O-Ring Size Calculator
Enter the bore or rod diameter, choose whether the groove is on a piston or in a housing round a rod, and whether the seal moves. The calculator suggests an O-ring size and the groove to cut for it.
- Piston or rod seal
- Static or dynamic
- Groove depth and width
Sizes in millimetres. Leave cross-section on automatic for a sensible default.
An O-ring seals by being squeezed in its groove. The groove is cut a little shallower than the ring's thickness, so the ring is compressed against the mating surface. Static seals use about 20% to 25% squeeze; moving seals use less to cut friction and wear.
O-ring sizing chart
Piston seal, static. Search for a bore.
| Bore (mm) | Cross-section | Groove dia | O-ring ID |
|---|---|---|---|
| 10 | 1.78 | 7.22 | 7.01 |
| 20 | 2.62 | 15.91 | 15.45 |
| 30 | 2.62 | 25.91 | 25.16 |
| 50 | 3.53 | 44.49 | 43.2 |
| 80 | 3.53 | 74.49 | 72.32 |
| 120 | 5.33 | 111.68 | 108.43 |
| 200 | 5.33 | 191.68 | 186.1 |
Groove diameter is on the piston. The O-ring is stretched about 3% to sit firmly in the groove.
Bore and O-ring
Piston, static. Slide the bore.
The drawing is a magnified cut through the groove, showing how the ring is squashed.
How to size an O-ring
Squeeze and stretch. Step through it.
Pick a cross-section
Thicker for bigger sizes.
Groove width is about 1.35 to 1.45 times the cross-section, leaving room for the ring to deform.
Nitrile, Viton or silicone
O-ring materials. Compare them.
The general choice for oils and water.
For high heat and chemicals.
Food grade, wide temperature range.
Hardness is usually 70 Shore A. Harder rings resist extrusion at high pressure.
Before fitting
Seal checks. Tick each item.
Tick each item as you go.
Lubricate O-rings with a compatible grease before fitting. A dry ring can twist or cut on sharp edges.
What you get
Seal design. Each tile explains one result.
ID × section.
To machine.
Sets squeeze.
Room to deform.
Percent.
Standard metric and AS568 inch O-rings come in fixed sizes. Choose the one nearest the calculated ID.
Frequently Asked Questions
No question matches that search.
How do I size an O-ring?
Choose a cross-section, set the groove depth for the squeeze you want and pick a ring slightly smaller than the groove diameter.
How much should an O-ring be compressed?
About 20% to 25% for static seals, 10% to 18% for moving ones.
How wide should an O-ring groove be?
About 1.35 to 1.45 times the cross-section.
How much can an O-ring stretch?
Up to about 5% when fitted, ideally 1% to 3%.
What O-ring size for a 50 mm bore?
About 43 × 3.53 mm for a static piston seal.
How do I measure an O-ring?
Inside diameter and the thickness of the cord (cross-section).
What are AS568 sizes?
The US standard list of inch O-ring sizes, numbered from 001 upwards.
A seal that holds
Choose the cross-section, cut the groove for the right squeeze and fit a ring sized to it. With the right material and a little grease, it will seal reliably.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.