Asking a seal supplier for an O-ring without knowing which size standard you are working with is like ordering a bolt without knowing the thread form. Three systems dominate the market: AS568 (inch, originally US), ISO 3601 metric (millimetres, European/global), and JIS B 2401 (Japanese metric). Equipment built in different regions tends to use different standards, and the three overlap without being identical.
This guide explains how each chart is organized, shows the five cross-section families they share, and gives you a practical procedure for finding a replacement. We stock all three standards in our O-ring product range.
All three standards describe a ring the same way — by ID (inside diameter), OD (outside diameter), and CS (cross-section), where CS = (OD − ID) / 2. The cross-section is the dimension that determines which groove family the ring fits; the ID determines which dash or metric size you pick within that family.
| Standard | Units | Typical Region / Origin | How sizes are designated |
|---|---|---|---|
| AS568 | Inch | USA; now a de facto global standard | Dash number, e.g. -214 |
| ISO 3601-1 (metric) | Millimetres | Europe and worldwide general industry | ID × CS, e.g. 20 × 3.55 |
| JIS B 2401 | Millimetres | Japanese machinery and automotive | P / G / V series + number, e.g. P20 |
An AS568 dash number is a hyphen followed by three digits. The hundreds digit tells you the cross-section family, and the last two digits identify the inside diameter. The five radial-seal families are:
| Series | Cross-section (in) | Cross-section (mm) | Typical use |
|---|---|---|---|
| -0XX | 0.070 in | 1.78 mm | Small instruments, compact fittings |
| -1XX | 0.103 in | 2.62 mm | Small hydraulic/pneumatic seals |
| -2XX | 0.139 in | 3.53 mm | The most common general-purpose family |
| -3XX | 0.210 in | 5.33 mm | Medium hydraulic cylinders, valves |
| -4XX | 0.275 in | 6.99 mm | Heavy-duty, larger grooves |
The -5XX to -8XX families reuse these same four cross-sections in larger-diameter face-seal arrangements, and the -9XX family covers very large diameters at 0.139 in cross-section. A few representative entries from the chart:
| Dash no. | ID (in) | CS (in) | ID (mm) |
|---|---|---|---|
| -006 | 0.114 | 0.070 | 2.90 |
| -010 | 0.239 | 0.070 | 6.07 |
| -111 | 0.487 | 0.103 | 12.37 |
| -214 | 0.984 | 0.139 | 24.99 |
Within each family, dash numbers ascend in fixed ID steps. You cannot always hit an exact target ID, so you pick the nearest dash and verify stretch and squeeze — the procedure explained in our O-Ring Design Reference series.
Metric O-rings are written as ID × CS in millimetres. The standard cross-sections are the metric counterparts of the AS568 families:
| Metric CS | 1.80 | 2.65 | 3.55 | 5.30 | 7.00 |
|---|---|---|---|---|---|
| Approx. inch equivalent | 0.070 | 0.103 | 0.139 | 0.210 | 0.275 |
| Typical metric sizes (ID × CS) | 8 × 1.80 | 12 × 2.65 | 20 × 3.55 | 40 × 5.30 | 80 × 7.00 |
Metric IDs run in incremental steps that do not line up exactly with AS568 IDs, so a metric ring and an inch ring that share a cross-section are not always interchangeable in the same groove.
JIS organizes sizes by duty rather than by cross-section: P-series for general reciprocating and rotating service, G-series for fixed (face) seals, and V-series for vacuum/flange service. Like metric ISO sizes, JIS rings are specified in millimetres and use the same 1.9 / 2.4 / 3.1 / 3.5 / 5.7 / 8.4 mm cross-section steps in their own tables. When replacing seals on Japanese-origin machines, expect to encounter JIS first.
A dash number, a metric size, and a JIS code can describe near-identical rings — but near-identical is not identical. When in doubt, send your old ring or a drawing to the manufacturer and let them cross-reference all three charts.
We supply AS568, metric and JIS O-rings in NBR, FKM, EPDM and silicone from stock, plus custom dimensions from 1 mm to large sizes. Browse the O-ring range or contact us with your size for a quick cross-reference and quote.