Engineering tool

O-ring groove design calculator

Check the O-ring that fits your gland, find listed sizes that land inside the usual squeeze and fill ranges, or size a groove from a dash number. Piston, rod and face seals, static or moving, in mm or inch.

Reference geometry check — not an engineering approval. Tolerances, swell, pressure and extrusion gaps, and the material choice stay with you; send the groove for an engineering review when the seal is critical.

1 The O-ring

2 The groove nominal dimensions

Face groove width = (groove OD − groove ID) ÷ 2.

A face groove is sized from the ring: the groove ID or OD is set from the ring, and the width follows.

3 Application & units
Seal type
Motion
Units

Worked example: AS568 -214 in a static piston groove — 31.75 mm bore, 26.11 mm groove, 4.8 mm wide. Change any field to use your own numbers.

All dimensions are nominal. A dash number fills in the AS568 nominal ID and CS from a public size reference.

Reference geometry check — not an engineering approval; tolerances, swell, pressure/extrusion gaps and material must be reviewed.

Answer first

What the numbers mean

Squeeze is how much the cross section is compressed between the groove floor and the mating surface: (installed CS − gland depth) ÷ installed CS. Static radial seals usually sit around 18–25 %, reciprocating seals 10–20 %, rotary 0–10 %, and face seals 20–30 %. Below the range the seal may not seat; well above it the rubber takes a set and the friction rises.

Stretch or interference on the diameter. On a piston the ring is stretched over the groove diameter — keep that under about 5 %, because stretching thins the section. For rod applications, this tool uses about 0–2 % interference on the ring ID as a reference window. It is not a guarantee against twisting: review installation, alignment, compatible lubrication and operating conditions. Face seals work the other way round: the ring has to press against one wall, so an internal-pressure groove is sized at or just inside the ring's outer edge, and an external-pressure groove at or just outside the ring ID.

Gland fill is the ring's cross-section area divided by the groove cross-section (depth × width). Keep it at or below about 75 % so roughly a quarter of the groove stays free for heat expansion and swell. Our model also thins the ring when it is stretched: installed CS ≈ CS × (1 − ½ × stretch), a first-order approximation — the real change depends on the compound and the fluid.

Gap and concentricity. The extrusion gap is the radial opening the rubber can be pushed into: (bore − piston Ø) ÷ 2 on a piston, (housing bore − rod) ÷ 2 on a rod. The source geometry tables use series-specific radial-gap references: 0.002 in (0.0508 mm) for 0.070/0.103/0.139 in cross-sections, and 0.003 in (0.0762 mm) for 0.210/0.275 in cross-sections. These are not universal pressure-safe limits. Review the actual maximum local gap, concentricity, pressure, temperature, compound hardness and motion before selecting support or backup rings. If the parts can float, the ring's squeeze drops on one side by the same amount.

Open the size finder Size charts Material guide

Questions

Groove design FAQ

Short answers to what buyers and designers ask before ordering.

Still unsure? Send the groove for an engineering review — we reply with the sizes and options that fit.

How is O-ring squeeze calculated?
Squeeze = (installed cross section − gland depth) ÷ installed cross section. On a piston the gland depth is (bore − groove diameter) ÷ 2; on a rod it is (groove diameter − rod) ÷ 2; on a face seal it is the groove depth. This tool uses installed CS ≈ nominal CS × (1 − ½ × stretch), a first-order stretch-thinning approximation. Engineer Pro uses free nominal CS without this correction, so the two model results can differ. Review compound, tolerances and service conditions.
What squeeze should a static seal have against a moving one?
Static radial seals commonly sit at 18–25 %, reciprocating seals at 10–20 %, rotary seals at 0–10 %, and face seals at 20–30 %. A moving seal carries less squeeze because friction and heat build up faster; insufficient positive squeeze can reduce sealing margin and increase leakage risk, depending on groove tolerances and operating conditions.
How much can I stretch an O-ring over the groove diameter?
Keep installed stretch at or below about 5 % for a piston groove, and rod-side interference on the ring ID around 0–2 %. Stretching thins the cross section, which reduces squeeze and speeds up ageing, so if the stretch is high, choose a bigger ring ID rather than a deeper groove.
What is gland fill and why keep it under 75 %?
Gland fill is the ring's cross-section area as a share of the groove area (depth × width). Below about 75 % roughly a quarter of the groove stays free, which leaves room for thermal expansion and for the swell that fluids and heat cause. A fuller groove raises friction and can make the ring extrude or split.
My groove is already machined — which O-ring fits it?
Use Find O-rings for my groove. Enter the groove dimensions and the tool searches every listed size, keeps the ones inside all of the reference ranges, and ranks them by how close they sit to the middle. Each row links to the exact product options, so the sizes are buyable rather than just theoretical.
Is the calculated groove a finished design?
No. The tool works with nominal dimensions and gives a starting point. Machining tolerances, the fluid and temperature, swell, pressure and the extrusion gap all shift the result, so a critical seal should be reviewed before release. The reference ranges come from public engineering practice, not from an official standard.