A simple law for the size of the intrinsic rotation observed in tokamaks in the absence of momentum injection is found: the velocity in the counter-current direction generated in the core of a tokamak is proportional to the ion temperature difference in the core divided by the plasma current, independent of the size of the device. The constant of proportionality is of the order of 10km.s-1 MA.keV-1. This law is derived using theoretical arguments and confirmed experimentally for several tokamaks of different size and heated by different mechanisms.