European standard NBN EN 1936

This standard indicates what percentage of the total volume (apparent volume) is made up by pores.
A distinction is made between two different types of porosity

• closed porosity: the pores in the stone are separated from each other;
• open porosity: the pore cavities are more or less linked by a series of chambers, allowing almost free passage to water.

As a technical characteristic, professional stone dealers will sometime quote an absorption level instead of a porosity level. This absorption level is expressed as a percentage of the mass (i.e., the mass of absorbed water in relation to the mass of the dry piece of tested stone).
Indicative porosity values: (in volume%):

Soft (white) limestone	5 to 50 %
Marble and compact limestone	0,2 to 5 %
Granite	0 to 2 %
Slate	< 3 %

There are large differences between the porosity levels for the various types of limestone. The percentage of pores gives a clear indication of the openness or density of the structure. These differences can be related to a number of factors, such as the depth of the strata from which the stone was extracted and the degree of metamorphosis which the stone underwent during its formation. Porosity levels will also often determine whether or not a stone can be polished - a finishing which is only suitable for stones of a sufficient density. Account must also be taken of the fact that stone types which absorb water with difficulty sometimes have difficulty in shedding this water at a later stage. This water retentiveness can be a disadvantage in terms of resistance to frost damage.