RKM code

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 informally as "R notation" is a notation to specify resistor and capacitor values defined since 1952 in the IEC 60062 standard of the International Electrotechnical Commission. Corresponding national standards include DIN 40825 of the Deutsches Institut für Normung, BS 1852 of British Standards Institution, and IS 8186 of the Bureau of Indian Standards; as well as the pan-European EN 60062 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. 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 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.
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, and 18R indicates.
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, BS 6430 and Health Level 7, which allow the prefix μ to be substituted by the letter 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.

Similar codes

Though non-standard, some manufacturers also use the RKM code to mark inductors with R indicating the decimal point in microhenry.
A similar non-standard notation using the unit symbol instead of a decimal separator is sometimes used to indicate voltages in contexts where a decimal separator would be impossible to use or inappropriate. Alternatively, letter P is seen being used instead of the V sometimes in device models and netnames. 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 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 the temperature coefficient of resistance :

Code letter	ppm/K
K	1
L	2
M	5
N	10
P	15
Q	25
R	50
S	100
U	250
Z	other

Production date codes

Twenty-year cycle code

First character: Year of production in twenty-year cycle
* A = 2030, 2010, 1990, 1970
* B = 2031, 2011, 1991, 1971
* C = 2032, 2012, 1992, 1972
* D = 2033, 2013, 1993, 1973
* E = 2034, 2014, 1994, 1974
* F = 2035, 2015, 1995, 1975
* H = 2036, 2016, 1996, 1976
* J = 2037, 2017, 1997, 1977
* K = 2038, 2018, 1998, 1978
* L = 2039, 2019, 1999, 1979
* M = 2020, 2000, 1980
* N = 2021, 2001, 1981
* P = 2022, 2002, 1982
* R = 2023, 2003, 1983
* S = 2024, 2004, 1984
* T = 2025, 2005, 1985
* U = 2026, 2006, 1986
* V = 2027, 2007, 1987
* W = 2028, 2008, 1988
* X = 2029, 2009, 1989
Second character: Month of production
* 1 to 9 = January to September
* O = October
* N = November
* D = December

Example: J8 = August 2017
Some manufacturers also used the production date code as a stand-alone code to indicate the production date of integrated circuits.
Some manufacturers specify a three-character date code with a two-digit week number following the year letter.
IEC 60062 also specifies a four-character year/week code.

Ten-year cycle code

First character: Year of production in ten-year cycle
* 0 = 2020
* 1 = 2021
* 2 = 2022, 2012
* 3 = 2023, 2013
* 4 = 2024, 2014
* 5 = 2025, 2015
* 6 = 2026, 2016
* 7 = 2017
* 8 = 2018
* 9 = 2019
Second character: Month of production
* 1 to 9 = January to September
* X = October
* Y = November
* Z = December

Example: 78 = August 2017
IEC 60062 also specifies a four-character year/week code.

Four-year cycle code

IEC 60062 also specifies a single-character four-year cycle year/month code.

Year	Month	Letter
1993 1997 2001 2005 2009 2013 2017 2021	1	A
1993 1997 2001 2005 2009 2013 2017 2021	2	B
1993 1997 2001 2005 2009 2013 2017 2021	3	C
1993 1997 2001 2005 2009 2013 2017 2021	4	D
1993 1997 2001 2005 2009 2013 2017 2021	5	E
1993 1997 2001 2005 2009 2013 2017 2021	6	F
1993 1997 2001 2005 2009 2013 2017 2021	7	G
1993 1997 2001 2005 2009 2013 2017 2021	8	H
1993 1997 2001 2005 2009 2013 2017 2021	9	J
1993 1997 2001 2005 2009 2013 2017 2021	10	K
1993 1997 2001 2005 2009 2013 2017 2021	11	L
1993 1997 2001 2005 2009 2013 2017 2021	12	M

Year	Month	Letter
1995 1999 2003 2007 2011 2015 2019 2023	1	a
1995 1999 2003 2007 2011 2015 2019 2023	2	b
1995 1999 2003 2007 2011 2015 2019 2023	3	c
1995 1999 2003 2007 2011 2015 2019 2023	4	d
1995 1999 2003 2007 2011 2015 2019 2023	5	e
1995 1999 2003 2007 2011 2015 2019 2023	6	f
1995 1999 2003 2007 2011 2015 2019 2023	7	g
1995 1999 2003 2007 2011 2015 2019 2023	8	h
1995 1999 2003 2007 2011 2015 2019 2023	9	j
1995 1999 2003 2007 2011 2015 2019 2023	10	k
1995 1999 2003 2007 2011 2015 2019 2023	11	l
1995 1999 2003 2007 2011 2015 2019 2023	12	m