The letter and digit code for resistance and capacitance values and tolerances, which is also known as RKM code or "R notation", is a notation to specify resistor and capacitor values defined in the international standard IEC 60062 (formerly IEC 62) since 1952. It is also adopted by various other standards including DIN 40825 (1973), BS 1852 (1974), IS 8186 (1976) and EN 60062 (1993). The significantly updated IEC 60062:2016 comprises the most recent release of the standard.
Contents
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 omits to always 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.
The code letters are loosely related to the corresponding SI prefix, but there are several exceptions, where the capitalization differs or alternative letters are used.
For example, 8K2 indicates a resistor value of 8.2 kΩ. Additional zeros imply tighter tolerance, for example 15M0.
When the value can be expressed without the need for a prefix, an "R" is used instead of the decimal separator. For example, 1R2 indicates 1.2 Ω, and 18R indicates 18 Ω.
For resistances, the standard dictates the use of the uppercase letters L (for 10−3), R (for 100 = 1), K (for 103), M (for 106), and G (for 109) to be used instead of the decimal point.
The usage of the letter R instead of the SI unit symbol Ω for ohms stems from the fact that the Greek letter Ω wasn't (and still isn't) part of most character sets and therefore is sometimes impossible to reproduce, in particular in some CAD/CAM environments. The letter R was chosen because visually it loosely resembles the Ω glyph, and also because it works nicely as a mnemonic for resistance in many languages.
The letters G and T weren't part of the first issue of the standard, which pre-dates the introduction of the SI system (hence the name "RKM code"), but were added after the adoption of the corresponding SI prefixes.
The introduction of the letter L in more recent issues of the standard (instead of an SI prefix m for milli) is justified to maintain the rule of only using uppercase letters for resistances (the otherwise resulting M was already in use for mega).
Similar, the standard prescribes the following lowercase letters for capacitances to be used instead of the decimal point: p (for 10−12), n (for 10−9), µ (for 10−6), m (for 10−3), but uppercase F (for 100 = 1) for Farad.
The letters p and n weren't part of the first issue of the standard, but were added after the adoption of the corresponding SI prefixes.
In cases, where the Greek letter µ is not available, the standard allows it to be replaced by u (or U, when only uppercase letters are available). This usage of u instead of µ is also in line with ISO 2955 (1974, 1983), DIN 66030 (Vornorm 1973; 1980, 2002) and BS 6430 (1983), which allow the prefix μ to be substituted by the letter u (or U) in circumstances, in which only the Latin alphabet is available.
Tolerance code
Letter code for resistance and capacitance tolerances:
Production date code
Example: V8 = August 2007