A thickness measurement device for measuring an absolute thickness of flat workpieces, with (a) a first arm and a second arm, which form an intermediate space for inserting the workpiece between them, (b) a first interferometer for - emitting a first light beam from the first arm onto the workpiece and - detecting a first phase (Φ o) between the first light beam and a first return beam reflected by the workpiece, (c) a second interferometer for - emitting a second light beam from the second arm onto the workpiece and - detecting a second phase (Φ o) between the second light beam and a second return light beam reflected by the workpiece, (d) a third interferometer for - forming a third light beam from one arm to another arm away from the workpiece, and - detecting a control phase (Φ ko) and (e) an evaluation device for determining the absolute thickness of the workpiece from interferometer data of the interferometer (26, 48, 62), characterised in that (f) the evaluation unit is configured to automatically - determine an effective phase (Φ eff) from the first phase (Φ o), the second phase (Φ u) and the control phase (Φ ko) and - determine the absolute thickness (I x) from a measurement phase (Φ mess), - the measurement phase (Φ mess) being a difference from the effective phase (Φ eff) with the workpiece in the intermediate space and the effective phase (Φ eff) without the workpiece in the intermediate space, - the effective phase (Φ eff) without the workpiece in the intermediate space being an offset phase. (a) a first arm and a second arm, which form an intermediate space for inserting the workpiece between them, (b) a first interferometer for - emitting a first light beam from the first arm onto the workpiece and - detecting a first phase (Φ o) between the first light beam and a first return beam reflected by the workpiece, - emitting a first light beam from the first arm onto the workpiece and - detecting a first phase (Φ o) between the first light beam and a first return beam reflected by the workpiece, (c) a second interferometer for - emitting a second light beam from the second arm onto the workpiece and - detecting a second phase (Φ o) between the second light beam and a second return light beam reflected by the workpiece, - emitting a second light beam from the second arm onto the workpiece and - detecting a second phase (Φ o) between the second light beam and a second return light beam reflected by the workpiece, (d) a third interferometer for - forming a third light beam from one arm to another arm away from the workpiece, and - detecting a control phase (Φ ko) and - forming a third light beam from one arm to another arm away from the workpiece, and - detecting a control phase (Φ ko) and (e) an evaluation device for determining the absolute thickness of the workpiece from interferometer data of the interferometer (26, 48, 62), - determine an effective phase (Φ eff) from the first phase (Φ o), the second phase (Φ u) and the control phase (Φ ko) and - determine the absolute thickness (I x) from a measurement phase (Φ mess), - the measurement phase (Φ mess) being a difference from the effective phase (Φ eff) with the workpiece in the intermediate space and the effective phase (Φ eff) without the workpiece in the intermediate space, the effective phase (Φ eff) with the workpiece in the intermediate space and the effective phase (Φ eff) without the workpiece in the intermediate space, - the effective phase (Φ eff) without the workpiece in the intermediate space being an offset phase.