Renard series
Renard series are a system of preferred numbers dividing an interval from 1 to 10 into 5, 10, 20, or 40 steps.^{[1]} This set of preferred numbers was proposed in 1877 by French army engineer Colonel Charles Renard.^{[2]}^{[3]}^{[4]} His system was adopted by the ISO in 1949^{[5]} to form the ISO Recommendation R3, first published in 1953^{[6]} or 1954, which evolved into the international standard ISO 3.^{[1]} The factor between two consecutive numbers in a Renard series is approximately constant (before rounding), namely the 5th, 10th, 20th, or 40th root of 10 (approximately 1.58, 1.26, 1.12, and 1.06, respectively), which leads to a geometric sequence. This way, the maximum relative error is minimized if an arbitrary number is replaced by the nearest Renard number multiplied by the appropriate power of 10. One application of the Renard series of numbers is to current rating of electric fuses. Another common use is the voltage rating of capacitors (e.g. 100 V, 160 V, 250 V, 400 V, 630 V).
Base series
The most basic R5 series consists of these five rounded numbers, which are powers of the fifth root of 10, rounded to two digits. The Renard numbers are not always rounded to the closest threedigit number to the theoretical geometric sequence:
 R5: 1.00 1.60 2.50 4.00 6.30
Examples
 If some design constraints were assumed so that two screws in a gadget should be placed between 32 mm and 55 mm apart, the resulting length would be 40 mm, because 4 is in the R5 series of preferred numbers.
 If a set of nails with lengths between roughly 15 and 300 mm should be produced, then the application of the R5 series would lead to a product repertoire of 16 mm, 25 mm, 40 mm, 63 mm, 100 mm, 160 mm, and 250 mm long nails.
 If traditional English wine cask sizes had been metricated, rundlet (68 liters), barrel (119 liters), tierce (159 liters), hogshead (239 liters), puncheon (318 liters), butt (477 liters) and tun (954 liters) could have become 63 (or 60 by R″5), 100, 160 (or 150), 250, 400, 630 (or 600) and 1000 liters, respectively.
Alternative series
If a finer resolution is needed, another five numbers are added to the series, one after each of the original R5 numbers, and one ends up with the R10 series. These are rounded to a multiple of 0.05. Where an even finer grading is needed, the R20, R40, and R80 series can be applied. The R20 series is usually rounded to a multiple of 0.05, and the R40 and R80 values interpolate between the R20 values, rather than being powers of the 80th root of 10 rounded correctly. In the table below, the additional R80 values are written to the right of the R40 values in the column named "R80 add'l". The R40 numbers 3.00 and 6.00 are higher than they "should" be by interpolation, in order to give rounder numbers.
In some applications more rounded values are desirable, either because the numbers from the normal series would imply an unrealistically high accuracy, or because an integer value is needed (e.g., the number of teeth in a gear). For these needs, more rounded versions of the Renard series have been defined in ISO 3. In the table below, rounded values that differ from their less rounded counterparts are shown in bold.



As the Renard numbers repeat after every 10fold change of the scale, they are particularly wellsuited for use with SI units. It makes no difference whether the Renard numbers are used with metres or millimetres. But one would need to use an appropriate number base to avoid ending up with two incompatible sets of nicely spaced dimensions, if for instance they were applied with both inches and feet. In the case of inches and feet a root of 12 would be desirable, that is, ^{n}√12 where n is the desired number of divisions within the major step size of twelve. Similarly, a base of two, eight, or sixteen would fit nicely with the binary units commonly found in computer science.
Each of the Renard sequences can be reduced to a subset by taking every nth value in a series, which is designated by adding the number n after a slash.^{[4]} For example, "R10″/3 (1…1000)" designates a series consisting of every third value in the R″10 series from 1 to 1000, that is, 1, 2, 4, 8, 15, 30, 60, 120, 250, 500, 1000.
See also
 Preferred numbers
 Preferred metric sizes
 125 series
 E series (preferred numbers)
 Logarithm
 Decibel
 Neper
 Phon
 Nominal Pipe Size (NPS)
 Geometric progression
References
 ^ ^{a} ^{b} ISO 3:197304  Preferred Numbers  Series of Preferred Numbers. International Standards Organization (ISO). April 1973. Retrieved 20161218. (Replaced: ISO Recommendation R31954  Preferred Numbers  Series of Preferred Numbers. July 1954. (July 1953))
 ^ Kienzle, Otto Helmut (20131004) [1950]. Written at Hannover, Germany. Normungszahlen [Preferred numbers]. Wissenschaftliche Normung (in German). 2 (reprint of 1st ed.). Berlin / Göttingen / Heidelberg, Germany: SpringerVerlag OHG. ISBN 9783642998317. Retrieved 20171101. (340 pages)
 ^ Paulin, Eugen (20070901). Logarithmen, Normzahlen, Dezibel, Neper, Phon  natürlich verwandt! [Logarithms, preferred numbers, decibel, neper, phon  naturally related!] (PDF) (in German). Archived (PDF) from the original on 20161218. Retrieved 20161218.
 ^ ^{a} ^{b} "preferred numbers". Sizes, Inc. 20140610 [2000]. Archived from the original on 20171101. Retrieved 20171101.
