O-rings are the most common sealing element in the world, yet sizing errors remain one of the top reasons buyers receive parts that do not fit. Because an O-ring is defined by just two dimensions-inner diameter and cross-section-and because several regional standards coexist, specifying the right size and tolerance is where most projects succeed or stall. This guide explains the standards buyers actually meet and how to order with confidence.
How an O-Ring Is Dimensioned
Every O-ring is specified by two measurements: the inner diameter (ID) and the cross-section (CS), sometimes called width or thickness. A size is written as ID × CS. The CS determines the seal's squeeze in the groove, while the ID must match the groove or shaft it sits against. Ordering by these two numbers-plus material and standard-removes ambiguity.
The Main Sizing Standards
Three standards cover the vast majority of global supply. Knowing which one your design uses prevents costly cross-standard mistakes.
| Standard | Unit Base | How Sizes Are Referenced | Common In |
|---|---|---|---|
| AS568 | Inch (imperial) | Dash numbers, e.g. -214 | USA, aerospace, fluid power |
| ISO 3601-1 | Metric (mm) | G-series & A-series tables | Europe, general industry |
| JIS B 2401 | Metric (mm) | P / G / V type codes | Japan, Asian supply chains |
A critical point: an AS568 dash size and a metric size that look similar are not interchangeable. Always state the standard when you order.
Understanding Tolerance Classes
Rubber shrinks slightly after molding and curing, so tolerances are wider than for metal parts. ISO 3601-1 defines four tolerance classes-A (tightest) through D (loosest)-while AS568 publishes its own fixed table per dash size. Tighter classes cost more and are rarely needed for static seals. The table below shows typical ISO 3601-1 Class A values; always consult the full standard for production.
| Dimension | Size Range | Class A Tolerance (±) |
|---|---|---|
| Inner Diameter | Up to 10 mm | 0.10 mm |
| Inner Diameter | 10–32 mm | 0.13–0.15 mm |
| Cross-Section | 1.0–1.8 mm | 0.08 mm |
| Cross-Section | 2.65–3.53 mm | 0.10 mm |
Hardness and Squeeze
O-rings are usually specified in Shore A hardness, commonly 70A for general NBR. Hardness affects both sealing and tolerance: softer compounds conform better to rough surfaces but may be harder to hold to tight IDs. Target squeeze matters too-roughly 15–30% of cross-section for static seals and 10–20% for dynamic applications. Groove dimensions should be taken from the same standard (AS568 or ISO 3601-2) used for the ring.
Five Mistakes Buyers Should Avoid
• Measuring a used ring. Stretched or compressed rings recover 3–5%; measure a relaxed new sample where possible.
• Mixing standards. An AS568 dash and a near-metric size are not the same part.
• Over-tolerancing. Specifying Class A everywhere adds cost without improving a static seal.
• Ignoring material shrinkage. Post-cure shrinkage varies by compound and must be built into tooling.
• Skipping the groove. A perfect ring fails in a wrongly sized groove; size both together.
What to Put in Your O-Ring RFQ
A complete O-ring inquiry prevents back-and-forth and tooling errors:
• ID and CS, or the standard dash / metric size code
• Standard referenced (AS568 / ISO 3601 / JIS B 2401)
• Material and Shore A hardness (e.g. NBR 70A, FKM 75A)
• Tolerance class required, if tighter than standard
• Quantity, color and any markings
• Application and media (oil, chemical, temperature, static vs dynamic)
Sourcing O-Rings That Actually Fit
Working with a manufacturer that masters both metric and imperial standards-and that controls shrinkage at the tooling stage-keeps your ID and CS within spec batch after batch. Explore our full O-ring and seal range, or send your drawing or sample for a matched quotation.
Need the right O-ring size?
Send us your ID, cross-section, material and standard-our team will confirm the closest standard size or quote a custom mold.
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