Static O-Ring Groove Calculator
Design o-ring grooves from first principles. Set your squeeze, fill, and stretch targets — the tool calculates the groove depth, width, and ID/OD to match. Includes tolerance verification so you know the seal still works at the edges of your machining tolerances.
Design Targets
Adjust these to calculate groove dimensions. Ranges are per Parker O-Ring Handbook for static seals.
How It Works
This calculator derives groove dimensions from three fundamental design parameters:
Squeeze (15–25%)
Squeeze determines groove depth. Squeeze is the percentage of the o-ring cross-section that gets compressed when the joint is assembled. A 20% squeeze on a 0.139″ CS o-ring means 0.028″ of compression, giving a groove depth of 0.111″.
- Below 15%: Risky — machining tolerances and thermal expansion could eliminate your seal. The o-ring may not maintain contact.
- Above 25%: The o-ring works harder, generating more heat and taking a permanent compression set faster. Good for initial sealing force, bad for long-term life.
- Sweet spot (18–22%): Reliable seal with good fatigue life.
Gland Fill (75–85%)
Fill determines groove width. Fill is the percentage of the groove cross-section occupied by the compressed o-ring. The remaining 15–25% is empty space for the rubber to flow into during compression and thermal expansion.
- Below 75%: Too much empty space. The o-ring can twist and roll in the groove, causing spiral failure — especially in dynamic applications.
- Above 85%: Not enough room. Thermal expansion or fluid absorption can cause hydraulic lock — the o-ring has nowhere to go and the joint won’t close properly.
Stretch (varies by seal type)
Stretch determines groove ID. The o-ring should be slightly smaller than the groove so it stays in position during assembly and doesn’t bunch up.
- Face seal (0–2%): The o-ring sits in a flat groove held by gravity. Minimal stretch needed — just enough to keep it from falling out.
- Piston seal (2–5%): The o-ring sits in a groove on the piston. Without stretch, it falls off during assembly. 3% is typical.
- Rod seal (1–3%): The o-ring sits in a bore groove. The OD is slightly compressed to hold it in place.
Warning: Excessive stretch changes the effective cross-section of the o-ring. Above 5% stretch, the CS thins enough to affect your squeeze calculation. The tool assumes stretch is within recommended limits.
Understanding Diametral Clearance
Diametral clearance is the gap between the o-ring groove and the mating surface — the space the o-ring could potentially extrude into under pressure.
Why it matters: Under pressure, the o-ring acts like a fluid and tries to flow into any available gap. If the clearance is too large, the rubber extrudes into it, gets pinched during cycling, and eventually fails. This is called nibbling or extrusion failure — you’ll see little chunks torn from the o-ring.
| Pressure Range | Max Diametral Clearance | Backup Ring? |
|---|---|---|
| ≤ 1,500 psi | 0.005″ | No — o-ring alone handles sealing |
| 1,500–5,000 psi | 0.003″ | Yes — PTFE backup ring on the low-pressure side prevents extrusion |
| > 5,000 psi | 0.002″ | Yes — backup rings on both sides |
Harder durometer o-rings (Shore A 80–90) resist extrusion better than soft ones (Shore A 50–60). See the O-Ring Material Selection Guide for durometer and material guidance.
Related Resources
- AS568 O-Ring Size Index — complete tables for all 5 series
- Static O-Ring Groove Design Guide — theory and worked examples
- O-Ring Material Selection — NBR, FKM, EPDM, Silicone, PTFE compared
Have an existing groove? Use the O-Ring Groove Identifier to reverse-engineer which AS568 o-ring fits your groove geometry.