Experiments on moisture uptake of monolithic and composite materials are generally performed by immersing rectanguloid (square plate) samples in water. An edge correction factor is derived which, in a mathematically simple way, takes water uptake through all 6 faces (2 broad and 4 smaller faces) into account. Analysis shows this edge correction factor to be very accurate (deviations typically less than 2%). New expressions for moisture uptake in composites with unidirectionally aligned fibres are derived, by incorporating this edge correction factor as well as proper boundary conditions which depend on volume fraction of fibres. Experimental data on moisture uptake in these types of composite samples is successfully analysed using these expressions.