How to Measure an O-Ring: ID, Cross-Section and AS568 Sizes
Measure the inside diameter and cord cross-section, and this tool returns the outside diameter and the nearest AS568 standard cross-section group — plus how to measure correctly and when a ring is metric or custom.
Built for: identifying unknown O-rings, writing replacements into specs, and checking rings against groove drawings.
O-Ring Measurement Calculator
How it works — formula, worked example & reference
Formula: OD = ID + 2 × CS. The cross-section is measured once but adds to the diameter on both sides of the ring.
Worked example: ID = 1.049 in, CS = 0.139 in: OD = 1.049 + 0.278 = 1.327 in. The 0.139 in cross-section is the AS568 1/8″ nominal group.
| AS568 cross-section group | Nominal | Actual CS |
|---|---|---|
| -0xx series | 1/16″ | 0.070 in |
| -1xx series | 3/32″ | 0.103 in |
| -2xx series | 1/8″ | 0.139 in |
| -3xx series | 3/16″ | 0.210 in |
| -4xx series | 1/4″ | 0.275 in |
Dash-number ID lookups come from the AS568 standard table; confirm final sizes against the standard or the mating groove drawing.
These results are engineering estimates for preliminary material selection; final validation is the responsibility of your engineering team.
Common questions
How do I measure an O-ring?
Measure the inside diameter and the cross-section (thickness of the cord). The outside diameter follows as OD = ID + 2×CS. Measure an unstretched ring, ideally with calipers on a flat surface.
What are standard O-ring sizes?
In the US, AS568 defines standard dash sizes in five cross-section groups: 1/16″ (0.070), 3/32″ (0.103), 1/8″ (0.139), 3/16″ (0.210) and 1/4″ (0.275). The calculator snaps your measured cross-section to the nearest group.
Can I use this for metric O-rings?
The geometry is identical — enter millimeters consistently and read the result in millimeters. Metric standards (ISO 3601) use different standard cross-sections: 1.78, 2.62, 3.53, 5.33 and 6.99 mm.
What if my ring is square-cut or a custom gasket?
The OD = ID + 2×CS relationship holds for any ring of uniform cross-section. For die-cut flat gaskets, send the drawing — flat gaskets are H-O’s core work.