Gregory Bender

BS 4518 O-ring standard specifications (metric)

I learned much of this information from the Precision Associates website.

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About the BS 4518 standard for O-rings

The BS 4518 standard for O-rings is produced by the British Standards Institution. The BS 4518 specification describes 203 O-rings. Each size is designated by a four digit number indicating the O-ring ID in tenths of millimeters followed by a hyphen and two digits indicating the O-ring cross-section, also in tenths of a millimeter.

In contrast to the AS 568 A inch system, the nominal and physical dimensions of metric O-rings match precisely. That is, a 1 mm O-ring will have a cross-section of 1 mm. The space that is being filled must be about 10% to 15% smaller than the cross-section of the O-ring in order for O-ring to seal properly.

Sizes within the BS 4518 standard for O-rings

Size reference codeDecimal size in millimetersDecimal size in inches
0031-161.6 mm3.1 mm0.063 inch0.122 inch
0041-161.6 mm4.1 mm0.063 inch0.161 inch
0051-161.6 mm5.1 mm0.063 inch0.201 inch
0061-161.6 mm6.1 mm0.063 inch0.24 inch
0071-161.6 mm7.1 mm0.063 inch0.28 inch
0081-161.6 mm8.1 mm0.063 inch0.319 inch
0091-161.6 mm9.1 mm0.063 inch0.358 inch
0101-161.6 mm10.1 mm0.063 inch0.398 inch
0111-161.6 mm11.1 mm0.063 inch0.437 inch
0121-161.6 mm12.1 mm0.063 inch0.476 inch
0131-161.6 mm13.1 mm0.063 inch0.516 inch
0141-161.6 mm14.1 mm0.063 inch0.555 inch
0151-161.6 mm15.1 mm0.063 inch0.594 inch
0161-161.6 mm16.1 mm0.063 inch0.634 inch
0171-161.6 mm17.1 mm0.063 inch0.673 inch
0181-161.6 mm18.1 mm0.063 inch0.713 inch
0191-161.6 mm19.1 mm0.063 inch0.752 inch
0221-161.6 mm22.1 mm0.063 inch0.87 inch
0251-161.6 mm25.1 mm0.063 inch0.988 inch
0271-161.6 mm27.1 mm0.063 inch1.067 inch
0291-161.6 mm29.1 mm0.063 inch1.146 inch
0321-161.6 mm32.1 mm0.063 inch1.264 inch
0351-161.6 mm35.1 mm0.063 inch1.382 inch
0371-161.6 mm37.1 mm0.063 inch1.461 inch
0036-242.4 mm3.6 mm0.094 inch0.142 inch
0046-242.4 mm4.6 mm0.094 inch0.181 inch
0056-242.4 mm5.6 mm0.094 inch0.22 inch
0066-242.4 mm6.6 mm0.094 inch0.26 inch
0076-242.4 mm7.6 mm0.094 inch0.299 inch
0086-242.4 mm8.6 mm0.094 inch0.339 inch
0096-242.4 mm9.6 mm0.094 inch0.378 inch
0106-242.4 mm10.6 mm0.094 inch0.417 inch
0116-242.4 mm11.6 mm0.094 inch0.457 inch
0126-242.4 mm12.6 mm0.094 inch0.496 inch
0136-242.4 mm13.6 mm0.094 inch0.535 inch
0146-242.4 mm14.6 mm0.094 inch0.575 inch
0156-242.4 mm15.6 mm0.094 inch0.614 inch
0166-242.4 mm16.6 mm0.094 inch0.654 inch
0176-242.4 mm17.6 mm0.094 inch0.693 inch
0186-242.4 mm18.6 mm0.094 inch0.732 inch
0196-242.4 mm19.6 mm0.094 inch0.772 inch
0206-242.4 mm20.6 mm0.094 inch0.811 inch
0216-242.4 mm21.6 mm0.094 inch0.85 inch
0246-242.4 mm24.6 mm0.094 inch0.969 inch
0276-242.4 mm27.6 mm0.094 inch1.087 inch
0296-242.4 mm29.6 mm0.094 inch1.165 inch
0316-242.4 mm31.6 mm0.094 inch1.244 inch
0346-242.4 mm34.6 mm0.094 inch1.362 inch
0356-242.4 mm35.6 mm0.094 inch1.402 inch
0376-242.4 mm37.6 mm0.094 inch1.48 inch
0396-242.4 mm39.6 mm0.094 inch1.559 inch
0416-242.4 mm41.6 mm0.094 inch1.638 inch
