O-rings
Metric O-rings
A metric O-ring is specified by its inside diameter and its cross-section in millimeters, for example 25 x 3.55, and the size lists behind that callout are ISO 3601-1 and the national systems it descended from or sits beside: DIN 3771, JIS B 2401, BS 1806 and BS 4518. Conquest Seal supplies metric O-rings in nitrile, FKM, EPDM, silicone, HNBR, FFKM and the other compound families, and matches an AS568 size to a metric callout when one exists.
- Callout
- ID x CS mmfor example 25 x 3.55
- ISO 3601-1 metric list
- 360 sizesfive cords, 1.8 to 7.0 mm
- ISO size code
- C0250group letter + ID in tenths of a mm
- Nearest AS568
- 255 of 346sizes within 2% on ID
What metric means on an O-ring print
A metric callout gives the nominal inside diameter and the cross-section in millimeters, ID x CS, and the two numbers together are the size. ISO 3601-1 adds a size code for each entry in its metric list: a cross-section group letter A to E followed by the inside diameter in tenths of a millimeter, so C0250 is 25.0 mm ID on the 3.55 mm cord. Send whichever form the print uses and the other is derived.
One trap is worth naming. Some charts titled ISO 3601 metric sizes are in fact the AS568 inch series converted to millimeters. A true metric size sits on one of the five metric cords, 1.8, 2.65, 3.55, 5.3 or 7.0 mm; an AS568 size converted to millimeters sits on 1.78, 2.62, 3.53, 5.33 or 6.99 mm.
The metric size systems: ISO 3601-1, DIN 3771, JIS B 2401, BS 1806 and others
Each system designates a size differently, and the number on an old drawing tells you which list it came from. The standards belong to their publishers and none of their tables is reproduced here; what follows is how each callout is built and where it stands today.
| System | Issuing body, region | How a size is designated | Status and notes |
|---|---|---|---|
| ISO 3601-1 | ISO, international (also BS ISO 3601-1:2012+A1:2019) | cross-section group letter A to E plus the ID in tenths of a mm (C0250 = 25.0 x 3.55) | current; 360 metric sizes on the 1.8, 2.65, 3.55, 5.3 and 7.0 mm cords |
| DIN 3771 | DIN, Germany | nominal ID x CS in mm (40 x 3.55) | withdrawn; DIN 3771-1:1984-12 was replaced by DIN ISO 3601-1:2010-08. Its Part 1 sizes are to ISO 3601-1, so an old German callout usually lands on an ISO size exactly |
| JIS B 2401 | JSA, Japan | series letter plus an index number close to, but not equal to, the ID in mm (P22 = 21.8 x 2.4); P dynamic, G static, V vacuum | status not verified here; P cords 1.9, 2.4, 3.5, 5.7 and 8.4 mm, G 3.1 and 5.7, V 4.0, 6.0 and 10.0 |
| BS 1806 | BSI, United Kingdom | dash number; the same dash numbers and dimensions as AS568 | identical with the Parker® 2-xxx series, which is the AS568 list |
| SAE AS568 | SAE, United States | three-digit dash number, a catalog lookup (-214 = 0.984 in ID x 0.139 in) | current; the inch system, 349 general-purpose sizes plus 20 boss sizes |
Common metric cross-sections and the AS568 cord they sit beside
The five ISO 3601-1 metric cords each sit within tolerance of an AS568 cord, so the cords interchange and the question is only the inside diameter. The JIS cords are a separate family, and so are the cords of two older national series that still turn up on prints, BS 4518 (United Kingdom) and SMS 1586 (Sweden), which the Parker handbook (section 8.2) lists at 1.6, 2.4, 3.0, 5.7 and 8.4 mm: a 2.4 or 5.7 mm ring is not an ISO or AS568 cord.
