How do I select the right O-ring size?

Start by identifying the seal application (piston, rod, or face seal), the housing bore or shaft diameter, and the required cross-section. For piston seals, the O-ring OD should match the bore — so ID = bore − 2×CS. For rod seals, the O-ring ID should match the rod diameter. Then select the nearest standard ID from ISO 3601 tables. Target 15–25% squeeze for reliable sealing.

What is squeeze percentage and why does it matter?

Squeeze is the percentage by which the O-ring cross-section is compressed in the groove. Squeeze % = (CS − compressed height)/CS × 100. For dynamic seals (piston/rod), target 10–20%. For static seals (face/flange), 15–30%. Too little squeeze causes leakage; too much causes excessive friction, heat, and premature wear.

What is the effect of stretch on O-ring performance?

Stretch occurs when the O-ring is installed on a diameter larger than its free-state ID. Up to 5% stretch is acceptable for most applications. Stretch reduces the effective cross-section and increases stress in the elastomer. Excessive stretch (>5%) can cause premature failure, especially with hard compounds like FKM/Viton.

What materials are available for O-rings?

Common O-ring materials include NBR (Nitrile, −40 to +120°C, oil-resistant), FKM/Viton (−20 to +200°C, chemical-resistant), EPDM (−50 to +150°C, water/steam), Silicone (−60 to +200°C, food-grade), and FFKM/Kalrez (−25 to +325°C, universal chemical resistance). Material selection depends on temperature, media, and pressure.

When are backup rings needed?

Backup rings (anti-extrusion rings) are needed when the pressure exceeds the O-ring's extrusion resistance — typically above 3.5 MPa (500 psi) for static seals or 7 MPa (1000 psi) for dynamic seals, depending on hardness and gap. Use one backup ring on the low-pressure side, or two for bi-directional pressure. Material is typically PTFE or polyamide.

Free Engineering Tool

O-Ring Size Selection

Find the right standard O-ring size per ISO 3601 for piston, rod, or face seal applications. Get groove dimensions, squeeze percentage, and stretch — instantly.

ISO 3601-1 AS568 CS 1.78 – 6.99 mm

Application

Cross-Section CS (mm)

Bore / Shaft Diameter (mm)

Target Squeeze (%)

Quick presets

Results

Recommended O-Ring

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Groove Depth

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Groove Width

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Actual Squeeze

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Stretch / Compression

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O-Ring ID (standard)

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O-Ring OD (calculated)

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ID Tolerance (ISO 3601)

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ISO 3601 Standard Cross-Sections

CS (mm)	AS568 Series	CS Tolerance	Typical ID Range	Groove Depth (20% sq.)	Groove Width
1.78	-0XX	±0.08	1–25 mm	1.42	2.50
2.62	-1XX	±0.09	9–76 mm	2.10	3.60
3.53	-2XX	±0.10	15–133 mm	2.82	4.80
5.33	-3XX	±0.13	19–267 mm	4.26	7.15
6.99	-4XX	±0.15	50–292 mm	5.59	9.40

O-Ring Squeeze

Squeeze is the diametral compression of the O-ring cross-section when installed in the groove. It creates the initial seal contact stress.

Dynamic seals (piston/rod): 10–20% squeeze recommended
Static seals (face/flange): 15–30% squeeze recommended

Groove Dimensions

The groove width must allow the O-ring to expand laterally when squeezed, plus accommodate thermal expansion and swell.

Stretch & Compression

For piston seals, the O-ring is stretched over the piston. For rod seals, it is compressed into the bore groove.

Max stretch: 5% (standard elastomers), 3% (PTFE, hard compounds)
Max compression: 3% of ID

Example — Piston Seal, Bore \u00d850 mm, CS 3.53 mm

Target: O-ring OD ≈ bore = 50 mm

Required ID = 50 − 2 × 3.53 = 42.94 mm

Nearest standard ID: 42.86 mm → OD = 42.86 + 2 × 3.53 = 49.92 mm

Stretch = (43.94* − 42.86) / 42.86 × 100 ≈ 2.5% ✓

Groove depth (20% squeeze) = 3.53 × 0.80 = 2.82 mm

Groove width = 3.53 × 1.35 = 4.77 mm

*groove bottom diameter depends on piston OD and groove depth

⚠️ Note: Always verify groove fill ratio (O-ring volume vs groove volume). The O-ring should fill 70–85% of the groove. Under-filling causes instability; over-filling prevents thermal expansion and causes extrusion.

ℹ️ Tip: For pressures above 3.5 MPa (static) or 7 MPa (dynamic), add a backup ring on the low-pressure side to prevent O-ring extrusion into the gap.

O-Ring Size Selection | ISO 3601 | Free Online Tool

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026