O-Ring Groove Design Calculator.
Size the gland for a piston, rod or face seal. Calculates groove depth and width, actual squeeze, gland fill percentage, installation stretch and the maximum extrusion gap for your pressure and durometer.
Application
O-Ring and Duty
Ring area = π·W² / 4
Gland fill % = ring area / (B × G) × 100
Stretch % = (groove root dia − ring ID) / ring ID × 100
How an O-Ring Gland Actually Works
An O-ring does not seal because it is squeezed. It seals because the squeeze creates an initial contact stress, and system pressure then adds to that stress by pushing the elastomer against the low-pressure side of the groove. The rubber behaves as a near-incompressible fluid: it transmits the pressure applied to it into the sealing interface. That is why a correctly designed O-ring seals better at high pressure than at low.
Everything in gland design follows from that: enough squeeze to seal at zero pressure, enough free volume for the rubber to expand into, and a small enough clearance gap that the rubber cannot escape.
Squeeze
Squeeze % = (W − G) / W × 100
Where W is the O-ring cross-section and G is the groove depth. Recommended bands depend on duty:
| Application | Squeeze % | Why |
|---|---|---|
| Static radial (piston / rod) | 15 – 25 | Reliable seal, moderate compression set |
| Static face seal | 18 – 30 | Highest available — no motion to resist |
| Dynamic reciprocating | 10 – 20 | Limits friction, heat and wear |
| Slow oscillating | 12 – 20 | Compromise between seal and drag |
| Vacuum | 20 – 30 | Needs high contact stress, no assisting pressure |
Small cross-sections need proportionally more squeeze because compression set consumes a larger fraction of them. A 1.78 mm ring at 15% squeeze has only 0.27 mm of interference — after thermal set that margin can vanish.
Gland Fill — the Number Most Designs Get Wrong
The groove must be wider than the ring is thick, because a compressed O-ring bulges sideways and expands with temperature and fluid absorption. Gland fill is the ratio of ring cross-sectional area to groove cross-sectional area:
Fill % = (π·W²/4) / (B × G) × 100
- Under 60% — the ring can roll or spiral in a dynamic groove and may nibble itself apart.
- 60 – 85% — the correct working range. Target 75%.
- Over 90% — dangerous. Thermal expansion or fluid swell has nowhere to go, and the ring hydraulically locks the groove, extruding into the clearance or splitting the gland.
Elastomers expand roughly ten times more than steel per degree, and many compounds swell several percent in service fluid. A groove filled to 95% at assembly can exceed 100% at operating temperature.
Stretch and Compression
For a piston seal the ring is stretched over the groove. Keep installation stretch between 1% and 5%. Above 5% the cross-section thins measurably (roughly half the stretch percentage), which quietly reduces your squeeze below the design value.
For a rod or bore seal the ring sits in a groove in the housing and its OD is compressed. Keep that compression under about 3%, otherwise the ring buckles in the groove instead of seating.
Extrusion Gap
The clearance between the moving and static parts is where a seal dies. Under pressure the elastomer flows into it, gets nibbled by the reversing motion, and fails. The permissible gap falls sharply with pressure and rises with hardness:
| Pressure (bar) | 70 Shore A | 80 Shore A | 90 Shore A |
|---|---|---|---|
| up to 35 | 0.25 mm | 0.35 mm | 0.45 mm |
| up to 70 | 0.20 mm | 0.28 mm | 0.38 mm |
| up to 105 | 0.13 mm | 0.20 mm | 0.30 mm |
| up to 140 | 0.08 mm | 0.15 mm | 0.23 mm |
| up to 210 | backup ring | 0.10 mm | 0.15 mm |
| over 210 | backup ring | backup ring | 0.10 mm |
These are total diametral gaps at the worst-case tolerance combination, not the nominal clearance. Add the eccentricity the parts can actually reach — a piston free to sit against one side doubles the gap on that side. Where the gap cannot be held, fit an anti-extrusion (back-up) ring of PTFE on the low-pressure side, or both sides for alternating pressure.
AS568 Cross-Sections
| Dash series | W (inch) | W (mm) | Typical use |
|---|---|---|---|
| 004 – 050 | 0.070 | 1.78 | Small bores, instrument fittings |
| 102 – 178 | 0.103 | 2.62 | General hydraulic and pneumatic |
| 201 – 284 | 0.139 | 3.53 | Medium cylinders, flanges |
| 309 – 395 | 0.210 | 5.33 | Large cylinders, pressure vessels |
| 425 – 475 | 0.275 | 6.99 | Very large diameters |
Groove Finish and Details
- Groove side and bottom finish — Ra 0.8 to 1.6 µm for static, Ra 0.4 to 0.8 µm for dynamic.
- Dynamic sealing surface — Ra 0.2 to 0.4 µm. Too smooth is also wrong: a mirror finish below Ra 0.1 µm cannot hold a lubricant film and causes stick-slip.
- Groove corner radius — 0.1 to 0.25 mm depending on section. Sharp corners cut the ring.
- Lead-in chamfer — 15° to 20°, at least 1.5 × W long, with all edges broken. Most installation damage happens here.
- Groove sides — 0° to 5° draft. Anything more lets the ring climb out of the groove.
Worked Example
A static piston seal in a Ø50 mm bore using a 2.62 mm section ring at 22% squeeze and 75% gland fill.
- Groove depth G = 2.62 × (1 − 0.22) = 2.04 mm
- Squeeze in absolute terms = 2.62 − 2.04 = 0.58 mm
- Ring area = π × 2.62² / 4 = 5.39 mm²
- Groove width B = 5.39 / (2.04 × 0.75) = 3.52 mm
- Groove root diameter = 50 − 2 × 2.04 = 45.92 mm
- At 100 bar with 70 Shore A the diametral gap must stay under 0.13 mm
Related Tools
For the cylinder the seal runs in, see the Cylinder Force Calculator. For the clearance fit that sets the extrusion gap, use ISO 286 Fits. For the groove finish spec see Surface Finish, and to confirm the gap under worst-case tolerances run a Tolerance Stack-Up.