AS568 O-Ring Size Chart — Dash Numbers

O-Ring Size Chart (AS568)

Complete AS568 rubber o-ring size chart with all 400+ standard dash numbers. Look up inner diameter (ID), outer diameter (OD), and cross-section width in inches or millimeters. Filter by cross-section series, search by dash number or dimension, and toggle for a metric o-ring size chart in mm.

AS568 standard o-ring sizes per ANSI/SAE AS568B

Source: ANSI/SAE AS568B, Aerospace Size Standard for O-Rings. Groove dimensions: Machinery’s Handbook 29th Ed., Table 2, pp. 2589–2590 (Auburn Manufacturing Co.).

O-Ring Dimension Diagram ID OD OD CS

How to Use This O-Ring Size Chart

Select a cross-section series from the dropdown to see all dash numbers for that ring width, or type a dash number, ID, or OD value into the search box to find a specific size. Click any column header to sort the table by that value. The outer diameter is calculated as OD = ID + 2 × CS. Use the unit toggle to switch between inches and millimeters. Click “More Cols” to reveal recommended groove dimensions for static and reciprocating seals.

This chart shows actual AS568 dimensions, not nominal fractions. Many references label cross-sections with approximate fractions (e.g., “1/16″” for the zero series), but the actual standard CS is 0.070″ — not 0.0625″. The nominal fractions are legacy labels; always use the actual decimal values shown here for groove design and procurement specifications.

To measure an existing o-ring, lay it flat without stretching and measure the inside distance across the center (ID) and the cross-section thickness (CS), as shown in the diagram above. Match these measurements to the closest standard dash number in the table. An o-ring that has been in service may have taken a compression set and measure slightly smaller than its nominal size.

O-Ring Dimension Formula

OD = ID + 2 × CS

where:
OD = outer diameter [length] — e.g. in, mm
ID = inner diameter [length] — e.g. in, mm
CS = cross-section width (ring thickness) [length] — e.g. in, mm

Geometric relationship per ANSI/SAE AS568B. See Machinery’s Handbook 29th Ed., p. 2587, Fig. 2 for ring cross-section diagram.

O-Ring Squeeze and Groove Design

The initial squeeze — the difference between the cross-section width and the gland depth — is the most important factor in o-ring seal performance. The gland depth is the groove depth plus the radial clearance gap. The ideal squeeze is approximately 20% of the cross-section, meaning the gland depth should be about 80% of the ring cross-section width. The groove width varies from about 1.3× to 2.0× the cross-section width depending on ring size, to allow for thermal expansion and fluid swell.

Gland Depth by Cross-Section (from MH29 Table 1)

CS (in) Reciprocating Gland Depth (in) Static Gland Depth (in)
0.070 0.055 – 0.057 0.050 – 0.052
0.103 0.088 – 0.090 0.081 – 0.083
0.139 0.121 – 0.123 0.111 – 0.113
0.210 0.185 – 0.188 0.170 – 0.173
0.275 0.237 – 0.240 0.226 – 0.229

Source: Machinery’s Handbook 29th Ed., p. 2588, Table 1 (Auburn Manufacturing Co.)

O-ring grooves require tight tolerances on groove depth and width to achieve the correct squeeze. CNC machining is the standard method for cutting o-ring glands to the dimensions in Table 2, with typical tolerances of ±0.001 inch on groove depth and width for reliable sealing. Pi Fabricators regularly machines o-ring grooves into large, often custom low-volume pneumatic actuators for the wood products industries.

Note on 900-series: Dash numbers 901–932 are o-ring boss (ORB) gaskets designed for straight-thread tube fittings. These o-rings seat in a groove that is machined into the fitting boss itself — not into your part. The groove dimensions for ORB ports are defined by SAE J1926-1, not by the o-ring sizing standard, which is why groove data is not shown for the 900-series in this chart.

Example: Selecting an O-Ring for a 1-Inch Bore

You need a static face seal for a hydraulic manifold with a 1.000-inch bore. You want a 200-series o-ring (0.139″ cross-section) because the groove has room for this standard CS. Filter the table by “201–284 (0.139″ CS)” and look for an ID near 1.000 inches. Dash number 214 (ID 0.984″, OD 1.262″) is the closest standard size. With a static gland depth of 0.111–0.113 inches (per Table 1 above), the squeeze on this ring is approximately 19–20% — right in the ideal range.

