Procure a small sample of the stone you would like to test.
Measure your stone's dry weight. This can be done by placing it on a scale.
Immerse your stone in water until it stays submerged.
Obtain it's weight, and divide the difference of it's current weight and the original by the unit weight for water or 62.3 lb. per foot cubed. Obtain the volume of the the rock it self: The formula for this is base x base x height. Divide this number into the volume of the water. This will yield the porosity in percentage form.