How to Measure a Bearing: Bore, OD and Width
Three numbers identify a bearing
Every metric rolling bearing is described by three dimensions: the bore (d) — the hole that fits the shaft, the outside diameter (D), and the width (B) across the two side faces. Measure these three accurately and you can identify almost any standard deep groove ball bearing, even one with no readable markings left.
All measurements here are in millimetres, because standard bearing dimensions are metric worldwide — even in countries that use inches for everything else.
What you need
A digital or vernier caliper is the right tool. A ruler or tape measure is not accurate enough: bearing bores are machined to hundredths of a millimetre, and a 1 mm reading error can point you at the wrong size.
Before measuring:
- Clean the bearing. Wipe off grease and dirt — old grease can add a visible fraction of a millimetre to a width reading.
- Work at room temperature. A bearing pulled hot from a machine reads slightly oversize.
- Take each measurement twice, rotating the bearing 90° between readings, and use the average.
Measuring the bore (d)
Use the caliper’s inside jaws (the small upper jaws). Insert them into the bore, open them until they touch both sides, and rock the caliper gently to find the largest reading — that is the true diameter.
The bore is the most important number, because it is encoded directly in the designation. For the common bore codes:
| Bore code | Bore (mm) |
|---|---|
| 00 | 10 |
| 01 | 12 |
| 02 | 15 |
| 03 | 17 |
| 04 and up | code × 5 |
So a 20 mm bore means bore code 04, and the designation ends in “04” — 6204, 6304, 6004 and so on. The full system is explained in How to Read a Bearing Designation.
Measuring OD (D) and width (B)
For the outside diameter, close the caliper’s main jaws across the outer ring. Measure on the cylindrical outer surface, square to the axis.
For the width, measure across the two flat side faces of the rings — outer face to outer face. Do not include the slight protrusion of a rubber seal; measure metal to metal if you can feel the ring faces through the seal, or compare with an open section of the bearing.
From numbers to a designation
Three measurements usually land you on exactly one standard size. Some common combinations for deep groove ball bearings:
| You measured (d × D × B, mm) | Likely designation |
|---|---|
| 10 × 30 × 9 | 6200 |
| 20 × 47 × 14 | 6204 |
| 25 × 52 × 15 | 6205 |
| 30 × 62 × 16 | 6206 |
| 40 × 80 × 18 | 6208 |
| 50 × 90 × 20 | 6210 |
The outside diameter tells you the dimension series. A 20 mm bore, for example, comes in several OD/width combinations: 42 × 12 mm is a 6004, 47 × 14 mm is a 6204, and 52 × 15 mm is a 6304 — same shaft, increasingly heavy bearing. That trade-off is covered in detail in 6204 vs 6304. This is why the bore alone is never enough: write down all three numbers before you order.
Common traps
- Inch-size bearings. If your readings come out as odd decimals — a 15.875 mm bore, for instance — you probably have an inch-series bearing (15.875 mm is exactly 5/8”). These follow different designation systems; measure carefully and do not round to the nearest millimetre.
- Tapered bores. Some bearings (suffix K) have a bore machined on a 1:12 taper for adapter sleeves. A caliper reads different values at each face; if that is what you see, do not average them — the bearing is a tapered-bore type.
- Worn or damaged bearings. If the shaft wore the bore out of round, your measurement reflects the damage, not the original size. In that case measure the shaft and housing seats instead — they define what the replacement must be.
- Shield recesses and snap-ring grooves are not part of D. Measure the largest cylindrical surface only.
When the numbers still do not match
If your three dimensions sit between catalogue sizes, the bearing may be a special, an inch size, or simply too worn to measure reliably. At that point, photographs of the bearing plus your three readings are what a supplier needs to cross-reference it.
Next step: once you have a designation, check its full dimensions and ratings on the detail pages — for example the 6204 — or use the Bearing Selection guide for preliminary selection.