O-ring dimensions by standard

O-ring dimension tables for three standards — AS568 (inch), ISO 3601 (metric) and JIS B 2401 (Japanese). Each size lists the inner diameter, cross-section and outer diameter, with a link to the matching product in the catalogue.

AS568
The inch-based SAE O-ring standard, split by cross-section into five series (dash numbers 001–475).
349 sizes
Open standard
ISO 3601
The international metric O-ring standard with series A–E by cross-section (identical to DIN 3771).
360 sizes
Open standard
JIS B 2401
The Japanese metric O-ring standard with series P (dynamic), S (small static) and G (static).
234 sizes
Open standard

Which standard are you most likely holding

The standard is set by the machine and the groove, not by the ring itself. The origin of the equipment usually narrows it down on the first try.

European machinery — ISO 3601 (DIN 3771): European drawings and manufacturer catalogues state the size metrically: inner diameter × cross-section in millimetres. DIN 3771 uses the same metric numbering as ISO 3601, so one table covers both designations.
American equipment — AS568 (BS1806): Dash numbers such as AS568-214 turn up on hydraulics, pneumatics and agricultural machinery of American origin, and in imported service kits. The British BS1806 standard uses the same numbers.
Japanese equipment — JIS B 2401: The P, S and G designations appear on Japanese machinery, hydraulics and engines. The number after the letter is a nominal size, not the exact inner diameter — P3, for example, has an inner diameter of 2.8 mm.
Free metric sizes — outside the standards: A large share of the rings sold every day belongs to none of these three tables: cross-sections of 3, 4 or 6 mm are not defined by any of the standards, even though they are produced as a matter of course. If your size is not in the tables, that is not an error — enter it directly in the catalogue search.

How to read an O-ring size code

Each of the three standards writes the size down differently, but all of them rest on the same two numbers — the inner diameter (ID) and the cross-section (CS).

Metric notation: ID × CS: A size written 20 × 3 mm means a 20 mm inner diameter and a 3 mm cord thickness. The first number is the inner diameter, not the outer one — mixing the two up is the most common ordering mistake.
AS568: the three-digit dash number: The series a number falls into fixes the cross-section: 001–050 is 1.78 mm, 102–178 is 2.62 mm, 201–284 is 3.53 mm, 309–395 is 5.33 mm and 425–475 is 6.99 mm. The exceptions are the three smallest sizes 001, 002 and 003, which carry narrower cross-sections (1.02, 1.27 and 1.52 mm).
ISO 3601: a letter plus four digits: The letter A to E marks the cross-section (1.80 · 2.65 · 3.55 · 5.30 · 7.00 mm), and the four digits are the inner diameter in tenths of a millimetre. Code B0530 therefore means a 2.65 mm cross-section and a 53.0 mm inner diameter.
JIS B 2401: the letter P, S or G: P is the dynamic series, S the small static series and G the static series. The cross-section is not constant within a series: in the P series it changes with diameter (1.9 to 8.4 mm), while the G series runs 3.1 mm up to G145 and 5.7 mm from G150. That is why the tables list it row by row.

The outer diameter is not an independent figure in any of the standards. OD = ID + 2 × CS always holds, and that is exactly how the tables derive it.

AS568 vs. ISO 3601: what actually differs

AS568 was written in inches. The millimetre values in the table are conversions (inches × 25.4), which is why they are not round: 1.78 · 2.62 · 3.53 · 5.33 · 6.99 mm. ISO 3601 was metric from the start and its cross-sections are round: 1.80 · 2.65 · 3.55 · 5.30 · 7.00 mm.

The cross-sections therefore differ by hundredths of a millimetre, but the inner-diameter ladders do not line up. AS568-214 measures 24.99 × 3.53 mm; the closest ISO 3601 size is C0250 at 25.0 × 3.55 mm. For other dash numbers the metric table has no comparable size at all.

So do not convert between standards by eye. The size converter handles the whole dash range 001 to 475 and finds the closest equivalent in ISO 3601 / DIN 3771 and JIS B 2401 for any size you enter — and says so when nothing falls inside tolerance.

Open the size converter

From a size to an actual ring

Pick the standard, open the series that matches your cross-section and find your row in the table. Every row carries the inner diameter, cross-section, outer diameter and a link to the matching product in the catalogue; where there is no exact match, the row offers the nearest size or a catalogue search instead.

If you do not know the code and only have the old ring, start by measuring it. A worn ring is often flattened or swollen, so the measured size need not match the original — measure several pieces and check them against the groove. When you are designing a new groove, the calculator works out its dimensions for a given cross-section per ISO 3601-2.

The size is only half the brief. The other half is the medium and the temperature — they decide the material (NBR, FKM, EPDM, silicone). Check compatibility with your specific fluid in the chemical resistance table.

Common questions about sizes and standards

Is AS568-214 the same as the metric size 25 × 3.5 mm?

Not exactly. Per the standard, AS568-214 has a 24.99 mm inner diameter and a 3.53 mm cross-section — values converted from inches. The closest metric size under ISO 3601 is C0250 (25.0 × 3.55 mm). The difference is hundredths of a millimetre, but on a drawing these are two different standards. The size converter gives you the exact nearest equivalent.

My drawing says DIN 3771. Where do I find that size?

In the ISO 3601 table. DIN 3771 uses the same A to E series numbering and the same inner diameters, so a DIN size is found in the ISO 3601 table without any conversion.

My size is in none of the tables. What does that mean?

It means you have a free metric size outside the standards. Cross-sections such as 3, 4 or 6 mm are not defined by any of them, even though they are routinely made and sold. Enter the inner diameter and cross-section in the catalogue search — a size page such as 20 × 3 mm shows every material available in that size.

Which number should I quote when ordering?

The inner diameter and cross-section in millimetres, or the standard code (AS568-214, B0530, P22). The outer diameter is not needed — it follows from ID + 2 × CS. If you are unsure, give both: the measured size and the code from the drawing.