Reference guide
AS568 Dash Numbers Explained
How the three-digit AS568 dash number encodes an O-ring's cross-section and inside diameter, what the 2-XXX and mil-spec prefixes add, and how to find yours.
An AS568 dash number is a catalog lookup, not a formula: each three-digit number maps to exactly one nominal inside diameter and one cross-section. The first digit tells you the cross-section family (0 for 1/16 in, 1 for 3/32 in, 2 for 1/8 in, 3 for 3/16 in, 4 for 1/4 in, 9 for the tube-fitting boss series), and within a family a higher number means a larger inside diameter. AS568-214, for example, is a 1/8 in cross-section ring (0.139 in, 3.53 mm) with a 0.984 in (24.99 mm) inside diameter. The current revision, SAE AS568F, defines 349 general-purpose sizes plus 20 boss seals, 369 in all, from a 0.029 in inside diameter at dash -001 to 25.94 in at the top of the 3/16 in and 1/4 in families. This guide decodes the number, the prefixes other makers put in front of it, and the mil-spec part numbers that carry it.
The dash-number families
Counts and ranges below are Conquest's own tally of the SAE AS568 dimensional data; the tolerances widen with diameter, which is why the range is given rather than one figure.
| Dash range | Sizes | Nominal CS | Actual CS (in / mm) | Inside diameters (in) | ID tolerance (in) |
|---|---|---|---|---|---|
| -001 to -050 | 50 | 1/16 in | 0.070 / 1.78 (below -004: 0.040, 0.050, 0.060) | 0.029 to 5.239 | 0.004 to 0.037 |
| -102 to -178 | 77 | 3/32 in | 0.103 / 2.62 | 0.049 to 9.737 | 0.005 to 0.055 |
| -201 to -284 | 84 | 1/8 in | 0.139 / 3.53 | 0.171 to 17.955 | 0.005 to 0.085 |
| -309 to -395 | 87 | 3/16 in | 0.210 / 5.33 | 0.412 to 25.940 | 0.005 to 0.120 |
| -425 to -475 | 51 | 1/4 in | 0.275 / 6.99 | 4.475 to 25.940 | 0.033 to 0.120 |
| -901 to -932 | 20 | tube-fitting boss series | 0.056 to 0.118 by size / 1.42 to 3.00 | 0.185 to 2.337 | 0.005 to 0.018 |
Outside diameter is never in the callout; it is the inside diameter plus twice the cross-section. Note the gaps in the numbering: there is no -051 to -101, no -179 to -200, no -285 to -308 and no -396 to -424. A dash number in one of those gaps is not an AS568 size, though BS 1806 does use some of them for its own "between" sizes. The 1/16 in family begins at -004; the three sizes below it (-001, -002, -003) sit on smaller cords of their own. The full table with tolerances is the AS568 size chart.
Worked examples: reading five callouts
| Dash | Family | ID (in / mm) | CS (in / mm) | ID tolerance (in / mm) | Metric neighbor |
|---|---|---|---|---|---|
| -001 | smallest, below 1/16 in | 0.029 / 0.74 | 0.040 / 1.02 | 0.004 / 0.10 | none; no ISO metric cord this small |
| -010 | 1/16 in | 0.239 / 6.07 | 0.070 / 1.78 | 0.005 / 0.13 | ISO A0060, 6.0 x 1.8 mm, within 2 percent |
| -112 | 3/32 in | 0.487 / 12.37 | 0.103 / 2.62 | 0.005 / 0.13 | ISO B0125, 12.5 x 2.65 mm, within 2 percent |
| -214 | 1/8 in | 0.984 / 24.99 | 0.139 / 3.53 | 0.010 / 0.25 | ISO C0250, 25.0 x 3.55 mm, within 2 percent |
| -325 | 3/16 in | 1.475 / 37.47 | 0.210 / 5.33 | 0.015 / 0.38 | ISO D0375, 37.5 x 5.3 mm, within 2 percent |
| -425 | 1/4 in | 4.475 / 113.67 | 0.275 / 6.99 | 0.033 / 0.84 | ISO E1150, 115 x 7 mm, within 2 percent |
| -901 | boss series | 0.185 / 4.70 | 0.056 / 1.42 | 0.005 / 0.13 | none; metric fittings use ISO 6149 |
The metric neighbors come from a size-by-size comparison of the AS568 list against the ISO 3601-1 metric list, computed for this site; 254 of the 349 general-purpose sizes have an ISO metric ring within 2 percent on the same cord. The metric O-ring sizes guide explains the systems.
What do the Parker® 2-XXX and other prefixes mean?
Parker's 2-XXX numbers are the AS568 sizes with a prefix (2-214 is AS568-214); 3-XXX are boss gaskets, 5-XXX are Parker specials outside the standard, and 8-XXX are military cross-references. A full Parker callout adds the compound, for example 2-214 N0674-70: N for nitrile, 0674 the compound number, 70 the Shore A durometer. Other makers use the same dash number with their own compound codes, so the dash size is the part of the callout that travels between suppliers. The 2-XXX to AS568 cross-reference chart lists the sizes with the metric conversion.
