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. Another advantage is the easier sortability of values which helps to optimize the bill of materials by combining similar part values to improve maintainability and reduce costs. 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 or F 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 is absent from most older character encodings (though it is present in the now-ubiquitous Unicode) 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), BS 6430 (1983) and Health Level 7 (HL7), which allow the prefix μ to be substituted by the letter u (or U) in circumstances in which only the Latin alphabet is available. Several manufacturers of resistors utilize the RKM code as part of the components' manufacturer's part numbers (MPNs).
Similar codes Though non-standard, some manufacturers also use the RKM code to mark inductors with R indicating the decimal point in microhenry (e.g. 4R7 for 4.7 μH). A similar non-standard notation using the unit symbol instead of a decimal separator is sometimes used to indicate voltages (i.e. 0V8 for 0.8 V, 1V8 for 1.8 V, 3V3 for 3.3 V or 5V0 for 5.0 V) in contexts where a decimal separator would be impossible to use or inappropriate (e.g. in signal or pin names, in variable names, in file names, or in labels or subscripts). Alternatively, letter P (presumably standing for "positive voltage" or "power supply rail") is seen being used instead of the V sometimes in device models and netnames (i.e. 1P8 for 1.8 V, 3P3 for 3.3 V). Respectively, both variants are also used as part of the MPN codes of zener diodes and voltage regulators by some manufacturers.
Tolerance code Letter code for resistance and capacitance tolerances:
Before the introduction of the RKM code, some of the letters for symmetrical tolerances (viz. G, J, K, M) were already used in US military contexts following the American War Standard (AWS) and Joint Army-Navy Specifications (JAN) since the mid-1940s.
Temperature coefficient code Letter codes for the temperature coefficient of resistance (TCR):
Production date codes
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