An apparatus for calibration of a fluid-operated power torque tool having a housing and a drive has a non-turnable torque measuring unit for measuring a torque generated by the tool, a non-turnable abutment unit, a first connecting element configured for connecting the drive of the tool with the torque measuring unit, for measuring a torque generated by the tool when a fluid pressure is applied to the tool, an adjustable braking unit configured for providing an adjustable braking force, a second connecting element configured for connecting the housing of the tool with the adjustable braking unit to apply the adjustable braking force to the housing so as to increase a turning resistance to turning of the housing of the tool, so that when the tool is operated the housing of the tool starts turning at a force determined by the braking unit while the drive of the tool apply an opposite and equal force to the measuring unit, so that when the braking force is increased a fluid pressure required for the tool increases and the tool can be calibrated, by determining the generated torque that corresponds to the applied fluid pressure, without inertia.