| Cross-section, mm | System and series | AS568 cord it sits beside, mm | Cords interchange? |
|---|---|---|---|
| 1.6 | BS 4518, SMS 1586 | none | separate family |
| 1.78 | AS568 0xx series (0.070 in) | 1.78 | the AS568 cord |
| 1.8 | ISO 3601-1 group A | 1.78 | yes, within tolerance |
| 1.9 | JIS B 2401 P series | 1.78 | marginal overlap |
| 2.4 | JIS B 2401 P series, BS 4518, SMS 1586 | 2.62 | no |
| 2.62 | AS568 1xx series (0.103 in) | 2.62 | the AS568 cord |
| 2.65 | ISO 3601-1 group B | 2.62 | yes, within tolerance |
| 3.0 | BS 4518, SMS 1586 | none | separate family |
| 3.1 | JIS B 2401 G series | none | no counterpart |
| 3.5 | JIS B 2401 P series | 3.53 | yes, within tolerance |
| 3.53 | AS568 2xx series (0.139 in) | 3.53 | the AS568 cord |
| 3.55 | ISO 3601-1 group C | 3.53 | yes, within tolerance |
| 4.0 | JIS B 2401 V series | none | no counterpart |
| 5.3 | ISO 3601-1 group D | 5.33 | yes, within tolerance |
| 5.33 | AS568 3xx series (0.210 in) | 5.33 | the AS568 cord |
| 5.7 | JIS B 2401 P and G series, BS 4518, SMS 1586 | 5.33 | no |
| 6.0 | JIS B 2401 V series | 6.99 | no |
| 6.99 | AS568 4xx series (0.275 in) | 6.99 | the AS568 cord |
| 7.0 | ISO 3601-1 group E | 6.99 | yes, within tolerance |
| 8.4 | JIS B 2401 P series, BS 4518, SMS 1586 | none | separate family |
| 10.0 | JIS B 2401 V series | none | no counterpart |
Nearest AS568 size for a metric callout: worked examples
Because the cords interchange, an AS568 size can stand in for an ISO 3601-1 metric size when the inside diameters are close enough. The rule used here comes from published stretch guidance: within 2 percent on inside diameter the two sizes are dimensionally close and the gland still has to be checked; 2 to 5 percent means the cord flattens and the groove must be recomputed; more than 5 percent is not a substitute. The examples are our own compilation from the two size lists, not a table from either standard.
| AS568 dash | AS568 ID x CS, mm | Nearest ISO 3601-1 | ISO ID x CS, mm | ID difference, % | Verdict |
|---|---|---|---|---|---|
| -006 | 2.90 x 1.78 | A0028 | 2.8 x 1.8 | -3.45 | check gland |
| -010 | 6.07 x 1.78 | A0060 | 6.0 x 1.8 | -1.15 | within 2 percent on inside diameter |
| -112 | 12.37 x 2.62 | B0125 | 12.5 x 2.65 | +1.05 | within 2 percent on inside diameter |
| -114 | 15.54 x 2.62 | B0160 | 16.0 x 2.65 | +2.96 | check gland |
| -206 | 12.29 x 3.53 | C0140 | 14.0 x 3.55 | +13.91 | not a substitute |
| -214 | 24.99 x 3.53 | C0250 | 25.0 x 3.55 | +0.04 | within 2 percent on inside diameter |
| -218 | 31.34 x 3.53 | C0315 | 31.5 x 3.55 | +0.51 | within 2 percent on inside diameter |
| -425 | 113.67 x 6.99 | E1150 | 115.0 x 7.0 | +1.17 | within 2 percent on inside diameter |
Across the 346 AS568 general-purpose sizes on the five standard cords (-001 to -003 sit below them), 255 have an ISO 3601-1 metric size within 2 percent, 21 fall in the check-gland band, 20 are not substitutes and 50 sit outside the reach of their cross-section group. Each AS568 size page carries its own dimensions; the metric size guide walks through reading and converting a callout.
How to specify a metric O-ring
Send the inside diameter and the cross-section in millimeters, or the ISO 3601-1 size code, exactly as the print shows them. Add the compound and hardness, the quantity, and the tolerance class or quality grade if the print calls one out. If the callout is an old DIN, JIS or BS number, send it as printed with the drawing and Conquest Seal reads it against the published size lists before quoting.
A metric size that has a close AS568 counterpart is quoted both ways when the two sizes are within 2 percent on inside diameter, so you can see both options side by side. The print governs; any change of size or standard is the buyer's engineering decision. A size that only exists on a JIS or BS 4518 cord is quoted as that size.
