How to measure an O-ring —
ID, OD and cross section
Get all three right and the replacement is in the correct size range. Get one wrong and the O-ring may fall out of the groove, have too little room to compress, or extrude under pressure. Here is how to measure each dimension correctly — from a good O-ring, a used one, or the groove itself when the original is gone.
The three dimensions — what each one means
The inside diameter is the measurement across the hole — the open centre of the O-ring. This is the dimension that determines how the O-ring sits in the groove. In many static applications, the O-ring ID is selected slightly smaller than the groove diameter so the ring stays located during assembly.
Measure the inside diameter without stretching the O-ring. Lay it flat, keep it circular, and use the calliper lightly across the internal opening — do not force the inside jaws apart, because that can stretch the rubber and overstate the ID. Measure at two perpendicular diameters — if they differ significantly, the O-ring has distorted in service and the measurements should be treated as approximate.
The outside diameter is the total width across the O-ring including the cord material. In many standard O-ring references, OD is derived from ID and cross section rather than specified independently. It still matters for groove design — the groove must allow the O-ring to sit without being pinched on the outer edge.
Measure across the outermost points with external calliper jaws. Note that a used O-ring may have swollen or deformed, giving an OD that no longer reflects the nominal size. If the OD measures significantly larger than expected, check whether material swelling has occurred.
The cross section is the diameter of the cord — the circular cross section of the rubber itself. It determines how much material is available to compress in the groove. Too little cross section for the groove depth and the O-ring has insufficient squeeze. Too much and the O-ring is over-compressed, which accelerates compression set and can prevent the joint from closing correctly.
Measure cross section across the cord itself at the thickest undamaged point, using the external calliper jaws without over-compressing the rubber. On a used O-ring, avoid measuring at points that show visible compression marks from the groove.
Standard O-ring size references
O-ring sizes follow standard systems. The most common references are metric (defined by ID and CS), BS 1806 (UK inch-based standard) and AS 568 (US standard). Metric sizing is common across European heating and many general industrial O-ring applications.
Example metric O-ring size references
| Common designation | ID (mm) | CS (mm) | OD (mm) |
|---|---|---|---|
| 10 × 2 | 10.0 | 2.0 | 14.0 |
| 12 × 2 | 12.0 | 2.0 | 16.0 |
| 14 × 2 | 14.0 | 2.0 | 18.0 |
| 16 × 2 | 16.0 | 2.0 | 20.0 |
| 18 × 2 | 18.0 | 2.0 | 22.0 |
| 20 × 2 | 20.0 | 2.0 | 24.0 |
| 20 × 3 | 20.0 | 3.0 | 26.0 |
| 25 × 3 | 25.0 | 3.0 | 31.0 |
| 30 × 3 | 30.0 | 3.0 | 36.0 |
| 40 × 3 | 40.0 | 3.0 | 46.0 |
| 50 × 3 | 50.0 | 3.0 | 56.0 |
| 15 × 2.5 | 15.0 | 2.5 | 20.0 |
Listed sizes are common metric references only. O-ring size in specific equipment varies by manufacturer. Always confirm against the equipment documentation or measure the actual groove before ordering.
When the original O-ring is not available — measuring from the groove
Measuring from the groove
When the original O-ring is missing, damaged beyond measurement, or has been incorrectly sized by a previous repair, measure the groove directly. This is a field identification method, not a gland design calculation — groove interpretation depends on whether the O-ring seals in a face groove, internal groove, external groove or dynamic gland:
- Groove ID: the inside diameter of the groove. Groove ID is used as the starting reference, but the final O-ring ID depends on the seal geometry and intended installation stretch.
- Groove depth: this gives an indication of the required cross section. Groove depth helps estimate the required cross section, but the final squeeze depends on the seal design standard and the application.
- Groove width: Groove width must allow the compressed O-ring to deform laterally without overfilling the gland. The exact width depends on the gland design standard.
Cross-reference your groove measurements against the nearest standard O-ring size. The nearest standard size is usually the best starting point for specification, subject to groove fit and application requirements.
The relationship that saves time: OD = ID + (2 × CS). If you can measure any two of the three dimensions accurately, the third is calculated. A measured OD of 26 mm and CS of 3 mm gives an ID of 20 mm — that is a standard metric 20 × 3 O-ring.
Measuring a used or damaged O-ring
Used O-rings present specific measurement challenges. The material may have swollen, compressed, cracked or permanently deformed in service. Treat measurements from used O-rings as reference starting points, not confirmed nominal dimensions.
- Swelling: if the O-ring material has swollen from contact with an incompatible fluid, all three dimensions will be larger than nominal. Swelling is a sign of material incompatibility — the replacement O-ring needs a different material, not just the same size.
- Compression set: the cross section will appear smaller than nominal at points that were under load in service. Measure at undamaged points and use the highest consistent reading.
- Out-of-round ID: measure ID at two perpendicular diameters. If the two ID readings differ noticeably, the O-ring has distorted and groove measurement becomes more reliable than ring measurement.
- Cracking or tearing: a cracked O-ring that cannot be laid flat cannot be measured reliably. In this case, measure the groove and cross-reference against size charts.
Frequently asked questions
How do I find the right O-ring size?
Measure inside diameter (ID), outside diameter (OD) and cross section (CS). Use light calliper contact so the O-ring is not stretched or compressed during measurement. If the original O-ring is missing or distorted, measure the groove as a starting reference and confirm the result against the nearest standard O-ring size and the actual seal design.
What is the difference between O-ring ID and OD?
ID is the inside diameter — the measurement across the hole in the centre of the O-ring. OD is the outside diameter — the total width across the O-ring including the cord material. The relationship between the two is: OD = ID + (2 × cross section). For example, an O-ring with 20 mm ID and 3 mm cross section has an OD of 26 mm.
Can I measure a used O-ring to find the replacement size?
Yes, with caution. A used O-ring may have swollen, shrunk or deformed in service. Measure the ID across at least two diameters, without stretching the ring, to check for out-of-round distortion. For cross section, measure at a point away from any visible damage or compression marks. Use the measurements as a starting reference and cross-check against the nearest standard size on a size chart — do not treat the exact measured dimension as the finished specification without confirming against the standard.
ID, OD and CS. All three. From an undamaged point on an undistorted O-ring — or from the groove when the original is gone.
Know any two of the three dimensions and the third follows. Measure at multiple points to check for distortion. Cross-reference against the nearest standard size rather than ordering to exact measured dimensions. The nearest standard size is usually the best starting specification, subject to groove fit and application requirements.