cubic meter (g/m3) or kilograms per cubic meter (kg/m3). Unlike relative humidity, it measures an absolute amount. It does not take into account how far the air is from saturation, but only indicates how much water vapor is actually contained.

This distinction is very important. Warm air can hold much more water vapor than cold air, so even if the relative humidity in both rooms is the same (50%), there could be large differences in actual water content. Absolute humidity eliminates this apparent difference caused by temperature and shows the actual content directly.

Differences between absolute humidity, relative humidity, dew point and specific humidity.

These terms describe the same water vapor, just from different perspectives.

Quantity	What it measures	Typical unit
Absolute humidity	Mass of vapor per volume of air	g/m³
Relative humidity	How saturated the air is, vs its maximum at that temperature	%
Dew point	Temperature the air must cool to for saturation	°C
Mixing ratio	Mass of vapor per mass of dry air	g/kg
Specific humidity	Mass of vapor per mass of moist (total) air	g/kg

Relative humidity is the ratio of actual vapor pressure to saturation vapor pressure at that temperature.

RH = 100\% \times \frac{P_a}{P_s}

Thus a relative humidity of 40% means that the actual amount of water present is 40% of the maximum amount the air can hold before condensation begins.

Formula for calculating absolute humidity:

The ideal gas equation is applied only for water vapor. The mass of the water vapor in a given volume is obtained by dividing the actual vapor pressure by the specific gas constant of water vapor (461.5 J·kg−1·K−1) multiplied by the thermodynamic temperature.

AH = \frac{P_a}{R_v \, T} = \frac{P_a}{461.5 \, T}

where Pa is the actual vapor pressure in pascals, T is temperature in kelvins, and the result is given in kg/m3. When calculating using relative humidity instead of actual vapor pressure, RH × Ps / 100 can be used in place of the actual vapor pressure, where Ps is saturation vapor pressure. By substituting a Magnus–Tetens approximation for Ps one gets a simple formula that is widely used in meteorology.

AH\;(\text{g/m}^3) = \frac{6.112 \; e^{\frac{17.67\,T}{T + 243.5}} \times RH \times 2.1674}{273.15 + T}

In this formula T is given in degrees Celsius and the relative humidity (RH) is entered as a normal numeric value. The constant 2.1674 combines the molar mass of water with the gas constant. The exponent represents the saturation vapor pressure in hectopascals, which is also why warmer air can hold more water vapor.

Example calculation:

Let's take an air with a temperature of 25°C and relative humidity of 60% as an example. First, we calculate the saturation vapor pressure.

P_s = 6.112 \; e^{\frac{17.67 \times 25}{25 + 243.5}} \approx 31.7 \text{ hPa}

The actual vapor pressure is a multiple of 60%, i.e. about 19.0 hPa. We substitute this into the formula.

AH = \frac{31.7 \times 60 \times 2.1674}{273.15 + 25} \approx 13.8 \text{ g/m}^3

Thus this air contains approximately 13.8 grams of water vapor per cubic meter. At this temperature the absolute humidity is about 23 g/m³. The dew point is around 16.7 °C, which is the temperature at which the water vapor begins to condense.

The warmer air is, the more water vapor it can hold.

The saturation vapor pressure increases almost exponentially with temperature, so that the water vapor capacity of air approximately doubles for a temperature increase of about 10 °C. The following table shows how quickly this limit rises.

Temperature	Saturation vapor pressure	Maximum absolute humidity
0 °C	6.1 hPa	4.8 g/m³
10 °C	12.3 hPa	9.4 g/m³
20 °C	23.4 hPa	17.3 g/m³
30 °C	42.5 hPa	30.4 g/m³
40 °C	73.8 hPa	51.1 g/m³

The morning fog, the water droplets on a cold drink glass and the sticky feeling of a sultry afternoon all rely on the same physical principle. When warm, moist air cools to its dew point temperature, the water vapor it can no longer hold condenses into water droplets, fog or rain.

Applications of absolute humidity

Absolute humidity indicates the actual amount of water vapour and is therefore an important indicator in many cases than relative humidity. Absolute humidity is considered when selecting air conditioning (HVAC) and dehumidifiers. It is monitored in museums, archives and server rooms to protect sensitive materials. In greenhouses and mushroom farms it is adjusted to promote growth. Meteorologists and epidemiologists use it to study weather conditions and the spread of airborne viruses. It also plays an important role in industrial drying processes, semiconductor wafer fabrication plants and paint shops.

Dew point as an indicator of comfort

Compared to relative humidity, the dew point often better reflects how the air actually feels on human skin because it correlates directly with actual water vapor content. Here are some rough guidelines:

Dew point	How it feels
Below 10 °C	Dry and comfortable
10–16 °C	Pleasant
16–18 °C	Getting sticky
18–21 °C	Humid and uncomfortable
21–24 °C	Oppressive
Above 24 °C	Sweltering

This tool is for educational and planning purposes only. The Magnus-Tetens approximation equation has an error of less than one percent in the range of about -45 °C to 60 °C. Outside this range or for precise engineering calculations a full model should be used to calculate moist air properties.

Frequently asked questions

How to calculate absolute humidity from relative humidity?: First, the saturation vapor pressure is determined from air temperature and multiplied by the relative humidity value to obtain actual vapor pressure. The result is then divided by the specific gas volume of water vapor (461.5 J/(kg·K)) multiplied by the temperature in kelvins.
How do you calculate absolute humidity from vapor pressure?: Divide the actual vapor pressure by the specific volume of water vapor (461.5 J/(kg·K)) multiplied by the temperature in kelvins. The result is the mass of water vapor per cubic meter of air.
What is the unit of measurement for absolute humidity?: Since the absolute humidity is a mass concentration per unit volume, it is expressed in grams per cubic meter (g/m3) or kilograms per cubic meter (kg/m3).
What does a humidity of 40% mean?: Relative humidity indicates the saturation level of water vapour in the air. A relative humidity of 40% means that the air actually contains 40% of the maximum amount of water vapour it can hold at that temperature before condensation occurs.
What is the difference between absolute and relative humidity?: Absolute humidity is the actual amount of water vapour in a given volume of air and does not change when the air warms or cools. Relative humidity is a ratio. If you warm up the same volume of air, the relative humidity can decrease even if the absolute humidity stays the same.

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Disclaimer: This calculator is provided for general informational and educational purposes only. Our calculators are under active development, and results may be inaccurate, incomplete, or unsuitable for your situation. Always verify the figures independently and seek advice from a qualified professional before relying on them. We make no warranties and accept no liability for any loss or decision arising from use of this tool.

References

Vaisala: Humidity Conversion Formulas
Industry reference for saturation vapor pressure and humidity conversions.
Bolton, D. (1980): The Computation of Equivalent Potential Temperature
Source of the 6.112 / 17.67 / 243.5 Magnus-Tetens coefficients.