 ^ ISO 17:197304  Guide to the use of preferred numbers and of series of preferred numbers. International Standards Organization (ISO). April 1973. Archived from the original on 20171102. Retrieved 20171102.
[…] Preferred numbers were first utilized in France at the end of the nineteenth century. From 1877 to 1879, Captain Charles Renard, an officer in the engineer corps, made a rational study of the elements necessary in the construction of […] aircraft. He computed the specifications […] according to a grading system […]. Recognizing the advantage to be derived from the geometrical progression, he adopted […] a grading system […] that would yield a tenth multiple of the value […] after every fifth step of the series […] Renard's theory was to substitute […] more rounded but […] practical values […] as a power of 10, positive, nil or negative. He thus obtained […] 10 16 25 40 63 100 […] continued in both directions […] by the symbol R5 […] the R10, R20, R40 series were formed, each adopted ratio being the square root of the preceding one […] The first standardization drafts were drawn up on these bases in Germany by the Normenausschuss der Deutschen Industrie on 13 April 1920, and in France by the Commission permanente de standardisation in document X of 19 December 1921. […] the commission of standardization in the Netherlands proposed their unification […] reached in 1931 […] in June 1932, the International Federation of the National Standardizing Associations organized an international meeting in Milan, where the ISA Technical Committee 32, Preferred numbers, was set up and its Secretariat assigned to France. On 19 September 1934, the ISA Technical Committee 32 held a meeting in Stockholm; sixteen nations were represented: Austria, Belgium, Czechoslovakia, Denmark, Finland, France, Germany, Hungary, Italy, Netherlands, Norway, Poland, Spain, Sweden, Switzerland, U.S.S.R. With the exception of the Spanish, Hungarian and Italian […] the other delegations accepted the draft […] Japan communicated […] its approval […] the international recommendation was laid down in ISA Bulletin 11 (December 1935). […] After the Second World War, the work was resumed by ISO. The Technical Committee ISO/TC 19, Preferred numbers, was set up and France again held the Secretariat. This Committee at its first meeting […] in Paris in July 1949 […] recommended […] preferred numbers defined by […] ISA Bulletin 11, […] R5, R10, R20, R40. This meeting was attended by […] 19 […] nations: Austria, Belgium, Czechoslovakia, Denmark, Finland, France, Hungary, India, Israel, Italy, Netherlands, Norway, Poland, Portugal, Sweden, Switzerland, United Kingdom, U.S.A., U.S.S.R. During […] subsequent meetings in New York in 1952 and […] the Hague in 1953, […] attended also by Germany, […] series R80 was added […] The draft thus amended became ISO Recommendation R3. […]
(Replaced: ISO Recommendation R171956  Preferred Numbers  Guide to the Use of Preferred Numbers and of Series of Preferred Numbers. 1956. (1955) and ISO R17/A11966  Amendment 1 to ISO Recommendation R171955. 1966.)  ^ De Simone, Daniel V. (July 1971). U.S. Metric Study Interim Report  Engineering Standards (PDF). U.S. Government Printing Office. Washington, USA: The National Bureau of Standards (NBS). NBS Special Publication 34511 (Code: XNBSA). Archived (PDF) from the original on 20171103. Retrieved 20171103.
Further reading
 Hirshfeld, Clarence Floyd; Berry, C. H. (19221204). "Size Standardization by Preferred Numbers". Mechanical Engineering. New York, USA: The American Society of Mechanical Engineers. 44 (12): 791–. [1]
 Hazeltine, Louis Alan (January 1927) [December 1926]. "Preferred Numbers". Proceedings of the Institute of Radio Engineers. Institute of Radio Engineers (IRE). 14 (4): 785–787. doi:10.1109/JRPROC.1926.221089. ISSN 07315996.
 Van Dyck, Arthur F. (February 1936). "Preferred Numbers". Proceedings of the Institute of Radio Engineers. Institute of Radio Engineers (IRE). 24 (2): 159–179. doi:10.1109/JRPROC.1936.228053. ISSN 07315996.
 Van Dyck, Arthur F. (March 1951) [February 1951]. "Preferred Numbers". Proceedings of the IRE. Institute of Radio Engineers (IRE). 39 (2): 115. doi:10.1109/JRPROC.1951.230759. ISSN 00968390.
 ISO 497:197305  Guide to the choice of series of preferred numbers and of series containing more rounded values of preferred numbers. International Standards Organization (ISO). May 1973. Archived from the original on 20171102. Retrieved 20171102. (Replaced: ISO Recommendation R4971966  Preferred Numbers  Guide to the Choice of Series of Preferred Numbers and of Series Containing More Rounded Values of Preferred Numbers. 1966.)
 Tuffentsammer, Karl; Schumacher, P. (1953). "Normzahlen – die einstellige Logarithmentafel des Ingenieurs" [Preferred numbers  the engineer's singledigit logarithm table]. Werkstattechnik und Maschinenbau (in German). 43 (4): 156.
 Tuffentsammer, Karl (1956). "Das Dezilog, eine Brücke zwischen Logarithmen, Dezibel, Neper und Normzahlen" [The decilog, a bridge between logarithms, decibel, neper and preferred numbers]. VDIZeitschrift (in German). 98: 267–274.