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O-Ring Size and Gland Squeeze Basics

Guide · Updated October 28, 2026

Standard inch O-rings are sized by AS568 dash numbers. The first digit of the three-digit number tells you the cross section, and the rest step through inside diameters. A seal works by being squeezed between the bottom of the groove (the gland) and the mating surface, so the groove depth you machine sets the squeeze. Too little squeeze leaks; too much overloads the rubber, makes assembly hard and can fill the groove completely. The squeeze ranges below are the static radial gland ranges published in seal maker design tables.

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Step by step

Measure the O-ring: Measure cross section with a caliper lightly, or better an optical comparator; measure ID on a cone or with a pi tape.
Find the series: Match the cross section to the AS568 series.
Pick the squeeze: Use the recommended range for the cross section and seal type.
Calculate gland depth: Gland depth = CS × (1 − squeeze).
Check gland fill: O-ring area should be about 75 to 85% of groove area or less, leaving room for swell.
Finish the groove: Keep the groove bottom and sides smooth and break sharp edges so the ring is not cut during assembly.

AS568 series and cross sections

-001 to -050: 0.070 in cross section (1.78 mm)
-102 to -178: 0.103 in (2.62 mm)
-201 to -284: 0.139 in (3.53 mm)
-309 to -395: 0.210 in (5.33 mm)
-425 to -475: 0.275 in (6.99 mm)

Examples: -010 is 0.239 ID × 0.070 CS, -110 is 0.362 × 0.103, -210 is 0.734 × 0.139 and -214 is 0.984 × 0.139. Nominal cross section tolerances are about ±0.003 in for 0.070 rings, ±0.004 for 0.139 rings and ±0.005 for 0.210 rings.

Typical static radial squeeze

0.070 CS: 22 to 32%
0.103 CS: 17 to 24%
0.139 CS: 16 to 23%
0.210 CS: 15 to 21%
0.275 CS: 15 to 20%

Face seals (axial glands) usually run a little more squeeze, about 20 to 30%. Dynamic seals run less, often 10 to 20%, to limit friction. Small cross sections need a higher percentage because tolerances are a bigger share of the ring.

Formulas

Squeeze % = (CS − gland depth) ÷ CS × 100
Gland depth = CS × (1 − squeeze)
For a radial groove on a piston: gland depth = (bore − groove diameter) ÷ 2
O-ring area = π × CS² ÷ 4
Gland fill % = O-ring area ÷ (groove width × gland depth) × 100
Stretch on a piston groove: (groove diameter − O-ring ID) ÷ O-ring ID, normally 1 to 5%

Worked example: -214 on a piston

A -214 ring (0.984 ID × 0.139 CS) on a piston in a 1.250 bore. Aim for 20% squeeze: gland depth = 0.139 × 0.80 = 0.111 in. Groove diameter = 1.250 − 2 × 0.111 = 1.028 in. Stretch = (1.028 − 0.984) ÷ 0.984 = 4.5%, within the 1 to 5% guideline. Ring area = 0.785 × 0.139² = 0.01517 in². With a 0.187 in wide groove, gland area = 0.187 × 0.111 = 0.0208 in², so fill = 73%.

Worked example: checking an existing groove

A groove measures 0.054 in deep for a 0.070 ring. Squeeze = (0.070 − 0.054) ÷ 0.070 = 22.9%, in range. Add the worst case: a 0.067 ring in a 0.056 deep groove is only 16% squeeze, below the 22% minimum, so tighten the groove depth tolerance or move the nominal to 0.052 in.

Finish and edges

Seal makers commonly call for groove surfaces around 32 µin Ra for static seals and 16 µin Ra or better on dynamic sealing surfaces, with groove edges broken about 0.005 in and a lead-in chamfer of 15 to 20 degrees on the bore so the ring is not cut on assembly.

Common problems

Measuring cross section with a caliper squeezed hard (rubber compresses), forgetting tolerance stack-up on gland depth, overfilling the groove so the ring extrudes, sharp groove edges and tool marks across the seal path. Check groove depth with a depth mic and diameters with a mic or bore gage.

FAQ

What does the first digit of an AS568 number mean?

The cross section series: 0 = 0.070, 1 = 0.103, 2 = 0.139, 3 = 0.210, 4 = 0.275 in.

How much squeeze should an O-ring have?

For a static radial seal, roughly 15 to 30% depending on cross section.

How deep is a groove for a 0.139 O-ring?

About 0.107 to 0.117 in for 16 to 23% squeeze.

How do I check groove depth?

Check the gage itself before trusting a measurement: gage blocks set mics, calipers and indicators; build any stack free at gageblockcalc.com.

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General shop information only — follow your drawing, tool maker's data and your quality system.