O-Ring Size Charts & Standards (AS568 & Metric)
AS568 and metric O-Ring size charts, dash numbers, groove design, and how to measure an existing seal.
Looking for a dash number, ID × CS, or back-up ring dimensions? Use the Master O-Ring Size Chart (under Sizing in the nav). For anti-extrusion sizes, select the AS568 Backup standard chip on that chart.
What is AS568?
AS568 is the SAE Aerospace Standard that defines inch-based O-Ring sizes used
throughout North America. Each size has a dash number (e.g. -214) that encodes
the cross-section series and the inside diameter.
The five cross-section series
| Series | Dash range | Cross-section (in) | Cross-section (mm) |
|---|---|---|---|
| 0xx | -001 to -0xx | 1/16 (.070) | 1.78 |
| 1xx | -102 to -178 | 3/32 (.103) | 2.62 |
| 2xx | -201 to -284 | 1/8 (.139) | 3.53 |
| 3xx | -309 to -395 | 3/16 (.210) | 5.33 |
| 4xx | -425 to -475 | 1/4 (.275) | 6.99 |
Within each series, the dash number increases with inside diameter. A -214, for example,
is a 1/8" cross-section with a 1" inside diameter. Tube-fitting boss seals (900 series,
AS568-901…932) are a separate filter — ORB Boss — in the
How to measure an O-Ring
You need two dimensions:
- ID (inside diameter): measured across the inside of the ring
- CS (cross-section): the thickness of the cord itself
Measure a used O-Ring with caution: swelling or compression set can change its dimensions. When possible, measure the groove instead and pick the size from the groove dimensions.
Metric sizes
Metric O-Rings are specified directly as ID x CS in millimeters (e.g. 25 x 3).
Common metric cross-sections are 1.5, 2, 2.5, 3, 3.5, 4, 5, and 6 mm. Japanese (JIS) and
European (ISO 3601) standards each define their own preferred series.
Groove design (static seals)
The O-Ring doesn’t seal on its own, it seals because the groove squeezes it. Two rules govern the groove:
- Squeeze it, but leave room. The groove is machined shallower than the cord so the O-Ring is compressed (squeezed) against the sealing face. But it must also be wider than the cord, because the groove volume has to exceed the O-Ring volume.
- Fill no more than ~90%. The largest ring should fill no more than 90% of the smallest groove. That headroom lets the elastomer expand from heat and from swelling in the sealed media without over-filling the groove and pushing itself out.
Guideline groove dimensions, axial (face) static seals
Straight from the Freudenberg standard, matched to the five AS568 cross-sections:
| Cord Ø (in) | Groove depth h (in) | Groove width b (in) | Corner radius r (in) | Resulting squeeze |
|---|---|---|---|---|
| 0.070 (1/16") | 0.047 | 0.106 | 0.012 | ~33% |
| 0.103 (3/32") | 0.075 | 0.142 | 0.012 | ~27% |
| 0.139 (1/8") | 0.106 | 0.181 | 0.024 | ~24% |
| 0.210 (3/16") | 0.165 | 0.260 | 0.024 | ~21% |
| 0.275 (1/4") | 0.228 | 0.323 | 0.039 | ~17% |
Note how percentage squeeze is higher for thin cords and eases off for fat ones, small cross-sections need proportionally more squeeze to seal reliably. Radial grooves (piston and rod seals) use slightly different depth/width values; ask us for the radial figures if you’re designing a dynamic gland.
Surface finish
The groove and sealing faces must be smooth enough not to leak, but not so polished that a dynamic seal has nothing to hold lubricant:
| Surface | Static, constant pressure | Static, pulsating | Dynamic |
|---|---|---|---|
| Sealing (contacted) face | Ra ≤ 63 µin (1.6 µm) | Ra ≤ 32 µin (0.8 µm) | Ra ≤ 16 µin (0.4 µm) |
| Groove base & flanks | Ra ≤ 125 µin (3.2 µm) | Ra ≤ 63 µin (1.6 µm) | Ra ≤ 32 µin (0.8 µm) |
Groove and surface-finish values: Freudenberg FST 2025 Technical Manual, ch. 6 (Static Seals), §8, pp. 613–620.
