AS568 is one of the most common size systems used to identify standard O-rings. Each O-ring is assigned a dash number, such as -214, that corresponds to a specific nominal inside diameter and cross section.

Need to identify a size quickly?

Use our searchable AS568 O-ring finder to search by dash number, inside diameter, outside diameter or cross section.

Open the O-Ring Size Finder

What Does an AS568 Dash Number Mean?

The dash number is simply the standard identifier for the O-ring. It does not directly describe the diameter in inches. Instead, the dash number points to a specific set of dimensions in the AS568 size chart.

For example, an AS568 -214 O-ring has a nominal inside diameter of approximately 0.984 in and a cross section of approximately 0.139 in.

Understanding ID, OD and Cross Section

Inside Diameter — ID

The inside diameter is the distance across the open center of the O-ring. This is usually the first dimension people measure when trying to identify an unknown O-ring.

Cross Section — CS

Cross section is the thickness of the O-ring itself. It is sometimes described as the cord diameter.

Outside Diameter — OD

Outside diameter is the full diameter from the outer edge on one side to the outer edge on the opposite side.

For a nominal O-ring size, OD can be calculated as:

OD = ID + (2 × Cross Section)

What Does “Actual Size” Mean?

When an O-ring chart lists the actual size, it is referring to the nominal physical dimensions associated with that dash number.

That typically includes:

  • Inside diameter
  • Cross section
  • Outside diameter

The dash number itself is only the identifier. The actual size is what tells you how large the O-ring is.

How to Measure an Unknown O-Ring

If you have an O-ring but do not know the dash number, measure the inside diameter and cross section as accurately as possible.

  1. Lay the O-ring flat without stretching it.
  2. Measure the inside diameter.
  3. Measure the cross-section thickness.
  4. Compare those dimensions with an AS568 size chart.

Calipers are usually more useful than a ruler when identifying smaller O-rings because small differences can separate one standard size from another.

Why Two O-Rings Can Look Almost the Same

Many AS568 sizes differ by only a small amount in inside diameter. Two O-rings can appear nearly identical by eye but still have different dash numbers.

This is why searching by an actual measured dimension can be faster than scrolling through a long size chart.

Common AS568 Cross-Section Families

Many standard AS568 sizes fall into common cross-section groups, including approximately:

  • 0.070 in
  • 0.103 in
  • 0.139 in
  • 0.210 in
  • 0.275 in

Knowing the cross section can quickly narrow the possible dash numbers.

Find the Correct O-Ring Size Faster

Instead of manually scanning hundreds of rows, use the Anco Precision O-ring finder to search by AS568 dash number or measured dimensions. The tool displays actual nominal ID, OD and cross section in both inches and millimeters.

Search AS568 O-Ring Sizes

Enter a dash number or dimension and instantly narrow the results.

Use the AS568 O-Ring Size Chart

O-ring dimensions shown in the finder are intended as a size identification reference. Verify current engineering requirements and applicable tolerances for critical sealing applications.