O-Ring Size Chart: Selected AS568 Sizes and Metric Cross-Section Comparisons

Use this O-ring size guide to compare inside diameter (ID) and cross-section (CS), the thickness of the cord. For a round, unstretched ring, outside diameter (OD) = ID + 2 × CS. AS568 sizes use dash numbers such as -210. Metric sizes are commonly written as ID × CS in millimeters.

This guide covers 10 selected AS568 dash sizes, the main cross-section groups, boss-seal examples and inch-to-metric cross-section comparisons. Use it to identify likely sizes and understand the numbers on a product listing. The linked handbook provides the wider size reference.

Why you need both ID and CS

Two rings can have nearly the same ID but different cross-sections. AS568 -214 has an ID of 0.984 in and a CS of 0.139 in. AS568 -318 has an ID of 0.975 in and a CS of 0.210 in.

The -318 cross-section is about 51% thicker: (0.210 − 0.139) ÷ 0.139 × 100. Both sizes have an ID tolerance of ±0.010 in, so their permitted ID ranges overlap. An ID measurement in that overlap cannot distinguish them; the CS measurement can.

Nominal fractions are another common source of confusion. A -210 is often labeled with a 3/4 in nominal ID, but its specified ID is 0.734 in ±0.010 in. That range is 0.724–0.744 in. A measured ID of 0.750 in is outside it. Use specified dimensions and tolerances, rather than fractional labels, to identify the size.

How does the AS568 dash number work?

SAE AS568 specifies O-ring dimensions, tolerances and dash numbers. Each dash number identifies an ID and a CS. In the 100, 200, 300 and 400 groups below, the first digit identifies the cross-section group. For example, -214 has a 0.139 in CS. The small -001 to -003 sizes and the 900-series boss seals are exceptions to the five-group summary.

The smaller -001, -002 and -003 sizes have cross-sections of 0.040, 0.050 and 0.060 in, respectively (1.02, 1.27 and 1.52 mm, rounded). These are exceptions to the 0.070 in group.

Selected AS568 O-ring sizes: 10 reference examples

The table uses handbook reference dimensions. Values in millimeters are calculated using 1 in = 25.4 mm and rounded to two decimal places. For additional sizes, see the Parker O-Ring Handbook, Section 9. Calculated OD values do not have an independent tolerance listed here.

ID tolerance varies with size. For example, it is ±0.005 in for -201 and ±0.055 in for -275. Browse the LILY O-ring catalog for available dash sizes and compounds.

What are the 900-series boss seals?

The AS568 900 series covers boss seals for straight-thread fittings. The dash numbers are not continuous, and CS varies by size. For many sizes, the last two digits correspond to tube OD in sixteenths of an inch. For example, -906 is associated with a 3/8 in tube; -916 with a 1 in tube. Tube OD is different from O-ring ID and port thread diameter.

For example, -916 has a reference ID of 1.171 in and CS of 0.116 in, approximately 29.74 mm and 2.95 mm. Its calculated OD is 1.403 in: 1.171 + 2 × 0.116. The BBHDNGJB product detail page lists these dimensions and a CS tolerance of ±0.004 in. Confirm the required ID tolerance against the applicable product drawing or specification.

Metric designations and inch-to-metric CS comparisons

A metric designation such as 18 × 3.55 mm means an 18 mm ID and a 3.55 mm CS. ISO 3601-1 specifies dimensions, tolerances and designation codes for industrial and aerospace O-rings, including dimensional tolerance classes A and B. A catalog’s ID × CS label alone does not establish standard compliance or a tolerance class.

Small CS differences do not establish interchangeability. Check ID and CS tolerances together with groove dimensions, installed stretch, squeeze and compound. For -210, the ID is 18.6436 mm ±0.254 mm, approximately 18.64 mm ±0.25 mm. A measured 19.00 mm ID lies outside that range.

JIS B 2401 is another O-ring standard. Check a JIS size code against its own dimensional table.

How to measure an O-ring

Use a calibrated caliper for an initial size check. A 0.01 mm display resolution does not mean the measurement is accurate to 0.01 mm. Jaw pressure, stretching and ring deformation can cause larger errors. For small, soft or borderline sizes, use a suitable optical or low-force measuring method and follow the agreed inspection procedure.

• Step 1: Clean the ring and let it relax. Remove loose dirt with a lint-free cloth. Use only a cleaner compatible with the compound, and measure the ring flat and unstretched at room temperature.

• Step 2: Measure CS at three or four points. Close the jaws until they just touch the rubber. Record the minimum, maximum and average; do not hide flattened or uneven sections by averaging. Compressing a 1.78 mm CS by 0.10 mm creates about 5.6% error. For the 0.070 in AS568 group, the CS tolerance is ±0.003 in (±0.0762 mm), about ±4.3%.

