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Porosity is the proportion of pore volume (open and closed) within the total volume of a material. Its counterpart is compactness, which indicates the degree to which the material is filled with solid matter and reflects the internal solid content. Its effect on material properties is the opposite of porosity. Compactness and porosity describe the density of a material from different aspects; porosity is usually used to express it.

Calculation of porosity
Void ratio = (1 - bulk density of granular material / apparent density of granular material) × 100%

Where P is the porosity of the material, in %.
Structure of voids
Connected pores: interconnected and open to the outside.
Closed pores: isolated and sealed from the outside.

Porosity of metal coatings
Porosity in a metal coating refers to the fine channels running from the coating surface down to the base metal. Coating porosity reflects the density of the coating surface and directly affects the protective capability of the coating, especially cathodic coatings. For coatings with special performance requirements, such as anti-carburizing or nitriding coatings, porosity measurement is also extremely important as a key indicator of coating quality.
