For sorption measurements many different methods are applicable, but nearly all of them require a correction of the raw experimental data by a factor including the sample's volume. This can be determined by a separate volume measurement for samples with constant volumes. If the sample volume is not constant because of swelling effects, the actual volume is usually calculated by suitable equations of state (EOS) for the sample-gas-system (if available) or estimated by optical or spectroscopic sample observation and assumptions about orientation of swelling. The advantage of the new approach presented here is that one measurement leads to both unknown quantities: sample volume and absorption. Not only is the volume measurement for constant sample volumes is obsolete, also the actual volume of swollen samples can be determined with minimal effort. The method can be applied to liquid and solid state samples without optical or spectroscopic sample observation or any assumption about orientation of swelling.