Reference guide
Metric O-Ring Sizes: How to Read and Convert Them
What a metric O-ring callout means, how ISO 3601, DIN 3771, JIS B 2401 and BS sizes differ, which cords interchange with AS568, and how close the nearest metric size is.
A metric O-ring is specified by its inside diameter and cross-section in millimeters, written ID x CS (for example 25 x 3.55). Two very different things get called "metric": an AS568 inch size expressed in millimeters, which is the same part with the same tolerances, and a true metric series size from ISO 3601-1, DIN 3771 or JIS B 2401 whose dimensions have no inch equivalent. Read the cross-section first. If it is 1.78, 2.62, 3.53, 5.33 or 6.99 mm, it is an AS568 size in millimeters and our AS568 chart lists it in both units. If it is 1.80, 2.65, 3.55, 5.30 or 7.00 mm, it is an ISO 3601 metric size.
If it is 1.9, 2.4, 3.5, 5.7 or 8.4 mm, it is JIS. This guide explains the systems, how the cords relate, and how to convert without guessing.
The five AS568 cross-sections in millimeters
The conversion is exact arithmetic (inches times 25.4) and the figures below are Conquest's own arrangement of the AS568 dimensional data.
| AS568 dash range | Nominal CS | Actual CS (in) | CS (mm) | ISO 3601 metric cord it overlaps |
|---|---|---|---|---|
| -004 to -050 | 1/16 in | 0.070 | 1.78 | Group A, 1.80 mm |
| -102 to -178 | 3/32 in | 0.103 | 2.62 | Group B, 2.65 mm |
| -201 to -284 | 1/8 in | 0.139 | 3.53 | Group C, 3.55 mm |
| -309 to -395 | 3/16 in | 0.210 | 5.33 | Group D, 5.30 mm |
| -425 to -475 | 1/4 in | 0.275 | 6.99 | Group E, 7.00 mm |
| -001, -002, -003 | below 1/16 in | 0.040, 0.050, 0.060 | 1.02, 1.27, 1.52 | none; no ISO metric cord this small |
| -901 to -932 | tube-fitting boss series | 0.056 to 0.118 | 1.42 to 3.00 | none; metric fittings are ISO 6149 |
Which metric standards exist, and how do you read their codes?
Each system writes its size differently, and the code format is the easiest way to tell them apart on a print. The descriptions below come from the published standards' catalog entries and from manufacturer size charts, never from the standards' own tables.
| Standard | Region and status | Cross-sections (mm) | How a size is written | Example |
|---|---|---|---|---|
| SAE AS568 | United States, current | 1.78, 2.62, 3.53, 5.33, 6.99 (plus the 900 boss series) | three-digit dash number, a catalog lookup | -214 = 24.99 x 3.53 mm |
| ISO 3601-1 | International, current; the metric list descends from DIN 3771, and the AS568 inch sizes were folded in from the 2008 edition at the class A tolerances | 1.80, 2.65, 3.55, 5.30, 7.00 | group letter A to E plus four digits = ID in tenths of a millimeter | C0250 = 25.0 x 3.55 mm |
| DIN 3771 | Germany, withdrawn, replaced by DIN ISO 3601-1; still on old drawings | 1.8 to 7.0 | ID x CS in millimeters, optionally a quality letter | 25 x 3.55 |
| JIS B 2401 | Japan, current | P: 1.9, 2.4, 3.5, 5.7, 8.4; G: 3.1, 5.7; S: 1.5, 2.0; V: 4.0, 6.0, 10.0 | series letter plus an index number that is close to, but is not, the ID | P22 = 21.8 x 2.4 mm |
| BS 1806 | United Kingdom, superseded by BS ISO 3601; still quoted | the AS568 cords | dash number; for sizes AS568 also lists, the number and dimensions match | -214 |
| BS 4518 | United Kingdom, legacy metric | 1.6, 2.4, 3.0, 5.7, 8.4 | four digits ID in tenths, hyphen, two digits CS in tenths | a code of the form 0245-24 reads 24.5 x 2.4 mm |
| Commercial "metric" (no standard) | Worldwide stock | 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0 and others | ID x CS with no standard named | 20 x 2 |
Three traps are worth naming. In ISO 3601 the letters A and B mean four unrelated things depending on where they sit: the cross-section group in the size code (A to E), the tolerance class (A is the tighter class and matches AS568; B is the economical class for true-metric sizes), the application series (G general purpose, A aerospace) and, in ISO 3601-3, the quality grade (N, S, CS). A JIS number is an index, not a diameter: P22 is 21.8 mm, G25 is 24.4 mm, and the offset changes with the cord. And a chart titled "ISO 3601 metric sizes" is often just the AS568 inch list in millimeters; test any chart by its cords.
