An O-ring size calculator for groove design
An O-ring seals by being squeezed between two surfaces, so the groove dimensions matter as much as the ring size. This O-ring size calculator takes the ring’s inside diameter and cross-section together with your groove or gland dimensions and reports the squeeze, the stretch, the groove fill and whether each falls inside a typical design range. It works for piston (internal), rod (external) and face seals, static or dynamic, in millimetres or inches.
You can also work backwards: enter the squeeze range you want and the calculator gives the gland depth, or enter a target stretch and it suggests the ring ID for your groove. A table lists the standard cross-sections, the AS568 inch series and the ISO 3601-1 metric series, with the groove depth each needs for your squeeze range.
The ring sizes themselves are not looked up. The AS568 dash-number table is a copyrighted standard, so read the inside diameter and cross-section from your supplier’s catalogue and enter them here. The guidance ranges are typical and editable; the manufacturer’s design charts for your compound and pressure take precedence.
How to use the O-Ring Size Calculator
- Choose the seal type (piston, rod or face), static or dynamic use, and the unit.
- Enter the O-ring ID and cross-section from the catalogue.
- Enter the groove: bore and groove root diameter for a piston seal, rod and groove outer diameter for a rod seal, or the depth for a face seal. Add the groove width for the fill.
- Adjust the squeeze, stretch and fill ranges if your manufacturer gives different figures.
- Read the squeeze, stretch and fill, and the ring ID that would give your target stretch.
O-Ring Size Calculator features
Three seal types
Piston, rod and face seals each use the right geometry.
Stretch correction
The stretched cross-section is used for the squeeze, because stretching thins the ring.
Gland fill
Ring area against groove area, with a limit you can set.
Cross-section chart
AS568 and ISO series with the depth for your squeeze range.
Squeeze, stretch and fill formulas
The squeeze is the compression of the cross-section between the groove bottom and the mating surface: (CS′ − depth) ÷ CS′. A ring stretched on a piston groove gets thinner, so CS′ = CS ÷ √(1 + stretch), because its volume stays the same. The stretch is (groove root diameter − ID) ÷ ID, and the gland fill is the ring’s cross-section area, π CS′² ÷ 4, divided by the groove area, depth × width.
Example: a 20 × 2.62 mm ring on a 20.4 mm groove root in a 24.6 mm bore. Stretch is 2%, the stretched cross-section is 2.594 mm, the gland depth is (24.6 − 20.4) ÷ 2 = 2.1 mm and the squeeze is 19.0%. In a 3.6 mm wide groove the fill is 70%. That is inside the 15–30% squeeze window used as a default for static seals.
| Quantity | Typical guide | Why it matters |
|---|---|---|
| Static squeeze | about 15–30% | enough force to seal without damaging the ring |
| Dynamic squeeze | about 10–25%, lower for large rings | less friction and wear on moving seals |
| Stretch | keep to 2–3%, at most about 5% | high stretch thins the ring and shortens life |
| Gland fill | below about 85% | room for thermal swell |
Limits
- The pressure, temperature, fluid and compound change the right squeeze; hard rings and high pressures need anti-extrusion back-up rings.
- Groove corner radii, surface finish and the lead-in chamfer are not checked.
- Rotary and vacuum seals have their own design rules.
- The ISO and AS568 series differ slightly (for example 2.62 mm and 2.65 mm); mixing them can leave a poor fit.
O-Ring Size Calculator FAQ
How do I calculate O-ring squeeze?
Squeeze = (cross-section − gland depth) ÷ cross-section. A 2.62 mm ring in a 2.1 mm deep gland, after allowing for stretch, is squeezed by about 19%.
What is the right O-ring stretch?
Keep it as low as possible, typically 2 to 3% and no more than about 5%, because stretching thins the cross-section and reduces the sealing force.
How do I find the O-ring size for my groove?
Measure the groove root diameter, choose the ID about 1 to 3% smaller than that for a piston seal, then pick the nearest catalogue size with the cross-section that gives your squeeze.
Does this include the AS568 size chart?
It includes the AS568 cross-section series, but not the full dash-number table of inside diameters, which is a copyrighted standard. Use your supplier’s catalogue for the sizes.
What is gland fill?
The share of the groove cross-section taken up by the ring. Below about 85% leaves room for the ring to swell with heat and fluid.
Are the ranges the same for every ring?
No. Squeeze guidance depends on the cross-section, whether the seal is static or dynamic and the pressure. The ranges here are editable; use the manufacturer’s chart for the final design.