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 , 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 , - 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. <EOS>