The capacity of the digestive tract is an important parameter in understanding digestive adaptations, particularly in herbivores. Measures of this capacity (‘gut fill’) are commonly performed in killed animals, which has ethical and logistical implications. Alternatively, dry matter gut contents (DMC) can be estimated in live animals from food intake, digesta retention and digestibility, based on physical principles (Holleman and White, Can. J. Zool. 67, 488–494, 1989). Although this method has been used to some extent, it still awaits thorough validation. Here we estimated DMC in seven tammar wallabies during 5-day feeding trials and compared the results to those gained from dissections immediately after the trials. Calculated DMC exceeded that actually measured by 29 ± 22%. A closer inspection of the data suggested that this was partly due to the fact that DMC as measured by dissection is susceptible to short-term influences such as daily variation in food intake, whereas the calculated DMC represents an integrative measure over the whole period of the feeding trial. Correlations between both the measured digesta retention times, and the calculated DMC, with the measured wet contents mass suggest that it is particularly the DMC determined via dissection that needs to be measured with care. For a comparison of gut capacities, the calculated DMC therefore can be considered adequate, but should for a more widespread use be validated in further studies including more species and experimental regimes controlling food intake variation. Additionally, we tested whether very small (100–500 μm) and small (500–1000 μm) particles were retained differently in the tammar wallabies. There was no indication of such a difference. Whether the macropod forestomach selectively passes a certain particle fraction (that represents microbes) with the generally faster-passing fluids remains to be investigated with even smaller markers, e.g. labelled bacteria.