Overview
ISO 497:1973 - Guide to the choice of series of preferred numbers and of series containing more rounded values - is a companion to ISO 17 that gives practical rules for selecting series of preferred numbers and for the exceptional use of more rounded values. The standard explains when and how to adopt rounded alternatives to preferred numbers, the limits of their use, and the consequences for scaling, precision and standardization.
Key topics and requirements
- Preference for preferred numbers: Use the canonical R series (preferred numbers) wherever possible. Preferred numbers give the best geometric progression, regular scaling and simplify calculations.
- Permitted rounded series: ISO 497 defines two levels of rounding - R' (first rounding) and R'' (second rounding) - and states that:
- R' may be used only when imperative reasons make preferred numbers impossible.
- R'' is a last-resort option and is more compromised in regularity and precision.
- Selection rule: Choose an appropriate ratio (series spacing) and the series precision/regularity that fit the application. Consider future insertion of intermediate values when selecting prototype values.
- Precision and regularity limits: The Annex quantifies permissible deviations and shows that more rounded values have greater irregularity and lower precision. Examples in the standard include deviations up to about 2.5% for R' and up to ~5.4% for some R'' values (the document cites specific cases such as a 5.36% deviation for value 1.5).
- Restricted use: Rounded values should be admitted only for imperative reasons (economic - e.g., existing tooling; or practical/psychological - e.g., whole-number tooth counts). Arbitrary deviations or mixed local choices are discouraged because they hinder national and international standardization.
Practical applications
- Mechanical design and manufacturing where a set of logically spaced sizes is needed (gears, shafts, bearings, fasteners, motor powers, pump outputs).
- Pricing lists, catalogs, and component dimensional series where regular geometric scaling simplifies multiplication/division and unit conversions.
- Situations requiring retention of legacy tooling or where whole-number counts are mandatory (e.g., gear teeth), provided the impact on series regularity is acceptable.
- Standards developers and national committees aligning product series
- Mechanical and design engineers planning scalable series of dimensions or capacities
- Manufacturing engineers and toolmakers managing legacy tooling choices
- Quality and procurement teams responsible for cataloging and interoperability
Related standards
- ISO 17 - Guide to the use of preferred numbers and of series of preferred numbers (ISO 497 is complementary)
- ISO 3 - Preferred numbers - series of preferred numbers
Using ISO 497:1973 helps maintain geometric scaling, calculation simplicity and international interoperability while providing guarded flexibility for exceptional rounding needs.