• Step 3: Measure ID in two or three directions. Use light contact with the inside jaws and do not pull the ring into shape. Record the range of readings. If it will not stay round without stretching, use an optical measurement or a sizing gauge.

• Step 4: Use a sizing tool for large rings. A suitable O-ring gauge or calibrated circumference tape can help estimate the size. An outside-perimeter measurement gives OD = circumference ÷ π, then ID = OD − 2 × CS, only for a circular, unstretched ring. A flexible tape pulled tightly around a floppy ring can distort it; treat that result as an estimate.

• Step 5: Compare both dimensions and tolerances. Identify possible CS groups, then compare ID with the permitted ranges. If several sizes fit, check the original part number or drawing. A tolerance overlap alone does not identify the intended nominal size.

Worked example: an unstretched ring measures 3.52 mm CS and 18.70 mm ID. For AS568 -210, the permitted CS range is approximately 3.43–3.63 mm and the ID range is 18.39–18.90 mm. Both readings fall within those ranges, so -210 is a dimensional candidate.

How to read the size fields on a product listing

The BBHDNGJB product listing illustrates the difference between size and compound properties. It lists a silicone AS568 -916 ring. Its downloadable dimensional sheet also shows ID 1.171 in, CS 0.116 in and OD 1.403 in.

After sizing: choose the O-ring compound

Choose the compound for the fluid or gas, temperature, pressure and seal motion. A dimensional match alone does not establish suitability. The following material, hardness and temperature values come from individual LILY product detail pages and apply to the named products. Review the chosen part’s current specification before ordering.

• NBR (Buna-N): BBHGNFE — Buna-N rubber, 70 Shore A, −40°F to 250°F (−40°C to about 121°C). The listing identifies this as an oil-resistant metal detector grade O-ring.

• FKM: BBGDNBAB — Viton® fluoroelastomer rubber, 70 Shore A, 25°F to 450°F (about −4°C to 232°C). The listing identifies this as a clean room O-ring.

• FFKM: BBAITBBG — Kalrez 6375 rubber, 75 Shore A, 30°F to 525°F (about −1°C to 274°C). This is a distinct compound from FKM; do not substitute one material name for the other.

• Silicone: BBHDNGJB — Silicone rubber, 50 Shore A, −60°F to 400°F (about −51°C to 204°C). Check wear, tear strength and media compatibility before selecting it for a moving seal.

• EPDM: BCIJNBAB — EPDM rubber, 70 Shore A, −70°F to 300°F (about −57°C to 149°C). The listing identifies this as a water- and steam-resistant high-temperature O-ring.

Need help identifying a replacement? Please contact us with your measurements and service details. A photo or groove drawing helps.

Frequently Asked Questions

Should I measure the old ring or the groove?

Start with the equipment drawing or original part number when available. A used ring may have swollen, stretched or taken compression set, leaving it flattened after removal. Groove and mating-part measurements can help reconstruct the required size, but groove width or depth is not itself an O-ring size. Check the appropriate gland-design guidance for the seal arrangement.

How should I store spare O-rings?

Keep them in protective, sealed packaging without stretching or deforming them. Store away from direct sunlight, heat and ozone sources. Retain the material and batch identification, and follow the supplier’s storage-life instructions. Inspect for cracks, stickiness or hardening before installation. A NASA failure assessment of nitrile/Buna-N O-rings attributed cracking in the examined seals to ozone attack. That case illustrates a material-specific risk; it does not establish a universal shelf life.

What information should I include when ordering an O-ring?

Specify the standard and dash number, or metric ID × CS with the required tolerances. Add the material or compound code, hardness, quantity and any required approvals or inspection documents. If the compound is unknown, supply the media, temperature, pressure and whether the seal is static or moving.

Can a joined cord ring replace a molded O-ring?

O-ring cord can be cut and joined for some applications, but the joint introduces a potential leak or failure point. It does not automatically provide the performance of a molded ring. Confirm joint quality, compound compatibility, groove fit and service conditions with the supplier. Ask about a custom molded size when a joined seal is unsuitable.

Technical references and calculation notes

Parker O-Ring Handbook (ORD 5700): Section 9 contains standard-size tables; the design sections explain gland fit and material selection. Use the standard revision required by your drawing or purchase specification.

ISO 3601-1:2012: inside diameters, cross-sections, tolerances and designation codes. ISO 3601-2:2025: housing dimensions for general applications. O-ring dimensions and groove dimensions are separate specifications.

NIST Guide to the SI, Appendix B.8: conversion factor 1 in = 25.4 mm. Millimeter values in this article are calculated from the inch values and rounded; use the original units for close tolerance decisions. SVG illustrations show relative geometry and are not measurement templates.