The NPL Gauge Block Interferometer, which is manufactured and sold under licence by Hexagon Metrology, was designed by NPL to measure the length of gauge blocks, length bars and Hoke gauges up to 300 mm in length. Up to 12 gauges can be wrung to a platen which is then placed in the instrument and measured after a suitable thermal settling period. After appropriate corrections have been applied, typical measurement uncertainties of 20 nm for a 1 mm gauge and 40 nm for a 100 mm gauge can be achieved, at 95 % confidence level.