Direct answer: use a metric O-ring when the drawing or groove is specified by inner diameter and cross section in millimeters. Use an AS568 O-ring when the equipment documentation gives an AS568 dash number. A nearby size from the other system is not automatically interchangeable.
Start with the O-Ring Size Guide and How to Measure an O-Ring. Confirm the groove or original documentation whenever the old seal is swollen, flattened, stretched, or damaged.
Metric and AS568 sizing compared
| Factor | Metric O-rings | AS568 O-rings |
|---|---|---|
| How size is written | Nominal ID × cross section in millimeters | Dash number mapped to defined nominal dimensions |
| Examples | 20 × 2.5 mm; 30 × 3 mm | -011; -206; -214 |
| Best starting point | Metric drawing, groove, or measured nominal dimensions | AS568 dash number or inch-series equipment documentation |
| Main risk | Choosing a nearby dash size that is not equivalent | Guessing a dash number from a worn seal |
What a metric O-ring size means
A metric size such as 20 × 2.5 mm normally describes a nominal 20 mm inner diameter and 2.5 mm cross section. Approximate outer diameter is calculated as OD = ID + 2 × CS, but groove dimensions and required squeeze still need to be checked.
What an AS568 dash number means
AS568 uses dash numbers to identify standardized nominal dimensions. For example, AS568-206 corresponds to a nominal 12.29 mm ID × 3.53 mm cross section. The dash number identifies size; it does not confirm material, hardness, tolerance, certification, or application suitability.
Why a close size can still leak
A small change in cross section changes squeeze, installation force, friction, and extrusion risk. A small change in inner diameter can make the ring too loose or require excessive stretch. Do not convert between metric and AS568 by eye.
How to choose the right standard
- Use the original drawing, parts list, or equipment documentation first.
- Stay with metric sizing when dimensions are specified in millimeters.
- Stay with AS568 when a dash number is provided.
- Measure both ID and cross section when only an old seal is available.
- Check the groove when the old seal is deformed.
- Confirm material and hardness after the size system is identified.
Material and hardness are separate decisions
The same nominal size can be offered in several material families and hardness levels. Use the Material Guide, Chemical Compatibility Guide, and O-Ring Hardness Guide. Generic material guidance is not proof for a specific compound or regulated application.
Buying path
- Size, material, and hardness confirmed: browse small packs or search by exact size.
- AS568 number confirmed: browse AS568 O-rings.
- Drawing, unusual size, larger quantity, or uncertain application: use Bulk Quote.
FAQ
Are metric and AS568 O-rings interchangeable?
Not automatically. Compare the exact nominal dimensions, groove, stretch, and squeeze before substituting one system for the other.
Does an AS568 number identify material?
No. It identifies a size. Material, hardness, compound, and documentation must be confirmed separately.
What if the old O-ring is deformed?
Measure the groove and check the original equipment information. A used seal may no longer represent its original dimensions.
See the Technical Information, Sources & Review Policy for how OXXRing distinguishes general guidance from product-specific proof.
