How do I calculate O-ring OD from ID and cross-section?
Outside diameter equals inside diameter plus twice the cross-section: OD = ID + 2 × CS.
Size calculation
O-Ring Size Calculator
Enter inside diameter and cross-section in millimetres or inches to calculate O-ring geometry and compare the nearest AS568 and ISO 3601 standard sizes.
Calculated geometry
- Outside diameter (OD)
- 32.100 mm
- Mean diameter
- 28.550 mm
- Mean circumference
- 89.692 mm
Nearest standard sizes
| Standard | Designation | ID × CS | Combined deviation |
|---|---|---|---|
| ISO 3601 | 25×3.55 | 25.00 × 3.55 mm | 0.000 |
| AS568 | -214 | 24.99 × 3.53 mm | 0.006 |
| ISO 3601 | 25×2.65 | 25.00 × 2.65 mm | 0.254 |
| AS568 | -120 | 25.07 × 2.62 mm | 0.263 |
| AS568 | -215 | 26.57 × 3.53 mm | 0.442 |
| AS568 | -213 | 23.39 × 3.53 mm | 0.454 |
A dimensional match does not verify material, hardness, tolerance grade or groove suitability.
What this calculator solves
Use it when a drawing gives ID and cross-section but you need OD, when a measured ring needs a likely standard designation, or when comparing an inch-family size with a metric candidate.
How to read the result
Outside diameter equals ID + 2 × CS. Mean diameter is ID + CS, and mean circumference is π × mean diameter. Nearest-size ranking uses both ID and cross-section; it is not an interchangeability approval.
Engineering checks after size identification
A size calculation does not verify squeeze, stretch, gland fill, material or tolerances. Continue to the seal-specific groove calculator and verify the current standard and supplier data before release.
Worked example: a 20 × 2.5 mm O-ring
Enter ID = 20 mm and CS = 2.5 mm. For a free, circular ring with a circular cross-section, the geometry is:
- Outside diameter
- 20 + 2 × 2.5 = 25 mm
- Mean diameter
- 20 + 2.5 = 22.5 mm
- Mean circumference
- π × 22.5 ≈ 70.686 mm
These are geometric values, not a standard designation or a sealing approval. An installed, stretched or swollen ring needs separate engineering checks.
Frequently asked questions
Is the nearest standard O-ring automatically interchangeable?
No. Confirm dimensional tolerances, material, hardness, groove squeeze, stretch and gland fill before substitution.