Metric O-Ring Size Chart - ISO 3601

24 April 2026

Verified by Rachel Mayfield, Supply Chain Analyst - April 2026

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Metric O-ring sizes per ISO 3601-1, the international standard for O-ring inside diameters and cross-sections in millimetres. Metric O-Ring Size Chart - ISO 3601 ISO 3601 is gradually replacing BS 1806 for new designs, though both systems remain in active use across UK industry. Tolerances shown are for Grade N (general industrial); Grade S (precision) tolerances are roughly half these values.

ID (mm) CS (mm) ID Tolerance (mm) Common Materials
2.0 1.0 +/- 0.08 NBR, FKM, EPDM
2.5 1.0 +/- 0.08 NBR, FKM, EPDM
3.0 1.5 +/- 0.08 NBR, FKM, EPDM
3.5 1.5 +/- 0.08 NBR, FKM
4.0 1.5 +/- 0.08 NBR, FKM, EPDM
5.0 1.5 +/- 0.09 NBR, FKM, EPDM
5.0 2.0 +/- 0.09 NBR, FKM, EPDM, PTFE
6.0 1.5 +/- 0.09 NBR, FKM, EPDM
6.0 2.0 +/- 0.09 NBR, FKM, EPDM
7.0 2.0 +/- 0.09 NBR, FKM, EPDM
8.0 2.0 +/- 0.09 NBR, FKM, EPDM
9.0 2.0 +/- 0.10 NBR, FKM, EPDM
10.0 2.0 +/- 0.10 NBR, FKM, EPDM, PTFE
10.0 2.5 +/- 0.10 NBR, FKM, EPDM
12.0 2.0 +/- 0.10 NBR, FKM, EPDM
12.0 2.5 +/- 0.10 NBR, FKM, EPDM
14.0 2.0 +/- 0.10 NBR, FKM
15.0 2.5 +/- 0.12 NBR, FKM, EPDM
16.0 2.0 +/- 0.12 NBR, FKM, EPDM
18.0 2.5 +/- 0.12 NBR, FKM, EPDM
20.0 2.5 +/- 0.13 NBR, FKM, EPDM, PTFE
20.0 3.0 +/- 0.13 NBR, FKM, EPDM
22.0 2.5 +/- 0.13 NBR, FKM, EPDM
25.0 3.0 +/- 0.13 NBR, FKM, EPDM
28.0 3.0 +/- 0.15 NBR, FKM, EPDM
30.0 3.0 +/- 0.15 NBR, FKM, EPDM
32.0 3.0 +/- 0.15 NBR, FKM, EPDM
35.0 3.5 +/- 0.15 NBR, FKM, EPDM
38.0 3.0 +/- 0.17 NBR, FKM, EPDM
40.0 3.0 +/- 0.17 NBR, FKM, EPDM, PTFE
42.0 3.5 +/- 0.17 NBR, FKM
45.0 3.0 +/- 0.17 NBR, FKM, EPDM
48.0 3.5 +/- 0.19 NBR, FKM, EPDM
50.0 3.0 +/- 0.19 NBR, FKM, EPDM
55.0 3.5 +/- 0.19 NBR, FKM, EPDM
60.0 3.5 +/- 0.21 NBR, FKM, EPDM
65.0 3.5 +/- 0.21 NBR, FKM, EPDM
70.0 4.0 +/- 0.23 NBR, FKM, EPDM
75.0 4.0 +/- 0.23 NBR, FKM, EPDM
80.0 4.0 +/- 0.25 NBR, FKM, EPDM
85.0 4.0 +/- 0.25 NBR, FKM
90.0 4.0 +/- 0.27 NBR, FKM, EPDM
95.0 4.0 +/- 0.27 NBR, FKM
100.0 4.0 +/- 0.29 NBR, FKM, EPDM

How to use the ISO 3601 O-ring chart

ISO 3601 metric O-rings are specified by two numbers: inside diameter and cross-section. For example, a 20 x 2.5 O-ring has a 20 mm inside diameter and 2.5 mm cross-section. Unlike BS 1806, there's no reference number system - you just quote the two dimensions.

