Capillary suction time (CST) and specific resistance to filtration (SRF) measurements are commonly used in the characterisation of sludges and for the prediction of efficient operation of dewatering processes such as centrifugation and filtration. Whilst occasionally useful in predicting trends, they do not assist in the design and optimisation of dewatering devices from first principles. Recent work in our laboratories has led to the development of a technique for the fast measurement of the permeability and compressibility of sludge. The use of a single volume-fraction dependent parameter, namely the solids diffusivity, D(ϕ), calculated from permeability and compressibility, enables full characterisation of the dewaterability of sludge. This allows different sludges to be compared in an unequivocal fashion. Data is presented for a range of sludges from different sources showing vastly different dewatering properties. The dewaterability of the different sludges is easily compared and the true role of flocculants in dewatering is highlighted.