To quantify the inaccuracy of pressure measurement by water-filled systems, with a view to recommending better practice. Papers examining quality of measurements were reviewed, and standards documents examined. Technical data from manufacturers were gathered and summarized. Measurements were made on two different urodynamic systems. Individual inaccuracies were combined using the root sum of squares method. The total inaccuracy in measurement may reach up to 9.6 cmH2 O on pdet , but with good practice this may be mitigated to 1.3 cmH2 O. Water-filled systems, while being the recommended method for urodynamic measurements, are subject to a potential inaccuracy during poor use of up to approximately 10 cmH2 O on pdet . Good practice can reduce this inaccuracy to a tolerable level, that is, approximately 1 cmH2 O.