Material selection depends on the media and temperature. NBR (Nitrile) suits most oils and fuels from -40 to +120C. FKM (Viton) handles higher temperatures up to +200C and aggressive chemicals. EPDM works with water, steam, and brake fluids from -50 to +150C but fails with mineral oils. PTFE covers the widest temperature range (-200 to +260C) but isn't elastic and needs careful groove design.

Related standards

References

  1. ISO 3601-1:2012 - O-rings - inside diameters, cross-sections, tolerances
  2. ISO 3601-2:2016 - O-ring housing dimensions
  3. British Standards Institution

Where this data comes from: Supplier listings are verified against Companies House public records. Product specifications reference published ISO, BS, and DIN standards. Trade body data is sourced from official member directories.

Frequently Asked Questions

What is the difference between ISO 3601 and BS 1806 O-rings?

ISO 3601 uses metric dimensions (whole millimetre IDs), while BS 1806 uses imperial-origin dimensions converted to metric. They aren't interchangeable - the sizes are different. New designs should specify ISO 3601 metric sizes, but existing equipment often uses BS 1806 so both systems remain in use.

What O-ring material should I use for hydraulic oil?

NBR (Nitrile) is the standard choice for mineral hydraulic oil, rated to +120C. For synthetic or fire-resistant hydraulic fluids (HFC, HFD), check the specific fluid compatibility. FKM (Viton) handles a wider range of fluids but costs 5-10x more than NBR.

What does O-ring Grade N and Grade S mean in ISO 3601?

Grade N is the standard industrial tolerance grade, suitable for most applications. Grade S is the precision grade with tighter tolerances (roughly half of Grade N), used where accurate sealing geometry matters - for example, high-pressure hydraulics or pneumatic valves.

What does the Metric O-Ring Size Chart - ISO 3601 specify?

The Metric O-Ring Size Chart - ISO 3601 specifies inside diameters and cross-sections for O-rings in millimetres, following international standard ISO 3601-1.

How are tolerances defined on the Metric O-Ring Size Chart - ISO 3601?

Tolerances on the Metric O-Ring Size Chart - ISO 3601 vary by grade; Grade N (general industrial) has a tolerance of +/- 0.08 mm for ID sizes up to 4.0 mm, increasing slightly thereafter.

What are some common materials used in metric O-rings as listed on the Metric O-Ring Size Chart - ISO 3601?

Common materials for metric O-rings include NBR (Nitrile Butadiene Rubber), FKM (Fluorocarbon), and EPDM (Ethylene Propylene Diene Monomer). Some sizes also use PTFE (Polytetrafluoroethylene).

How is the choice between ISO 3601 and BS 1806 made for new designs in UK industry?

For new designs, ISO 3601 is increasingly favoured over BS 1806, but both standards remain in use across UK engineering due to ongoing compatibility requirements.

Which UK suppliers can provide metric O-rings according to the Metric O-Ring Size Chart - ISO 3601?

UK suppliers such as Parker Hannifin and Trelleborg Sealing Solutions offer metric O-rings that conform to the specifications of the Metric O-Ring Size Chart - ISO 3601.

What are the cross-section sizes for ID 5.0 mm on the Metric O-Ring Size Chart - ISO 3601?

For an inside diameter (ID) of 5.0 mm, the Metric O-Ring Size Chart - ISO 3601 lists two cross-sections: 1.5 mm and 2.0 mm, both with a tolerance of +/- 0.09 mm.

Are Grade S tolerances different from those listed on the Metric O-Ring Size Chart - ISO 3601?

Yes, Grade S (precision) tolerances are approximately half those of Grade N as indicated in the Metric O-Ring Size Chart - ISO 3601, making them more suitable for high-accuracy applications.

References