ISO 10545-4 loads a tile resting on two supports until it breaks. Two figures result: the breaking strength in newtons, which depends on thickness, and the modulus of rupture in N/mm², which describes the material.
For dry-pressed porcelain (group BIa) the standard requires a breaking strength of at least 1,300 N for tiles 7.5 mm thick or more, and a modulus of rupture of at least 35 N/mm².
These are values for the tile alone. On a floor, the substrate and full adhesive coverage matter as much as the tile.