Which mil-spec part numbers carry the same dash size?
Military O-ring part numbers are the AS568 dash number with a material specification in front of it. The size is identical; the prefix tells you the compound and the fluid it was qualified for. Conquest's mil-spec chart and the Parker handbook spec summary give the pairings below.
| Part number form | Material specification | Compound | Qualified fluid or service | Size system |
|---|---|---|---|---|
| MS28775-XXX | MIL-P-25732, now AMS-P-25732 | Nitrile, 70 Shore A | MIL-H-5606 hydraulic fluid | AS568 dash |
| M83248/1-XXX | MIL-R-83248 / AMS-R-83248, Type I Class 1 | Fluorocarbon (FKM), 75 Shore A | High-temperature fluids | AS568 dash |
| M83248/2-XXX | MIL-R-83248 / AMS-R-83248, Type I Class 2 | Fluorocarbon (FKM), 90 Shore A | High-temperature, high-pressure | AS568 dash |
| AS3209-XXX | AMS 7276 | Fluorocarbon (FKM), 75 Shore A | A common aerospace Viton® callout | AS568 dash |
| M25988/1-XXX | AMS-R-25988 | Fluorosilicone, 70 Shore A | Fuel systems | AS568 dash |
| MS28778-XXX | AMS-P-5510 | Nitrile, 90 Shore A | Straight-thread boss fittings | boss series, its own numbering |
The mil-spec O-ring chart lists the specifications, and the M83248 and AMS7276 guides cover two common fluorocarbon callouts. Note the Parker handbook's own caution: a specification's temperature limits do not necessarily represent the operating limits of the seal.
How do I find the dash number of an O-ring I have?
Measure the inside diameter and the cross-section of an unused ring with calipers, taking the cross-section in two places. The cross-section alone narrows it to one family; the inside diameter picks the number off the chart. A metric callout with a cross-section of 1.78, 2.62, 3.53, 5.33 or 6.99 mm is an AS568 size expressed in millimeters. Sizes between the standard steps are custom or spliced rings, and a ring that measures oversize is usually a used one that has swollen or taken a set (see the compression set guide). Send the two measurements with your quote request and we identify the size.
Viton is a trademark of The Chemours Company; Parker is a trademark of Parker Hannifin Corporation. No affiliation or endorsement is implied.
Sources
- Parker O-Ring Handbook ORD 5700, section 2, read 2026-09-14
- AMS 7276
- AMS-P-25732
- AMS-P-5510
- AMS-R-25988
- AMS-R-83248
- AS3209
- AS568
- BS 1806
- ISO 3601-1
- ISO 6149
- MIL-H-5606
- MIL-P-25732
- MIL-R-83248
- MS28775
- MS28778
Reviewed by Grant Midstokke, Conquest Seal Corporation. Last updated September 2026. Spec values in this guide are drawn from the sources named in the sentence where each appears; the compound datasheet and your application conditions govern the final choice.
Frequently asked questions
Does the dash number tell me the material?
No. AS568 defines dimensions only. The material comes from the compound code or the material specification in front of the dash number.
What is the difference between AS568-214 and 2-214?
None in size. 2-214 is Parker's catalog prefix for the AS568-214 size; other makers use their own prefixes with the same three digits.
Is AS568B still current?
The revision letter has moved on; the current document is SAE AS568F. Drawings that say AS568A or AS568B call out the same dash sizes, which is why old prints still work.
Why is there no -100 or -200 size?
The families do not use every number. The 3/32 in family starts at -102 and the 1/8 in family at -201; the unused numbers are simply not sizes.
Are the -9xx boss seals in the metric standards?
No. ISO 3601-1 folded in the AS568 general-purpose sizes but excludes the boss-seal series (-901 to -932); metric fitting O-rings are covered by ISO 6149.
How much does the tolerance change across the chart?
It grows with the inside diameter. A -006 is 0.114 in inside diameter plus or minus 0.005 in, a -214 is 0.984 in plus or minus 0.010 in, a -349 at 4.475 in carries plus or minus 0.030 in, and a -395 at 25.94 in carries plus or minus 0.12 in. The cross-section tolerance is steadier: plus or minus 0.003 in on the 0xx and 1xx sections, 0.004 in on 2xx, 0.005 in on 3xx and 0.006 in on 4xx.
What are the -001, -002 and -003 sizes?
Three sizes below the 0xx cord with their own cross-sections: -001 is 0.029 in inside diameter with a 0.040 in section, -002 has a 0.050 in section and -003 a 0.060 in section. They have no ISO 3601 metric counterpart, so a metric drawing cannot call them out by equivalent.
What does a -9xx dash number look like in dimensions?
The 20 boss gaskets carry sections that match the tube-fitting port rather than the five general-purpose cords: -901 is 0.185 in inside diameter with a 0.056 in section, -910 is 0.755 in with a 0.097 in section, and -932 is 2.337 in with a 0.118 in section. The last two digits follow the tube size, so a -910 seals a 5/8 in port and a -932 a 2 in port.
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