The method measures consistency with a rotating consistency meter. The method comprises chopping with the teeth of a chopper structure optical radiation transmitted by an optical source to an optical detector while the shafts of the rotating consistency meter rotate. In addition, a chopper reference of the chopper structure chops optical radiation to form at least one reference pulse independent of the swivel between the shafts. The duration of each reference pulse is measured when measuring consistency. The change in the reference pulse is defined by comparing the duration of the measured reference pulse with the duration of the predefined reference pulse. The durations of the measuring pulses are corrected according to the change in the duration of the reference pulse.