Do AS568 and ISO 3601 metric cords interchange?
Yes, on cross-section. The published tolerance bands for the five AS568 cords and the five ISO 3601 metric groups overlap by 0.14 to 0.29 mm on every family, so the difference between a 3.53 mm and a 3.55 mm cord is a rounding difference, not a second cord. When an ISO metric ring will not work in an AS568 gland, the reason is always the inside diameter. JIS cords mostly do not interchange: only the 3.5 mm P cord overlaps an AS568 cord (3.53 mm); 2.4 against 2.62, 5.7 against 5.33 and 6.0 against 6.99 have no overlap at all. BS 4518's 1.6 / 2.4 / 3.0 / 5.7 / 8.4 mm family matches neither.
| AS568 cord (mm, band) | ISO 3601 group (mm, band) | Bands overlap by | Nearest JIS cord | JIS overlap |
|---|---|---|---|---|
| 1.78 (1.70 to 1.86) | A 1.80 (1.72 to 1.88) | 0.14 mm, interchange | 1.9 | 0.04 mm, marginal |
| 2.62 (2.54 to 2.70) | B 2.65 (2.56 to 2.74) | 0.14 mm, interchange | 2.4 | none |
| 3.53 (3.43 to 3.63) | C 3.55 (3.45 to 3.65) | 0.18 mm, interchange | 3.5 | 0.17 mm, interchange |
| 5.33 (5.20 to 5.46) | D 5.30 (5.17 to 5.43) | 0.23 mm, interchange | 5.7 | none |
| 6.99 (6.84 to 7.14) | E 7.00 (6.85 to 7.15) | 0.29 mm, interchange | 6.0 | none |
| 1.02, 1.27, 1.52 (-001 to -003) | none | no metric cord | S 1.5 | none |
How close is the nearest metric size to each AS568 size?
The comparison below was computed for this site across all 349 general-purpose AS568 sizes against the ISO 3601-1 metric list (360 sizes) and JIS B 2401 (258 sizes), judging inside diameter as stretch per the Parker handbook gland rules: within 2 percent the two sizes are dimensionally close and the gland still has to be checked, 2 to 5 percent needs the gland rechecked, more than 5 percent is a different size, not a substitute.
| Outcome | Against ISO 3601 metric | Against JIS B 2401 |
|---|---|---|
| ID within 2 percent, cords overlap | 254 of 349 | 14 of 349 |
| Check the gland (ID 2 to 5 percent off) | 22 | 6 |
| Not a substitute (ID more than 5 percent off) | 20 | 9 |
| Cord mismatch (cross-section bands do not overlap) | 0 | 97 |
| No size in that group at this diameter | 50 | 223 |
| No metric cord exists (-001, -002, -003) | 3 | not applicable |
AS568-015 and ISO A0140 both publish 14.00 mm, and AS568-214 (24.99 mm) sits 0.01 mm under ISO C0250 (25.00 mm), well inside either tolerance. So a print calling for C0250 in an AS568 gland is not a custom part. None of this is an approval: squeeze, gland fill and groove width still decide, and the groove design guide covers them.
How do I convert an AS568 size to metric?
Multiply the inch dimensions by 25.4 and keep two decimals. AS568-214, for example, is 0.984 in ID by 0.139 in CS, which reads 24.99 x 3.53 mm, and the tolerance converts the same way (0.010 in on the ID becomes 0.25 mm). The AS568 chart carries both units and the tolerances for every size, and the metric size chart lists the metric equivalents. Going the other way, an ISO 3601 metric code converts itself: the four digits are the inside diameter in tenths of a millimeter, so E4000 is a 400.0 mm ring on the 7.00 mm cord.
