The RKM code, 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 "R notation", is a notation to specify resistor and capacitor values defined in the international standard IEC 60062 since 1952. It is also adopted by various other standards including DIN 40825, BS 1852, IS 8186 and EN 60062. The significantly updated IEC 60062:2016 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. 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 explicitly and instead relies on implicit knowledge raised from the usage of specific letters either only for resistors or for capacitors, the case used, 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, 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, R, K, M, and G 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 is absent from most older character encodings 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, but were added after the adoption of the corresponding SI prefixes. The introduction of the letter L in more recent issues of the standard is justified to maintain the rule of only using uppercase letters for resistances. Similar, the standard prescribes the following lowercase letters for capacitances to be used instead of the decimal point: p, n, µ, m, but uppercase F 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. This usage of u instead of µ is also in line with ISO 2955, DIN 66030 and BS 6430, which allow the prefix μ to be substituted by the letter u in circumstances in which only the Latin alphabet is available.
Tolerance code
Letter code for resistance and capacitance tolerances: Before the introduction of the RKM code, some of the letters for symmetrical tolerances were already used in US military contexts following the American War Standard and Joint Army-Navy Specifications since the mid-1940s.
Temperature coefficient code
Letter codes for resistor temperature coefficients:
