AS568, ISO 3601 and JIS B 2401: converting O-ring sizes between standards

The same O-ring can be written four different ways: an American drawing says AS568-214, a European catalogue says ISO 3601 C0250 or DIN 3771 25.00 x 3.55, a Japanese parts list says JIS P25, and a maintenance fitter simply says 25 x 3.5. Until you can convert between these notations you are ordering blind — and tenths of a millimetre decide whether a substitute will seat in the groove. This article is a practical conversion guide: it explains the logic of all three standard families, lists the cross-section tables and shows a safe procedure for finding an equivalent across standards.

The common language: inner diameter × cross-section

Every standard encodes the same two numbers — the inner diameter (ID) and the cord thickness, i.e. the cross-section (CS). The outer diameter is always a derived value: OD = ID + 2 × CS. That is why the first step of every conversion is identical: translate the foreign designation into ID × CS in millimetres, and only then look for an equivalent. Inch values convert by multiplying by 25.4; catalogues list the millimetre values rounded to two decimal places.

AS568: the American inch standard with dash numbers

AS568 is an SAE standard, originally written for aerospace, today the default for the entire American industry. Sizes are designated by three-digit dash numbers from -001 to -475; 349 sizes are published, split into five series by cross-section. Within a series the cross-section is constant, so you read a dash number as the series (the hundreds digit) plus the diameter position (the rest).

Series 001 to 050: cross-section 0.070 in (1.78 mm). The three smallest sizes are the exception — 001, 002 and 003 carry narrower cross-sections of 0.040 / 0.050 / 0.060 in (1.02 / 1.27 / 1.52 mm).

Series 102 to 178: cross-section 0.103 in (2.62 mm).

Series 201 to 284: cross-section 0.139 in (3.53 mm) — the workhorse of general machinery and hydraulics.

Series 309 to 395: cross-section 0.210 in (5.33 mm).

Series 425 to 475: cross-section 0.275 in (6.99 mm) — the thickest cords, used for large static seals.

The largest published diameters: the 200 series ends at -284 with an ID of 17.955 in (456.06 mm), the 300 and 400 series at 25.940 in (658.88 mm). Watch out for older charts — a minority of catalogues print the 300/400 series tails a full inch lower (24.940 in); vendor stock lists resolve against 25.940 in.

How to read a dash number: the AS568-214 example

AS568-214 belongs to the 200 series, so the cross-section is 0.139 in. The table gives an inner diameter of 0.984 in. Converted: ID = 0.984 × 25.4 = 24.99 mm, CS = 3.53 mm, OD = 24.99 + 2 × 3.53 = 32.05 mm. If a British drawing says BS1806-214, it is the same ring — the British BS 1806 standard adopts AS568 dash numbers one to one.

ISO 3601 and DIN 3771: the metric world

ISO 3601-1 (2008 edition) is the metric standard of European industry. It groups sizes into five cross-section series: A = 1.80 mm, B = 2.65 mm, C = 3.55 mm, D = 5.30 mm and E = 7.00 mm. The size code is the series letter plus the inner diameter multiplied by ten and padded to four digits: B0530 is series B with a 53.0 mm ID, A0053 is series A with a 5.3 mm ID.

Diameter coverage: series A spans IDs from 1.8 to 125 mm, series B from 4.5 to 250 mm, series C from 14 to 355 mm, series D from 37.5 to 200 mm and series E from 109 to 400 mm. For most diameters two or three cross-section alternatives therefore exist, and the groove decides which one is right.

German drawings often reference DIN 3771 or DIN 3770. This is not yet another size table — DIN shares the ISO 3601 metric numbering and writes the ring directly as ID × CS, for example 53.00 x 2.65. If you can read an ISO code, you can read DIN.

JIS B 2401: the Japanese P, S and G series

The Japanese standard JIS B 2401 shows up on machines and pumps of Japanese origin and has three working series. P (moving, dynamic seals) with cross-sections of 1.9 / 2.4 / 3.5 / 5.7 / 8.4 mm depending on the diameter band, S (static, small cross-sections of 1.5 and 2.0 mm) and G (static) with a 3.1 mm cross-section up to G145 and 5.7 mm from G150.

The most important property of JIS: the number in the designation is the nominal diameter, not the actual ID. P3 actually has an ID of 2.8 mm, P25 has 24.7 mm and G25 has 24.4 mm. If you order a twenty-five by the Japanese designation and check it with callipers against a metric 25 x 3.5 ring, you will see a 0.3 mm difference — and that is correct, the standard is written exactly that way.

