Reference & Charts
O-Ring Sizes (AS568)
Decode the AS568 o-ring dash-number system: each series (the 100s digit) fixes the cross-section, from 1/16" up to 1/4". Look up which series a dash number belongs to and its exact cross-section in inches and millimetres.
5 entries · Last verified: 2026-07-10
AS568 is the standard dash-number system for o-rings. The number encodes the size: the first digit (the series) sets the cross-section — the thickness of the ring — and the remaining digits index the inside diameter within that series. So every −2xx o-ring has the same 0.139" (1/8") cross-section; only the ID changes across −201 to −284. This table maps each series to its cross-section; the exact inside diameter for a specific dash number comes from a full AS568 chart.
| O-Ring Series | Dash range | Cross-section(in) | Cross-section(mm) | Nominal |
|---|---|---|---|---|
| −004 to −050 | 004–050 | 0.07 in | 1.78 mm | 1/16" |
| −102 to −178 | 102–178 | 0.103 in | 2.62 mm | 3/32" |
| −201 to −284 | 201–284 | 0.139 in | 3.53 mm | 1/8" |
| −309 to −395 | 309–395 | 0.21 in | 5.33 mm | 3/16" |
| −425 to −475 | 425–475 | 0.275 in | 6.99 mm | 1/4" |
Frequently asked
- What cross-section is a −214 o-ring?
- It's in the 200 series, so the cross-section is 0.139" (3.53 mm, nominal 1/8"). The '14' indexes the inside diameter within that series — look it up on a full AS568 chart for the exact ID.
- Does the dash number tell me the inside diameter?
- Not directly. The series (first digit) gives the cross-section; the remaining digits index the ID within that series, but you need an AS568 ID chart to get the actual inside-diameter value. This table covers the cross-section half of the code.
- Which o-ring size is most common?
- The 200 series (1/8" cross-section) is the most widely used general-purpose range, followed by the 100 series (3/32"). Larger cross-sections (300, 400) are for bigger glands and diameters.
Sources
- SAE AS568 — aerospace size standard for o-rings (cross-section series)