Volume measurements in archaeobotany are not performed uniformly. The goal of this paper therefore is to test the different known methods and to define the obtained differences, in order to make the density values (remains per litre of sediment) for plant macroremains in the samples comparable between sites. Three methods of volume measurement were tested for a large number of samples of different sizes coming from two late Neolithic layers of the lakeshore site of Parkhaus Opéra (Zürich, Switzerland). The sampled layers were preserved in waterlogged conditions and there were samples rich in sand, loam, lake marl but mostly consisting of organic remains, including uncharred subfossil plant macroremains. In general, the classical volume (that is the upper limit of the sediment in water) measured before and after freezing as pre-treatment gave similar results. But a systematic difference was found between the classical volume measured after freezing and the displacement volume. This difference could be described by a proportionality factor of 1.5. This proportionality factor could be used to make data obtained with different methods of volume measurement comparable, although more evaluations are needed from other sites in order to test the generality of the factor proposed.