CTR K

O-Ring & Seals Calculator — Squeeze, Gland Fill & Extrusion Check (ISO 3601 / AS 568)

Governing standard: ISO 3601· ISO 3601-1/-2 (O-ring sizes, tolerances & gland dimensions) · AS 568 (aerospace size standard) · ASME VIII Div. 1 App. 2 (gasket bolt-load)

How ISO 3601 works — the method explained

The MechanixCalc O-Ring & Seals calculator sizes and verifies elastomeric seals to ISO 3601 and AS 568 — the governing international and aerospace standards for O-ring dimensions, tolerances and gland (groove) design. Enter the housing bore, groove geometry, system pressure, temperature and elastomer material, and the tool returns the compression squeeze, diametral stretch, gland fill percentage, extrusion threshold and back-up ring recommendation in a single pass, along with recommended groove dimensions for static and dynamic applications.

It is built for fluid-power, hydraulic and pneumatic engineers who need to select and verify O-ring seals for pistons, rods and face-seal glands — and who need a standards-cited calculation to hand a reviewer or include in a design file. A gasket bolt-load tab (ASME VIII Div. 1 App. 2) and a seal-type selection matrix are also included for broader sealing system design.

What this calculator does

  • ISO 3601 / AS 568 O-ring sizing and compression (squeeze) check for static and dynamic applications
  • Gland fill percentage and groove dimension design (depth H and width W) per ISO 3601-2
  • Extrusion threshold check with single-, double- or no-back-up-ring recommendation by elastomer and pressure
  • Material suitability across NBR, FKM, EPDM, PTFE, Silicone and HNBR — temperature and chemical range check
  • Groove design reference table across all four application types: static face, static bore, dynamic reciprocating and dynamic rotary
  • Gasket bolt-load calculator to ASME VIII Div. 1 Appendix 2 (seating and operating bolt-load, effective seating width)
  • Branded PDF engineering report with the full methodology and formula substitutions shown

Method & formulas

Compression (squeeze) and gland fill

O-ring sealing depends on the radial compression of the elastomer cross-section between the groove root and the mating bore or face. ISO 3601-2 specifies the groove depth H as a fraction of the nominal wire diameter CS so that the installed squeeze falls within the recommended range: 15–30% for static seals and 10–20% for dynamic reciprocating seals. MechanixCalc computes squeeze directly from the groove depth and O-ring wire diameter, then checks it against the application-type range.

Gland fill is the ratio of the O-ring cross-sectional area (circular section) to the groove cross-sectional area (rectangular slot). A fill between 60% and 85% leaves room for thermal expansion and prevents over-fill extrusion into the bore clearance while maintaining adequate sealing contact.

Compression squeeze
Squeeze (%) = (CS − H) / CS × 100

where CS = O-ring wire (cross-section) diameter (mm); H = groove depth (mm). Target: 15–30% static, 10–20% dynamic reciprocating, 5–15% dynamic rotary.

Gland fill
Fill (%) = (π/4 · CS²) / (W · H) × 100

where CS = wire diameter (mm); W = groove width (mm); H = groove depth (mm). Target: 60–85%.

Extrusion and back-up ring selection

At elevated pressures the elastomer can be forced into the diametral clearance gap between the rod/piston and its bore, causing permanent nibbling damage. Each elastomer has a characteristic extrusion pressure limit that depends on its hardness (Shore A) and the clearance gap. When the operating pressure exceeds this limit a PTFE back-up ring — placed on the low-pressure side of the O-ring — bridges the gap and blocks extrusion. At still higher pressures a double-sided arrangement (rings on both sides) is required; beyond about six times the extrusion limit a different seal technology should be considered.

Extrusion ratio
Ratio = P_MPa / P_extrusion_limit

where P_MPa = system gauge pressure (MPa = bar × 0.1); P_extrusion_limit = elastomer-specific extrusion pressure limit (MPa) at the design clearance gap. Ratio ≤ 1: no back-up ring; 1–3: single back-up ring; 3–6: double back-up rings; > 6: redesign.

Gasket bolt-load (ASME VIII Div. 1 App. 2)

Flanged joints use a separate seating check: the bolts must squeeze the gasket hard enough to seat it (seating condition, Wm2) and must continue to hold the operating pressure plus the residual gasket stress that prevents leakage (operating condition, Wm1). ASME VIII Div. 1 Appendix 2 defines the effective seating width b from the basic contact width b0 and computes the governing bolt load as the larger of Wm1 and Wm2. The calculator checks the actual bolt area against the required area and reports the margin.

