A calibration rotor for a horizontal balancer, configured to be driven by a driver. The calibration rotor includes a main barrel that has a longitudinal axis and the periphery of which includes points for attaching balance weights that are evenly distributed in an axial and angular manner about the axis. A rotation guide is arranged at each end of the main barrel. The rotor is modular, and the rotation guide includes adapters (40, 42) which are capable of being configured to be attached to the ends of the barrel. The guide is of a first type, suitable for rotatably guiding the barrel on rollers, or of a second type, suitable for rotatably guiding the half-shell barrel. Each type of adapter has the same predetermined moment of inertia and is interchangeable on the barrel.