The international method for measuring strength is given in standard UNI EN ISO 10545.4 and applies to all ceramic and porcelain stoneware tiles.
Using this method, every tile in the test batch is placed on two rollers, while increasing force is applied by a third roller along the whole middle line of the working surface until the tile breaks.
As the breaking value also depends on the shape of the tile, the standard requires that it is not directly the force, F which is calculated, but rather the breaking strength, S:
S = FL/b
i.e. breaking strength = force, distance between rollers/tile width