AS568 vs ISO 3601: two ways to call out the same ring
O-rings are catalogued under two dominant standards. AS568 (SAE AS568B) is the inch-based North American standard that assigns each ring a three-digit dash number — for example, a -214 or a -325. ISO 3601-1 is the international metric standard that lists each ring directly as its cross-section by its inside diameter in millimetres. The two systems overlap dimensionally for many common sizes, but their tolerance classes and naming conventions are different, so the standard printed on your drawing matters.
Cross-section series and the dash number
Within AS568, the first digit of the dash number tells you the cross-section series. The 0xx series uses a 1.78 mm (0.070 in) cross-section, 1xx uses 2.62 mm, 2xx uses 3.53 mm, 3xx uses 5.33 mm and 4xx uses 6.99 mm; the three smallest sizes (-001 to -003) use thinner sections. The remaining two digits step the inside diameter up within that fixed cross-section. Once you know the series, you can read a size purely from the number — a -2xx is always a 3.53 mm ring.
Reading inside, cross-section and outside diameter
Three numbers fully describe an O-ring: the inside diameter (ID), the cross-section or thickness (CS), and the outside diameter (OD). They are linked by a single formula — OD = ID + 2 × CS — so any one can be derived from the other two. This finder lets you search by code, filter by standard, or enter any combination of ID, CS and OD; matches are returned within a ±0.5 mm window, with the computed OD and published tolerances shown for each row. When you have the right size, send it straight to our team for a material recommendation and quote.