The second trap is the suffix letters. P10 and P10A share the same nominal and actual diameter (ID 9.8 mm) but differ in cross-section: P10 is 1.9 mm, P10A is 2.4 mm. The same suffix game applies to P22 (CS 2.4 mm) versus P22A (3.5 mm). Never ignore a letter in the designation.

Ranges: the P series runs from P3 (ID 2.8 mm) to P400 (ID 399.5 mm, cross-section 8.4 mm), the S series from S3 (ID 2.5 mm) to S150 (ID 149.5 mm) and the G series from G25 (ID 24.4 mm) to G300 (ID 299.3 mm).

Where the standards line up — and where they do not

AS568 and ISO 3601 cross-sections are deliberately close: 1.78 versus 1.80 mm, 2.62 versus 2.65 mm, 3.53 versus 3.55 mm, 5.33 versus 5.30 mm and 6.99 versus 7.00 mm. Differences of 0.02 to 0.05 mm are usually of no practical consequence in a static seal — but interchangeability is always decided by the groove drawing, not by the standards table. Be more careful with dynamic seals and small cross-sections.

JIS does not fit this pairing. The 3.5 mm cross-section sits close to 3.53/3.55 and an equivalent can usually be found, but 1.9 mm is far from 1.78/1.80, 2.4 mm is far from 2.62/2.65, and the 1.5 / 2.0 / 3.1 / 8.4 mm cross-sections have no counterpart in AS568 or ISO at all. A large share of Japanese sizes therefore cannot be replaced one to one by a catalogue American or European ring.

A cross-standard match around the 25 x 3.5 mm size: AS568-214 measures 24.99 × 3.53 mm, ISO 3601 C0250 measures 25.00 × 3.55 mm (DIN notation 25.00 x 3.55) and JIS P25 measures 24.70 × 3.50 mm. The diameters spread across 0.3 mm — in a static groove all three may work, but judge it against the groove drawing; in dynamic service the actual squeeze decides.

A safe conversion procedure in four steps

Step 1: write the size as ID × CS in millimetres. Multiply inches by 25.4. With JIS, look up the actual ID in the table — the nominal number is not enough.

Step 2: search the target standard with a sensible tolerance. Our cross-reference tool uses ±0.5 mm on the diameter and ±0.4 mm on the cross-section and ranks candidates by the sum of deviations — the same band is a good guide when searching a catalogue by hand.

Step 3: verify the result against the groove. A substitute with a cross-section deviation changes the squeeze; a static seal has more reserve, a dynamic one less. We covered the squeeze and gland-fill rules in the groove design article.

Step 4: do not forget the material. A size conversion says nothing about the rubber — AS568-214 exists in NBR, FKM and EPDM alike, and switching standards is no reason to switch material. Check media compatibility in the chemical resistance chart.

Frequently asked questions

Is AS568-214 the same as a metric 25 x 3.5 ring?

Not exactly. AS568-214 measures 24.99 × 3.53 mm. It is very close to 25 x 3.5 and the swap usually passes in an ordinary static groove, but formally it is a different size — the groove drawing has to decide.

What does DIN 3771 on a German drawing mean?

DIN 3771 adopts the ISO 3601 metric size series and designates the ring as ID × CS, for example 53.00 x 2.65. So look for a metric ring with those values; you do not need a separate conversion table.

Why does JIS P25 have a 24.7 mm diameter and not 25 mm?

Because the number in the Japanese designation is the nominal size, not the actual ID. Actual inner diameters in the P series run 0.2 to 0.5 mm below the nominal value (P3 = 2.8 mm, P25 = 24.7 mm).

Can I order a ring to BS 1806?

Yes — BS 1806 uses the same dash numbers as AS568, so BS1806-214 is dimensionally identical to AS568-214. Order by the dash number.

Convert your size yourself

You do not have to page through three standards: the size converter on oringy.sk translates dash numbers, ISO codes and millimetre notation, and shows the equivalents in AS568/BS1806, ISO 3601/DIN 3771 and JIS B 2401 side by side. Feed the measured or converted size into the configurator and it finds rings in stock. If you cannot identify an old ring — chewed up or hardened — the how-to-measure guide helps, and the failure analysis tool explains why it died. Match the material to your medium in the chemical resistance chart. And if you have a size that fits no standard at all, write to Filip — non-standard sizes can usually be sourced.