| Family | ASTM D1418 | Standard range, C | Typical media |
|---|---|---|---|
| Viton® | FKM | -26 to 204 | fuels, oils, heat |
| Buna-N | NBR | -34 to 100 | mineral oils, hydraulic fluid |
| EPDM | EPDM | -57 to 150 | water, steam, glycol; never petroleum oils |
| Silicone | VMQ | -54 to 204 | dry heat and cold, static service |
| FFKM | FFKM | -26 to 320 | aggressive chemistry and temperature beyond FKM (FFKM vs FKM) |
| HNBR | HNBR | -32 to 149 | oil with heat |
| Fluorosilicone | FVMQ | -73 to 177 | fuels at low temperature |
| Polyurethane | AU/EU | -40 to 82 | abrasion and high pressure |
| Aflas® | FEPM | -9 to 232 | steam, amines and sour gas; poor at low temperature |
| Butyl | IIR | -59 to 121 | gas impermeability, brake fluid and phosphate esters; never oil |
| Polyacrylate | ACM | -21 to 177 | hot transmission oil; poor at low temperature and in water |
| Ethylene-acrylic | AEM | -40 to 149 | hot automotive oil |
| SBR | SBR | -46 to 100 | the compound sheet governs |
| Natural rubber | NR | -50 to 70 | the compound sheet governs |
| Epichlorohydrin | ECO | -40 to 135 | the compound sheet governs |
Related pages
Larger and non-standard sizes: a metric O-ring beyond the ISO 3601-1 list, or on a diameter no list reaches, is supplied to order as a spliced and vulcanized O-ring in any compound family made as cord.
Frequently asked
What is a metric O-ring and how do I specify one?
A metric O-ring is sized in millimeters, by inside diameter and cross-section, on one of the metric size lists: ISO 3601-1 today, DIN 3771 and JIS B 2401 on older or Japanese drawings. Specify it as ID x CS in mm or as the ISO size code, add the compound, hardness and quantity, and quote the tolerance class or quality grade if the print names one.
Is a 25 x 3.55 O-ring the same as an AS568-214?
They are the nearest thing to the same size in the two systems: ISO 3601-1 C0250 is 25.0 mm ID on the 3.55 mm cord and AS568-214 is 24.99 mm ID on the 3.53 mm cord, so the inside diameters are 0.01 mm apart and the cords interchange within tolerance. The tolerances on the print decide whether the substitution is acceptable.
What does an O-ring 20 mm callout mean?
20 mm on its own is an inside diameter, not a size. Add the cross-section: 20 x 2.65 is the ISO 3601-1 size B0200, for example, and the same inside diameter can exist on other cords. The print or the groove gives the cross-section.
Why does my ISO 3601 chart show inch sizes?
It is most likely the AS568 inch series converted to millimeters. A chart that only shows 1.78, 2.62, 3.53, 5.33 and 6.99 mm cords is the inch series; the true ISO 3601-1 metric cords are 1.8, 2.65, 3.55, 5.3 and 7.0 mm.
Do metric O-rings come in the same compounds?
Yes. Nitrile, FKM, EPDM, silicone, HNBR, fluorosilicone, polyurethane and FFKM are supplied in metric sizes the same way as in AS568 sizes; the media and the temperature pick the family and the compound sheet sets the limit.
Related guides
Sources
SAE AS568 sizes as printed in Parker O-Ring Handbook ORD 5700, 2021 printing, section IX, pages 9-2 to 9-11, read 2026-09-27; ISO 3601-1 sizes from Precision Polymer Engineering, ISO 3601 O-ring Sizes reference table, read 2026-09-06; Parker O-Ring Handbook ORD 5700, 2021 printing, section IX, AS568 size tables, pages 9-2 to 9-11, read 2026-09-27; BS ISO 3601-1:2012+A1:2019 catalogue entry (BSI), https://landingpage.bsigroup.com/LandingPage/Standard?UPI=000000000030382295; DIN 3771-1:1984-12 catalogue entry, status withdrawn, replaced by DIN ISO 3601-1:2010-08 (DIN Media), https://www.dinmedia.de/en/standard/din-3771-1/1162892; Parker O-Ring Handbook ORD 5700, section 8.2, page 8-8 (BS 1806 identical with the Parker 2-xxx series; DIN 3771 Part 1 sizes to ISO 3601-1; BS 4518 and SMS 1586 cross-sections 1.6 / 2.4 / 3.0 / 5.7 / 8.4 mm); Precision Polymer Engineering, JIS B 2401 O-ring size chart (P, G and V series), https://www.prepol.com/documents/o-ring-sizes-jis-b-2401/; Parker O-Ring Handbook ORD 5700 compound family ranges. Dimensional facts are stated in our own words; no standard's text or table is reproduced.
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Send the metric callout as printed
ID x CS in millimeters or the ISO 3601-1 size code, the compound and the quantity. A Conquest representative replies with the quote, and with the AS568 counterpart when one is within 2 percent on inside diameter. Quotes answered within one business day. A person answers (714) 632-9830, Pacific business hours.
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