An O-ring only seals if the groove squeezes it the right amount and leaves room for it to move. Too little squeeze and it leaks; too much and it takes a compression set or extrudes; overfill the gland and thermal expansion has nowhere to go. This guide covers the two numbers that make or break a static O-ring seal — squeeze and gland fill — with an AS568 worked example.
The two numbers that matter
1. Squeeze
Squeeze is how much the cross section is compressed when the gland closes, expressed as a percentage of the free cross-section diameter (CS):
For a static seal, aim for roughly 15–30% squeeze. Dynamic (reciprocating) seals run lower — about 8–16% — to control friction and wear. Too little squeeze risks leakage; too much accelerates compression set.
2. Gland fill
Gland fill is the fraction of the groove volume occupied by the O-ring. An elastomer is nearly incompressible, so it needs empty space to flow into when it is squeezed and when it expands with temperature:
Keep fill in the 60–85% band. Below 60% the ring can roll or spiral; above 85% there is no room for thermal expansion and the ring can force the gland open or extrude past the clearance gap.
Worked example: AS568-214 static radial seal
An AS568 -214 O-ring has a nominal cross section CS = 0.139 in. We want a static seal with about 18% squeeze.
Step 1 — groove depth from target squeeze. Rearranging the squeeze formula:
Step 2 — verify actual squeeze. With a machined depth of 0.113 in:
That lands comfortably in the static 15–30% window.
Step 3 — set groove width for fill. To keep gland fill near 75%, a static groove is typically about 1.3–1.5× the cross section wide. Here that is roughly 0.181–0.209 in. Widening the groove drops fill (more empty volume); narrowing it raises fill toward the 85% ceiling.
| Parameter | Target (static) | This design |
|---|---|---|
| Squeeze | 15–30% | 18.7% ✓ |
| Gland fill | 60–85% | ~75% ✓ |
| Groove depth | — | 0.113 in |
| Groove width | ~1.3–1.5 × CS | 0.181–0.209 in |
Let the calculator do the geometry
Our free O-Ring Groove Calculator returns groove width, depth, squeeze, and gland fill for any AS568 size, static or dynamic. Measuring an existing groove instead? The O-Ring Groove Identifier works it backward to the AS568 dash number, and the Seal Compatibility tool checks your elastomer against the fluid. They all live under Seal Design.
Static vs. dynamic: pick your squeeze
A static seal never moves relative to its groove, so you can push squeeze up into the 15–30% range for a robust seal. A dynamic seal (a reciprocating rod or piston) slides, so high squeeze means high friction, heat, and wear — keep it around 8–16% and pay closer attention to surface finish and the clearance gap. The AS568 size and groove math are the same; only the targets shift.
Compound and durometer
Durometer (Shore A hardness) trades sealability for extrusion resistance. A soft 70A compound conforms and seals at low pressure but nibbles at high pressure; a hard 90A resists extrusion but needs more squeeze and a finer surface to seal. Match the elastomer family (Nitrile, FKM/Viton, EPDM, silicone) to the fluid and temperature first, then pick durometer for the pressure.
Common failure modes
- Compression set — the ring takes a permanent flat from too much squeeze, heat, or an incompatible compound.
- Extrusion / nibbling — high pressure forces the ring into the clearance gap; fix with a harder compound, tighter gap, or a back-up ring.
- Spiral failure — a dynamic ring twists and shreds; usually too little squeeze or a rough surface.
- Explosive decompression — absorbed gas expands when pressure drops fast, blistering the ring; use an ED-resistant compound.
Frequently asked questions
What squeeze should a static O-ring have?
For a static seal, target roughly 15–30% squeeze. Dynamic reciprocating seals run lower, about 8–16%, to limit friction and wear.
Why can’t I fill the groove completely?
An elastomer is nearly incompressible. If gland fill exceeds about 85%, thermal expansion has nowhere to go and the ring can force the gland open or extrude through the clearance gap. Keep fill in the 60–85% band.
How do I stop an O-ring from extruding?
Tighten the diametral clearance gap, use a harder (higher-durometer) compound, lower the pressure, or add a back-up ring on the low-pressure side of the groove.
Designing a gland? Get squeeze, fill, and every groove dimension for your AS568 size in one click.
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