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.
Machining a groove? The Groove Charts page has depth, width, radii, and finish tables for every common gland style (piston, rod, face, dovetail, hydraulic, pneumatic, and floating) on one page.
Buying cord stock? Cord Tolerances covers the RMA/ARPM and ISO 3302-1 extrusion classes (E1, E2, E3), how much wider cord runs than a molded ring, and what material and durometer do to the number.
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
Master O-Ring Size Chart. For SAE J1926 ports, materials, and a
Boss-only chart, see
SAE J1926 ORB Boss O-Rings.
How to measure an O-Ring
You need two dimensions: ID (inside diameter) and CS (cross-section). Measure a used O-Ring with caution: swelling or compression set can change its dimensions. When possible, measure the groove instead.
Full guide covering calipers, pi-tape, size cones, slide gauges, lab vision systems, and why we recommend Olypsys® first: How to Measure an O-Ring.
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.
