Vapor Quality Calculation – Definition and Formula

Saturated liquids and saturated vapors are very important terms in thermodynamics. Between these two phases, the ratio is generally calculated. Here you can find vapor quality calculations and a calculator which you can use in thermodynamics and fluid mechanics calculations.

What is the Vapor Quality?

So, in the phase change of the pure substances between the liquid and vapor phases, the ratio of the vapor to the total mass is the vapor quality. These calculations are very important for pure substances.

How to Calculate It?

Firstly, in the calculation of the vapor quality, we need to make calculations according to the volumes. The general formula for the this value is like this;

Vapor quality equation.

In here,

Vf is the total volume that saturated fluid has and also the Vfv is the Vv – Vf.

Vv is the total volume of the saturated vapor.

If you take a look at the T – v diagrams, you will understand it clearly.

Vapor quality from a tv diagram.
Image Source: Yunus A. Çengel, Michael A. Boles – Thermodynamics, An Engineering Approach-McGraw-Hill Science, 8th Ed., Pg: 128

All the units of specific volumes are m3/kg or ft3/lb.
By knowing the vapor quality of a liquid-vapor mixture, you can also calculate the total enthalpy of a system.

Enthalpy formula.

As you see above, you can choose a point in the T-v diagram, then you can calculate the enthalpy of it. In here;

hf is the enthalpy of the saturated liquid and hv is the enthalpy of the saturated vapor. Also, the unit of the enthalpy is kj/kg or BTU/lb. You can find the enthalpy by using vapor quality.

But you need to have the ability to look at the thermodynamic tables. In these thermodynamic tables of pure substances, you can find the enthalpies and specific volumes with the changing pressures and temperatures. Vapor quality is very important.


Above all, these are the general things about vapor quality.

So, do not forget to leave your comments and questions below about this value of vapor-liquid mixtures of pure substances.

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