0446-242.4 mm44.6 mm0.094 inch1.756 inch
0456-242.4 mm45.6 mm0.094 inch1.795 inch
0476-242.4 mm47.6 mm0.094 inch1.874 inch
0496-242.4 mm49.6 mm0.094 inch1.953 inch
0516-242.4 mm51.6 mm0.094 inch2.031 inch
0546-242.4 mm54.6 mm0.094 inch2.15 inch
0556-242.4 mm55.6 mm0.094 inch2.189 inch
0576-242.4 mm57.6 mm0.094 inch2.268 inch
0586-242.4 mm58.6 mm0.094 inch2.307 inch
0596-242.4 mm59.6 mm0.094 inch2.346 inch
0616-242.4 mm61.6 mm0.094 inch2.425 inch
0626-242.4 mm62.6 mm0.094 inch2.465 inch
0646-242.4 mm64.6 mm0.094 inch2.543 inch
0676-242.4 mm67.6 mm0.094 inch2.661 inch
0696-242.4 mm69.6 mm0.094 inch2.74 inch
0195-303 mm19.5 mm0.118 inch0.768 inch
0215-303 mm21.5 mm0.118 inch0.846 inch
0225-303 mm22.5 mm0.118 inch0.886 inch
0245-303 mm24.5 mm0.118 inch0.965 inch
0255-303 mm25.5 mm0.118 inch1.004 inch
0265-303 mm26.5 mm0.118 inch1.043 inch
0275-303 mm27.5 mm0.118 inch1.083 inch
0295-303 mm29.5 mm0.118 inch1.161 inch
0315-303 mm31.5 mm0.118 inch1.24 inch
0325-303 mm32.5 mm0.118 inch1.28 inch
0345-303 mm34.5 mm0.118 inch1.358 inch
0355-303 mm35.5 mm0.118 inch1.398 inch
0365-303 mm36.5 mm0.118 inch1.437 inch
0375-303 mm37.5 mm0.118 inch1.476 inch
0395-303 mm39.5 mm0.118 inch1.555 inch
0415-303 mm41.5 mm0.118 inch1.634 inch
0425-303 mm42.5 mm0.118 inch1.673 inch
0445-303 mm44.5 mm0.118 inch1.752 inch
0495-303 mm49.5 mm0.118 inch1.949 inch
0545-303 mm54.5 mm0.118 inch2.146 inch
0555-303 mm55.5 mm0.118 inch2.185 inch
0575-303 mm57.5 mm0.118 inch2.264 inch
0595-303 mm59.5 mm0.118 inch2.343 inch
0625-303 mm62.5 mm0.118 inch2.461 inch
0645-303 mm64.5 mm0.118 inch2.539 inch
0695-303 mm69.5 mm0.118 inch2.736 inch
0745-303 mm74.5 mm0.118 inch2.933 inch
0795-303 mm79.5 mm0.118 inch3.13 inch
0845-303 mm84.5 mm0.118 inch3.327 inch
0895-303 mm89.5 mm0.118 inch3.524 inch
0945-303 mm94.5 mm0.118 inch3.72 inch
0995-303 mm99.5 mm0.118 inch3.917 inch
1045-303 mm104.5 mm0.118 inch4.114 inch
1095-303 mm109.5 mm0.118 inch4.311 inch
1145-303 mm114.5 mm0.118 inch4.508 inch
1195-303 mm119.5 mm0.118 inch4.705 inch
1245-303 mm124.5 mm0.118 inch4.902 inch
1295-303 mm129.5 mm0.118 inch5.098 inch
1345-303 mm134.5 mm0.118 inch5.295 inch
1395-303 mm139.5 mm0.118 inch5.492 inch
1445-303 mm144.5 mm0.118 inch5.689 inch
1495-303 mm149.5 mm0.118 inch5.886 inch
1545-303 mm154.5 mm0.118 inch6.083 inch
1595-303 mm159.5 mm0.118 inch6.28 inch
1645-303 mm164.5 mm0.118 inch6.476 inch
1695-303 mm169.5 mm0.118 inch6.673 inch
1745-303 mm174.5 mm0.118 inch6.87 inch
1795-303 mm179.5 mm0.118 inch7.067 inch
1845-303 mm184.5 mm0.118 inch7.264 inch
1895-303 mm189.5 mm0.118 inch7.461 inch
1945-303 mm194.5 mm0.118 inch7.657 inch
1995-303 mm199.5 mm0.118 inch7.854 inch
2095-303 mm209.5 mm0.118 inch8.248 inch
2195-303 mm219.5 mm0.118 inch8.642 inch
2295-303 mm229.5 mm0.118 inch9.035 inch
2395-303 mm239.5 mm0.118 inch9.429 inch
2445-303 mm244.5 mm0.118 inch9.626 inch
2495-303 mm249.5 mm0.118 inch9.823 inch
0443-575.7 mm44.3 mm0.224 inch1.744 inch
0453-575.7 mm45.3 mm0.224 inch1.783 inch
493-575.7 mm49.3 mm0.224 inch1.941 inch
0523-575.7 mm52.3 mm0.224 inch2.059 inch
