A measurement of the pressure of hydraulic fluid in a fluid working machine can be determined from the timing of the movement of valves which selectively seal a working chamber of cyclically varying volume from a fluid gallery. The timing of opening of a low pressure valve, to connect a working chamber to a low pressure fluid gallery, following the closure of a high pressure valve to seal a working chamber can be used to estimate the pressure in the high pressure fluid gallery at the moment of closure of the high pressure valve.