Abstract: Pneumatic sheet thickness measuring apparatus includes a chamber to which air is supplied via a restriction, and a nozzle positioned adjacent a sheet to be measured via which nozzle air can escape from the chamber. The effective area from which air can escape with a sheet of nominal thickness is in the range of substantially 0.3 to 0.9, preferably 0.6, times the area of the restriction.Air is also supplied to a second restriction, chamber and nozzle providing the same area ratio and the difference in the pressures in the two chambers is monitored with a differential pressure transducer.The sheet to be measured is supported on a reference roller and the nozzle assembly is mounted above the roller on a carriage one end of which is carried by a bearing and the other end of which runs on a vertically adjustable rail.