Preferred Metric Sizing Charts

Preferred Metric Sizes

The preferred numbering system has played a major role in the development of metric standards. This is a geometrical series of numbers adopted worldwide. Its first known application was in the 1870’s by Charles Renard, a French army captain who reduced the different diameters of rope for military balloons from 425 to 17. The R5, R10 and R20 series refers to the Renard 5 (first choice sizes 60 % increments), Renard 10 (second choice sizes 25 % increments) and Renard 20 (third choice sizes 12 % increments) series of preferred numbers, see ISO 3.


Nominal metric sizes are identical where the metric system has been in use for several years. Here is how the preferred metric nominal sizes were developed and how these chosen sizes reflect preferred metric standard sizes for threaded fasteners, metal sheets, steel plates, bars, etc already in use throughout the world.


How do the preferred metric sizes relate to the customary inch sizes and the preferred numbers.





Fastener Sizes

The ANSI preferred metric sizes are identical to those in the ISO 497 R20'' rounded series selected years ago. The intent of the number series shown is to help reduce the number of standard sizes for screw threads, metal sheets and plates, round metal bars, lifting capacities, hydraulic cylinder diameters, etc.

The preferred size range from 4 through 40 mm may be extended to cover smaller or larger sizes by just multiplying or dividing sizes shown by 10. For instance, 60-mm sizes would be a preferred choice as would 2.5-mm devices.

For example, the six first choice thread sizes shown in TABLE 2 are recommended to replace the 61 other thread sizes listed. The first choice sizes are according to the R5" series of preferred sizes shown in TABLE 1. Less savings will be achieved if you select the twelve ISO (green) first and second choice sizes shown or the 14 ANSI (bold) first choice sizes. The cost reduction becomes substantial when you figure thousands of dollars savings for each unique fastener size that can be eliminated from the product design.


Click here for the FASTENER SIZES - TABLE 2



Metal Sheet Sizes

The metric coarse thread pitch is slightly smaller (finer) compared to the customary unified coarse thread pitch. For example; the metric coarse thread M8 has the thread pitch 1.25 mm and the inch thread 5/16-18 (7.94 mm) has the thread pitch 1.41 mm.

Preferred metric sizes for thin metal flat products (sheets) in the range from 0.4 through 4 mm follow the sizes shown in TABLE 3. We used to specify even gage numbers for cold rolled material years ago, and now with the First Choice selection in the ANSI B32.100 standard, it is recommended you specify the first choice sizes or in four gage number increments.


Click here for the METAL SHEET SIZES - TABLE 3



Steel Plate Sizes

Preferred metric, standard metric and customary inch steel plate sizes are shown in TABLE 4. It is recommended selections be made from the preferred metric column marked first and second choice. Selection of first choice sizes will further reduce the number of plate sizes from nine to five. This is a huge cost reduction compared with the 19 inch plate sizes that used to be stocked by a large USA company. Steel plate sizes shown are those specified in the ANSI B32.100 standard.


Click here for the STEEL PLATE SIZES - TABLE 4



Preferred Tolerance Zones

The ISO tolerance zones shown here are those specified in the ANSI B4.2 standard and tabulated in the reference material. Those shown in the green areas are those used in the preferred fits.


The ISO System of Limits and Fits has been in use in many countries for more than 70 years. Cold finished round bars held to the tolerance zones, h11, h9, h7 and h6, are covered in national standards throughout the world as shown in TABLE 7. GO-NOGO gages to inspect parts and reamers to produce the holes to these tolerance zones are frequently readily available. Designed software where you can enter metric or inch fit conditions, and compare them with a proven fit, has been available since 1997. A second online version was available for two years, and a new third online version will be released soon.


The ISO System of Limits and Fits offer industries in USA numerous savings opportunities. The required documentations to help cut costs in terms of engineering time, tooling and material stock has been prepared, and it is up to us to take advantage of the system. The software program, , make the limit dimensions quick to calculate and copy without errors, and can maximize those opportunities while saving time. Preferred Sizes and tolerances help reduce the number of different hole and shaft sizes used in products.





Round Cold Finished Steel Bars

Hole basis fits with hole tolerances H11, H9, H8 and H7 help rationalize on standard cutting tools and gages, whereas shaft basis fits with shaft tolerances h11, h9, h7 and h6 help rationalize on standard round steel bars available in all major industrial countries. The ANSI B32.100 standard specify the above shaft tolerances and diameters, and the sizes are according to those shown in TABLE 1. How the tolerance zones apply in national standards are shown in TABLE 7 below.





Ten Preferred Hole and Shaft Basis Fits

The ANSI standard and software have ten preferred hole and shaft basis fits ranging from LOOSE RUNNING to FORCE fits as shown in TABLE 8. It is recommended you use hole basis fits in most applications since it helps you reduce the cost of cutting tools and gages. However, it may be to your advantage to use shaft basis fits where you have a standard shaft size in a machine with bearings, couplings, sprockets, gears, and other components attached to it. Examples include knitting, printing, and farm machines. Each preferred fit has the same clearance or interference for hole or shaft basis fit listed on the same line. The UK and Australian standards shown above also have ten hole and shaft basis fits, but the fit H9/d9 and D9/h9 has replaced the H9/d10 and D10/h9. Input from the automotive industries in USA demanded we add the interference fit H7/u6 and U7/h6 to the ANSI B4.2 standard. We combined two clearance fits into one, and ended up with the ten Preferred Fits as shown below but with the same hole (H11, H9, H8 and H7) and shaft (h11, h9, h7 and h6) tolerance zones: