O-Ring & Seals Calculator — Squeeze, Stretch, Gland Fill & Extrusion Check (SAE AS568)

Sizes are identified by SAE AS568 dash number, the US/aerospace O-ring size designation — 202 sizes here, spanning the 000, 100, 200 and 300 series (wire diameters 1.78, 2.62, 3.53 and 5.33 mm), each carried with a nominal inside diameter, cross-section and tolerance for the calculation. Confirm the dimensions against your O-ring supplier's size chart before ordering. AS568 also defines a 400 series (6.99 mm wire) that this calculator does not currently stock, and the 000 series opens with three sizes (-001, -002, -003) whose wire diameters are 1.02, 1.27 and 1.52 mm rather than 1.78. AS568 is a DIMENSIONAL standard: it specifies sizes and tolerances, not squeeze, stretch or gland-fill limits. Those target bands are established gland-design practice and are badged as engineering estimates in the tool. The gasket bolt-load tab follows ASME VIII Div. 1 Appendix 2. If you need the metric ISO 3601-1 series (1.80 / 2.65 / 3.55 / 5.30 / 7.00 mm cross-sections), note that this calculator does not implement it — the size list uses AS568 designations throughout.

Established gland-design practice 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.

It models a piston (male) gland — groove in the piston, ring stretched onto the groove root, squeeze applied by the housing bore. First it picks the cross-section family from your groove depth (H = CS × 0.80 static, × 0.85 dynamic reciprocating, × 0.90 dynamic rotary), which prevents it proposing a wire far too thick or thin for the groove you have machined. Then it picks the nearest AS568 inside diameter in that family, measured against the groove root B − 2H rather than against the bore, so the stretch it reports is the stretch it selected for. Target stretch is 1–5%: too little and the ring can fall out of the groove during assembly, too much and the wire thins, which reduces the squeeze. A negative value means the ring is larger than the groove root and would buckle in it — the tool grades that as severely as over-stretch.

Because the permissible unsupported gap falls steeply with pressure. For a 70 Shore A compound without a back-up ring it is roughly 0.08 mm at 50 bar, 0.04 mm at 100 bar, 0.025 mm at 150 bar, 0.02 mm at 200 bar and 0.01 mm at 300 bar. The calculator interpolates that curve at your operating pressure rather than applying a single constant, so the same 0.05 mm clearance is comfortable at 50 bar and unacceptable at 200 bar. A PTFE back-up ring raises the allowable gap to roughly 0.3 mm.

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.

The choice depends on temperature range and chemical compatibility. NBR (nitrile) covers −40 to +120 °C and is excellent with mineral oils and water — the workhorse for hydraulics. FKM (fluoroelastomer) 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.

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