O-Ring Size Standards
The three size systems that cover most O-ring specifications worldwide — AS568, ISO 3601 and GB/T 3452.1 — and how to pick between them.
Three systems, two families
Most standard O-rings belong to one of two dimension families. The inch-based AS568 dash sizes use cross-sections of 1.78, 2.62, 3.53, 5.33 and 6.99 mm. The metric G-series shared by ISO 3601 and GB/T 3452.1 uses 1.8, 2.65, 3.55, 5.3 and 7.0 mm. The two families sit deliberately close together, but they are not interchangeable by default — groove design and tolerances differ.
If your machine heritage is North American or aerospace, you will usually meet AS568 dash numbers. European and Chinese fluid-power designs usually reference ISO 3601 or GB/T 3452.1. When in doubt, measure the ring, then let the size recommendation step in our calculators show the nearest candidates from every system with the exact deviation.
Standard guides
AS568 O-Ring Size Guide
Inch-based dash sizes: series, cross-sections and common sizes with metric conversions.
ISO 3601 O-Ring Size Guide
The international metric G-series: preferred inside diameters and cross-sections.
GB/T 3452.1 O-Ring Size Guide
The Chinese national standard, dimensionally aligned with the ISO 3601 G-series.
Match a size automatically
Every groove calculator includes an optional standard-size recommendation step: enter a target ID, cross-section or OD and get the closest candidates from all three standards with per-dimension deviations.
Data sources & verification status
All size entries are curated subsets marked as pending verification against the current standard editions. Counts:
- AS568 — 现行修订版待核对（常见引用为 AS568D） · 60 entries
- ISO 3601 — ISO 3601-1（版本待核对） · 70 entries
- GB/T 3452.1 — GB/T 3452.1-2005 · 70 entries
Standard size data is for preliminary engineering selection only. Final purchasing and design should be verified against the applicable standard edition, supplier data sheets and actual operating conditions.