Operating bolt-load (Wm1)
Wm1 = π/4 · G² · P_d + 2π · G · b · m · P_d

where G = mean gasket diameter (mm); P_d = design pressure (MPa); b = effective seating width (mm) per ASME App. 2; m = gasket factor (dimensionless, material-dependent).

Seating bolt-load (Wm2)
Wm2 = π · G · b · y

where y = minimum gasket seating stress (MPa, material-dependent per ASME App. 2 Table 2-5.1); other symbols as above.

Worked example

Check the squeeze and gland fill for an NBR O-ring with wire diameter CS = 3.0 mm installed in a static bore seal with groove depth H = 2.4 mm and groove width W = 4.5 mm.

Given

  • O-ring wire diameter CS3.0 mm
  • Groove depth H2.4 mm
  • Groove width W4.5 mm
  • Application typeStatic bore

Result

  • Compression squeeze20.0% (GOOD — within 15–30% static target)
  • Gland fill65.4% (GOOD — within 60–85% target)
  1. Compute compression squeeze: Squeeze = (CS − H) / CS × 100 = (3.0 − 2.4) / 3.0 × 100 = 0.6 / 3.0 × 100 = 20.0%.
  2. Check against target range for static applications (15–30%): 20.0% is within range — GOOD.
  3. Compute O-ring cross-section area: A_oring = π/4 × CS² = π/4 × 9.0 = 7.069 mm².
  4. Compute groove cross-section area: A_groove = W × H = 4.5 × 2.4 = 10.80 mm².
  5. Compute gland fill: Fill = A_oring / A_groove × 100 = 7.069 / 10.80 × 100 = 65.4%.
  6. Check against target range (60–85%): 65.4% is within range — GOOD.

Illustrative example using round numbers. The calculator also computes diametral stretch, contact pressure and extrusion ratio from your actual system pressure and clearance gap.

Frequently asked questions

Which standard does this O-ring calculator use?

The primary O-ring sizing (dimensions, tolerances, compression squeeze and groove design) follows ISO 3601-1 and ISO 3601-2, cross-referenced to SAE AS 568 (the aerospace O-ring size standard). The gasket bolt-load tab follows ASME VIII Div. 1 Appendix 2 for flanged joints. The governing standard and formula substitutions are shown in the generated PDF report.

What is the recommended O-ring compression (squeeze) for my application?

ISO 3601-2 targets 15–30% squeeze for static face and bore seals, 10–20% for dynamic reciprocating (piston and rod) seals, and 5–15% for dynamic rotary seals. Higher squeeze improves the sealing contact stress but increases friction, heat generation and wear — dynamic seals use less squeeze to limit these. The calculator flags GOOD, LOW or HIGH for your specific application type.

When is a back-up ring required?

When the system pressure exceeds the O-ring elastomer's extrusion limit for the installed clearance gap — typically around 5–10 MPa for NBR without a back-up ring. A single PTFE back-up ring on the low-pressure side extends the limit to roughly three times that value; double back-up rings (one each side) handle up to about six times. The calculator computes the extrusion ratio and recommends none, single, double or redesign based on the elastomer type and pressure.

How do I choose between NBR, FKM and PTFE for my seal?

The choice depends on temperature range and chemical compatibility. NBR (Buna-N) covers −40 to +120 °C and is excellent with mineral oils and water — the workhorse for hydraulics. FKM (Viton) extends to +200 °C and resists aggressive chemicals and fuels. PTFE is the widest-range material (−200 to +260 °C) and resists virtually all media, but it is non-elastic so it relies on compression-load rather than stretch for sealing. The calculator shows the temperature range for each material and warns if your operating temperature is out of range.

Is the O-Ring & Seals calculator free?

You can use it during a free 30-minute preview with no sign-up required. A free 14-day account trial unlocks every calculator with no credit card needed. The branded PDF engineering report and saved calculations are part of a paid plan.

Related calculators

Run the O-Ring & Seals on your own numbers

Free 30-minute preview — no sign-up. A free 14-day account trial unlocks every tool and the branded PDF report, no credit card required.

Start free