In place calibration of the permanent on-line force measurment system of a rolling mill is performed by replacing the working rolls with dedicated calibration rolls, which can be scrap working rolls. Discontinuities formed in the cylindrical surfaces of dedicated calibration rolls, such as flat surfaces or preferrably flat bottomed longitudinal grooves, are engaged by interface members which also engage confronting flat surfaces or grooves in calibration load cells inserted between the dedicated calibration rolls. The interface members, which are made of a softer material than both the calibration rolls and the load cells, cold flows to evenly distribute the load and eliminate the need for precision machining of the interfaces. The dedicated calibration rolls can also be fixed, such as by welding, to dedicated calibration roll chocks which mount the calibration rolls in the mill housing to further prevent roll rotation and ejection of the calibration load cells. In some instances, when the braking effect of the deenergized work roll drive motor is sufficient, calibration can be performed using the normal working rolls with the aluminum interfaces with the calibration load cells having concave grooves which receive the working rolls.