RKM code

The RKM code is a notation for the specification of resistors and capacitors, defined since 1952 by the International Electrotechnical Commission (IEC) in its standard IEC 60062 (formerly IEC 62). It is also referred to as "letter and numeral code for resistance and capacitance values and tolerances", "letter and digit code for resistance and capacitance values and tolerances", or informally as "R notation". Corresponding national standards include DIN 40825 (1973) of the Deutsches Institut für Normung, BS 1852 (1975) of British Standards Institution, and IS 8186 (1976) of the Bureau of Indian Standards; as well as the pan-European EN 60062 (1993) of the European Standards Organizations. The updated IEC 60062:2016, amended in 2019, comprises the most recent release of the standard.

Overview

Originally meant also as part marking code, this shorthand notation is widely used in electrical engineering to denote the values of resistors and capacitors in circuit diagrams and in the production of electronic circuits (for example in bills of material and in silk screens). This method avoids overlooking the decimal separator, which may not be rendered reliably on components or when duplicating documents.

The standards also define a color code for fixed resistors.

Part value code

For brevity, the notation often does not specify the unit (ohm or farad) explicitly and instead relies on implicit knowledge raised from the usage of specific letters either only for resistors or for capacitors, the case used (uppercase letters are typically used for resistors, lowercase letters for capacitors), a part's appearance, and the context.

The notation also avoids using a decimal separator and replaces it by a letter associated with the prefix symbol for the particular value.

This is not only for brevity (for example when printed on the part or PCB), but also to circumvent the problem that decimal separators tend to "disappear" when photocopying printed circuit diagrams.