Standardisation and Preferred Sizes

Design is a creative activity which naturally leads to diversity. If this is allowed to increase unchecked then difficulties will inevitably arise when it comes to purchasing and carrying stocks of components or spare parts. To prevent uncontrolled growth in variety, there have been standards, or preferred sizes for a long time and designers are encouraged to use these wherever possible. (In ancient Rome, standard pipe sizes were used for water supplies).

Preferred sizes are based on geometric series of numbers, three series designated the R5, R10 and R20 are used and described in ISO 3 - 1973. These series are based on the 5th, 10th and 20th root of 10 and are sometimes referred to as Renard Numbers:

R5 1 1.60 2.5 ....
R10 1 1.25 1.60 2.00 2.5 ....
R20 1 1.12 1.25 1.40 1.60 1.80 2.00 2.24 2.5 ....

Where sizes are given in mm, the following list shows preferred values:

0.05, 0.06, 0.08, 0.10, 0.12, 0.16, 0.20, 0.25, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, 1.0, 1.1, 1.2, 1.4, 1.5, 1.6, 1.8, 2.0, 2.2, 2.5, 2.8, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 8.0, 9.0, 10, 11, 12, 14, 16, 18, 20, 22, 25, 28, 30, 32, 35, 40, 45, 50, 60, 80, 100, 120, 140, 160, 180, 200, 250, 300.

It should be noted that even where a manufacturers catalogue lists a range of sizes, this does not mean they are all available for quick delivery. Items in regular demand will normally be quickly delivered, but items that are only bought infrequently, may have long delivery times, and this should always be checked where you are working to a tight timescale.

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David Grieve, 18th October 1999.