Knowledge · Cross Section
O-Ring Size Chart Guide: How to Match ID and Cross Section
How to match an O-ring by inner diameter and cross section, use OD as a check, and compare metric and AS568 sizing.
Direct answer: match an O-ring by inner diameter (ID) and cross section (CS). Use outer diameter (OD) as a check value: OD = ID + 2 × CS. Do not choose from OD alone, because different ID and CS combinations can produce the same outside diameter.
How O-ring sizes are written
A metric size such as 10.00 × 2.00 mm normally means 10.00 mm ID and 2.00 mm CS. Approximate OD is 14.00 mm.
Use this matching order
- Measure ID without stretching the ring.
- Measure CS gently in several places.
- Calculate or measure OD as a check.
- Compare with a standard size or original drawing.
- Confirm material and hardness separately.
- Choose the appropriate buying or review path.
Use the measurement guide for detailed instructions.
What if the measurement is close but not exact?
A used O-ring may swell, shrink, flatten, stretch, harden, or crack. Treat a damaged seal as a clue and compare it with the groove, original specification, drawing, part number, or an unused matching seal.
Use What to Do If You Don't Know Your O-Ring Size when the old seal is unreliable.
Metric vs AS568
Metric sizes are normally shown directly as ID × CS in millimetres. AS568 uses dash numbers mapped to defined inch-series nominal dimensions. A nearby metric and AS568 size are not automatically interchangeable.
Review Metric vs AS568 O-Rings or the AS568 dash-size guide.
Material and hardness still matter
The same nominal size can be offered in NBR, FKM, EPDM, silicone, FFKM, and different hardness levels. Use the Material Guide, Chemical Compatibility Guide, and Hardness Guide.
Choose the buying path
- ID, CS, material, and hardness confirmed: use the finder or browse small packs.
- Size or material uncertain: use the guides before ordering.
- Drawing, sample, unusual size, special material, critical application, or larger quantity: use Custom O-Ring Quote.
Information to send for review
- ID, CS, and OD if measured
- Metric or inch units
- AS568 or part number if known
- Material and hardness if known
- Fluid, temperature, pressure, and movement
- Quantity
- Photo, drawing, sample, or groove dimensions
FAQ
Is O-ring size always ID × CS?
Most metric listings use ID × CS. Confirm the listing format when using another standard or supplier.
Can OD identify the correct O-ring?
Not by itself. Use ID and CS as the primary dimensions.
Why does a used ring not match the chart?
Service exposure and permanent deformation may have changed its dimensions.
What if two listed sizes are close?
Compare the groove, original specification, stretch, squeeze, material, and operating conditions instead of choosing by appearance.