Frequently Asked Questions

What is an AS568 o-ring dash number?

An AS568 dash number is a standardized size designation defined by ANSI/SAE AS568B (formerly AS568A). Each dash number uniquely identifies an o-ring by its inner diameter (ID) and cross-section width. For example, dash number 230 specifies an o-ring with a 2.484-inch ID and 0.139-inch cross-section. The numbering is organized into series by cross-section: 001–050 (0.070″ CS), 102–178 (0.103″ CS), 201–284 (0.139″ CS), 309–395 (0.210″ CS), 425–475 (0.275″ CS), and 900-series (o-ring boss gaskets for straight-thread tube fittings per SAE J1926-1, with varying cross-sections).

How do I measure an o-ring to find its size?

Measure the inner diameter (ID) and cross-section width (CS) of the o-ring. Lay the o-ring flat without stretching and measure the inside distance across the center for the ID. For the cross-section, measure the thickness of the ring material itself. Then look up the closest AS568 dash number matching both dimensions. The outer diameter can be calculated as OD = ID + 2 × CS. If the o-ring has been in service, it may have taken a compression set and measure slightly smaller than nominal — compare against the standard size chart rather than assuming the measured value is exact.

How do I convert o-ring sizes to metric (mm)?

AS568 sizes are defined in inches. To convert to millimeters, multiply the inch dimension by 25.4. For example, dash number 210 has an ID of 0.734 inches = 18.64 mm and a cross-section of 0.139 inches = 3.53 mm. Use the unit toggle on this chart to switch between inches and millimeters. For metric-native o-ring sizes, the ISO 3601 standard defines sizes in millimeters directly — these are not always interchangeable with AS568 sizes.

What is the recommended o-ring squeeze percentage?

The ideal o-ring squeeze is approximately 20% of the cross-section width — meaning the gland depth (groove depth plus radial clearance) is about 80% of the o-ring cross-section. For reciprocating seals, squeeze ranges from about 10% to 21%, and for static seals from about 17% to 29%, depending on cross-section size (per Machinery’s Handbook Table 1). The groove width should be approximately 1.3× to 2.0× the cross-section width depending on ring size. Exact groove dimensions depend on the application type and are specified in gland design standards.

How do I calculate o-ring groove dimensions?

O-ring groove dimensions depend on the seal type (static, reciprocating, or rotary) and the o-ring cross-section. For a static face seal with a 0.139-inch CS o-ring (200-series), a typical gland depth is 0.111 to 0.113 inches (about 80% of CS) and groove width is 0.188 inches (about 1.35× CS). Diametral clearance should be 0.002 to 0.006 inches depending on system pressure. The Machinery’s Handbook provides detailed gland depth and groove width tables for all standard cross-sections per AS568.

Can I download this o-ring size chart as a PDF?

Yes. Click the Print / PDF button above the table to open your browser’s print dialog, then select “Save as PDF” as the destination. The print stylesheet automatically hides the interactive controls and formats the table for paper. You can filter by cross-section series first to print only the sizes you need.

What size o-ring does an SAE o-ring boss (ORB) fitting use?

SAE straight-thread o-ring boss (ORB) fittings per SAE J1926 use standard AS568 dash numbers. The o-ring sits in a groove on the fitting body and seals against a flat face on the port. Common ORB fitting sizes use dash numbers in the 100- and 200-series. For example, a -4 SAE ORB fitting (7/16″-20 thread) typically uses a dash 904 or equivalent o-ring. Consult the fitting manufacturer’s catalog for the specific dash number required for each port size.

Related Calculators

References

  • ANSI/SAE AS568B — Aerospace Size Standard for O-Rings. Defines all standard dash numbers, nominal dimensions, and tolerances for o-rings used in aerospace and industrial applications.
  • Oberg, E. et al. Machinery’s Handbook, 29th Edition, Industrial Press, 2012, pp. 2587–2592. O-ring design theory, gland depth tables (Table 1), groove dimensions (Table 2), and compound properties (Table 3).

Data last verified: March 2026

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