Caldera-forming explosive eruptions from rhyolitic volcanoes typically produce huge quantities of pyroclastic material, blanketing the landscape and triggering long-term, voluminous and widespread sedimentary responses. Pumice is a major component of such eruptions, but its behaviour in the sedimentary environment is little understood. Its most notable feature is its very low density, which frequently enables pumice to float on water for long periods of time before eventually waterlogging enough to sink. Once fully saturated, pumice behaves in a broadly similar way to normal clastic material in terms of its hydrodynamic behaviour, modified by its lower density. However, relatively little is known about its earlier behaviour in the sedimentary environment, and in particular the manner and rate by which initially dry pumice absorbs water. In this paper, we develop a mathematical model which predicts that the time required for a pumice clast to saturate is proportional to the square of its radius. Tests on samples of real pumice accord with predicted trends, and match lacustrine pumiceous facies observed in the field. The ability to predict the time required for large pumice clasts to sink may provide a tool for estimating the timescales over which certain rapid post-eruptive sedimentary processes operate.