The boule's hardness is expressed in the ROCKWELL HRC unit or in kg/mm². For example, 35 HRC corresponds to 110 kg/mm².
The higher the number, the harder the boule (FIPJP accepted minimum is 35 HRC, accepted maximum is 50 HRC).The harder the boule, the less likely it is to get damaged, and the higher its elasticity, and vice versa.Regular players who use soft boules should consider frequently changing their boules to avoid undermining their game.