Measuring a metric O-ring
Measure the inside diameter and the cross-section of an unused, uncompressed ring with calipers, and measure the cross-section in two places. A used ring reads oversize because it has swollen or taken a set; a ring stretched over a shaft reads undersize. Match the cross-section to a cord family first, then the inside diameter to a size, and send the ID x CS callout, the material and, if known, the standard with your quote request; we identify the standard size before anyone talks about tooling.
The print governs; any change of size or standard is the buyer's engineering decision.
Sources
- SAE AS568 dimensional data, converted at 25.4 mm per inch
- BS ISO 3601-1:2012+A1:2019, ISO 3601-1:2012, DIN 3771-1 and JIS B 2401-1:2012 catalog records, read 2026-09-06
- ISO 3601-1 and JIS B 2401 metric size lists as published in manufacturer size charts, read 2026-09-06
- Parker O-Ring Handbook ORD 5700A/US, section 3.5 (stretch), read 2026-08-12
- BS 1806, BS 4518, ISO 3601-3 and ISO 6149, named as callouts
Reviewed by Grant Midstokke, Conquest Seal Corporation. Last updated September 2026. Spec values in this guide are drawn from the sources listed above; the compound datasheet and your application conditions govern the final choice.
Frequently asked questions
Is a 25 x 3.5 O-ring an AS568-214?
Almost always. AS568-214 is 24.99 x 3.53 mm, ISO C0250 is 25.0 x 3.55 mm, JIS P25.5 is 25.2 x 3.5 mm, and all three cords overlap. Confirm with the gland before substituting a JIS ring, and check which tolerance class the print wants.
What does "class A" mean on an ISO 3601 O-ring?
It depends which letter is meant. As a tolerance class, A is the tighter class that matches AS568. As a cross-section group in the size code (A0250), it means the 1.80 mm cord. As an application series, A means aerospace. Ask which axis the drawing intends.
Is DIN 3771 still valid?
DIN 3771-1 was withdrawn and replaced by DIN ISO 3601-1, but because the ISO metric list descends from it, a DIN ID x CS callout almost always lands exactly on an ISO 3601 metric size.
Can I use a JIS G ring where a JIS P ring is called out?
No. P and G overlap in inside diameter but never in cord below 150 mm: P carries 1.9, 2.4, 3.5, 5.7 and 8.4 mm cords and G carries 3.1 mm. G50 is 49.4 x 3.1 mm and P50 is 49.7 x 3.5 mm.
Are metric tube-fitting O-rings in ISO 3601?
No. The boss-seal series of AS568 (dash -901 to -932, for SAE straight-thread fittings) has no ISO 3601 counterpart; metric fitting O-rings are defined in ISO 6149, a different document with a different size set.
How do I read an ISO 3601 size code like C0250?
The letter is the cross-section group and the four digits are the inside diameter in tenths of a millimeter: C0250 is a 25.0 mm inside diameter with the group C cord of 3.55 mm. Groups A to E are 1.8, 2.65, 3.55, 5.3 and 7.0 mm. For the inch-derived sizes in the same standard the size code is simply the AS568 dash number.
Why is a JIS P22 not a 22 mm O-ring?
Because the JIS number is a catalog index, not the inside diameter: P22 measures 21.8 mm inside diameter with a 2.4 mm cross-section. JIS P rings are for moving seals and G rings for fixed ones, and the two series use different cords, so the letter matters as much as the number.
Do the AS568 and ISO cords really interchange?
For the five main groups the tolerance bands overlap by 0.14 to 0.29 mm, so a 1.78 mm AS568 cord and a 1.8 mm ISO cord are the same cord in practice, and likewise 2.62 and 2.65, 3.53 and 3.55, 5.33 and 5.3, 6.99 and 7.0. JIS is different: its 2.4 mm cord has no overlap with the 2.62 mm AS568 band, so a 1xx ring does not swap into a JIS P groove.
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