Table 1 - Preferred Metric Sizes

Preferred Metric Sizes

The preferred numbering system has played a major role in the development of metric standards. This is a geometrical series of numbers adopted worldwide. Its first known application was in the 1870s by Charles Renard, a French army captain who reduced the different diameters of rope for military balloons from 425 to 17. The R5, R10 and R20 series refers to the Renard 5 (first choice sizes 60 % increments), Renard 10 (second choice sizes 25 % increments) and Renard 20 (third choice sizes 12 % increments) series of preferred numbers standardized in ASME Z17.1 and ISO 3.


Nominal metric sizes are identical where the metric system has been in use for several years. Here is how the preferred metric nominal sizes were developed and how these chosen sizes reflect preferred metric standard sizes for threaded fasteners, steel plates, sheets, bars, etc already in use throughout the world.