This British Standard provides a procedure for conversion which will give the essential accuracy required for precise dimensional interchangeability. The standard lays down an optimum fineness of rounding the converted size so that the conversion is made with the required precision and, at the same time, an excessive number of decimal places in the converted size is avoided. The procedure adopted is designed to ensure correct acceptance or rejection of a component, with respect to sizes, whether manufactured in one system of units or the other, provided that the methods of measurement used in the two systems are of comparable accuracy.

The conversion factor used

is: 1 inch = 25.4 millimetres.

This conversion factor is exact. It results from the legal definition of the yard in terms of the metre adopted by the USA in 1959 and by the UK in the Weights and Measures Act 1963. The values given in the tables in Appendix A are taken to a sufficient number of decimal places to cover the finest rounding required by the rules for conversion of inch and millimetre sizes.