0543-575.7 mm54.3 mm0.224 inch2.138 inch
0553-575.7 mm55.3 mm0.224 inch2.177 inch
0593-575.7 mm59.3 mm0.224 inch2.335 inch
0623-575.7 mm62.3 mm0.224 inch2.453 inch
0643-575.7 mm64.3 mm0.224 inch2.531 inch
0693-575.7 mm69.3 mm0.224 inch2.728 inch
0743-575.7 mm74.3 mm0.224 inch2.925 inch
0793-575.7 mm79.3 mm0.224 inch3.122 inch
0843-575.7 mm84.3 mm0.224 inch3.319 inch
0893-575.7 mm89.3 mm0.224 inch3.516 inch
0943-575.7 mm94.3 mm0.224 inch3.713 inch
0993-575.7 mm99.3 mm0.224 inch3.909 inch
1043-575.7 mm104.3 mm0.224 inch4.106 inch
1093-575.7 mm109.3 mm0.224 inch4.303 inch
1143-575.7 mm114.3 mm0.224 inch4.5 inch
1193-575.7 mm119.3 mm0.224 inch4.697 inch
1243-575.7 mm124.3 mm0.224 inch4.894 inch
1293-575.7 mm129.3 mm0.224 inch5.091 inch
1343-575.7 mm134.3 mm0.224 inch5.287 inch
1393-575.7 mm139.3 mm0.224 inch5.484 inch
1443-575.7 mm144.3 mm0.224 inch5.681 inch
1493-575.7 mm149.3 mm0.224 inch5.878 inch
1543-575.7 mm154.3 mm0.224 inch6.075 inch
1593-575.7 mm159.3 mm0.224 inch6.272 inch
1643-575.7 mm164.3 mm0.224 inch6.468 inch
1693-575.7 mm169.3 mm0.224 inch6.665 inch
1743-575.7 mm174.3 mm0.224 inch6.862 inch
1793-575.7 mm179.3 mm0.224 inch7.059 inch
1843-575.7 mm184.3 mm0.224 inch7.256 inch
1893-575.7 mm189.3 mm0.224 inch7.453 inch
1943-575.7 mm194.3 mm0.224 inch7.65 inch
1993-575.7 mm199.3 mm0.224 inch7.846 inch
2093-575.7 mm209.3 mm0.224 inch8.24 inch
2193-575.7 mm219.3 mm0.224 inch8.634 inch
2293-575.7 mm229.3 mm0.224 inch9.028 inch
2393-575.7 mm239.3 mm0.224 inch9.421 inch
2493-575.7 mm249.3 mm0.224 inch9.815 inch
2593-575.7 mm259.3 mm0.224 inch10.21 inch
2693-575.7 mm269.3 mm0.224 inch10.6 inch
2793-575.7 mm279.3 mm0.224 inch11 inch
2893-575.7 mm289.3 mm0.224 inch11.39 inch
2993-575.7 mm299.3 mm0.224 inch11.783 inch
3093-575.7 mm309.3 mm0.224 inch12.77 inch
3193-575.7 mm319.3 mm0.224 inch12.571 inch
3393-575.7 mm339.3 mm0.224 inch13.358 inch
3593-575.7 mm359.3 mm0.224 inch14.146 inch
3793-575.7 mm379.3 mm0.224 inch14.933 inch
3893-575.7 mm389.3 mm0.224 inch15.327 inch
3993-575.7 mm399.3 mm0.224 inch15.72 inch
4193-575.7 mm419.3 mm0.224 inch16.508 inch
4393-575.7 mm439.3 mm0.224 inch17.295 inch
4593-575.7 mm459.3 mm0.224 inch18.083 inch
4793-575.7 mm479.3 mm0.224 inch18.87 inch
4893-575.7 mm489.3 mm0.224 inch19.264 inch
4993-575.7 mm499.3 mm0.224 inch19.657 inch
1441-848.4 mm144.1 mm0.331 inch5.673 inch
1491-848.4 mm149.1 mm0.331 inch5.87 inch
1541-848.4 mm154.1 mm0.331 inch6.067 inch
1591-848.4 mm159.1 mm0.331 inch6.264 inch
1641-848.4 mm164.1 mm0.331 inch6.461 inch
1691-848.4 mm169.1 mm0.331 inch6.657 inch
1741-848.4 mm174.1 mm0.331 inch6.854 inch
1791-848.4 mm179.1 mm0.331 inch7.051 inch
1841-848.4 mm184.1 mm0.331 inch7.248 inch
1891-848.4 mm189.1 mm0.331 inch7.445 inch
1941-848.4 mm194.1 mm0.331 inch7.642 inch
1991-848.4 mm199.1 mm0.331 inch7.839 inch
2041-848.4 mm204.1 mm0.331 inch8.035 inch
2091-848.4 mm209.1 mm0.331 inch8.232 inch
2191-848.4 mm219.1 mm0.331 inch8.626 inch
2291-848.4 mm229.1 mm0.331 inch9.02 inch
2341-848.4 mm234.1 mm0.331 inch9.217 inch
2391-848.4 mm239.1 mm0.331 inch9.413 inch
2491-848.4 mm249.1 mm0.331 inch9.807 inch