Metric vs inch bearings: how to tell them apart

Most bearings sold worldwide today follow the ISO metric dimension plans — the same plans behind standard numbers such as 6205 or 22210. Alongside them, a large population of inch-series bearings is still in use, especially in North American machinery, older equipment and some automotive and agricultural applications. Mixing the two up is an easy mistake and an expensive one.

Two separate design systems

The key point: an inch bearing is not a metric bearing measured in inches, and a metric bearing is not an inch bearing measured in millimetres. Each was designed in its own unit, so its dimensions only come out as “clean” numbers in that unit.

Why a 6205 is not an inch bearing

The 6205 is 25 × 52 × 15 mm. In inches that is roughly 0.9843 × 2.0472 × 0.5906 in. Those are perfectly valid conversions — every size page on MyBearings shows them — but they are not inch sizes. A 25 mm bore is not a 1 inch (25.4 mm) bore: the 0.4 mm difference is far beyond any shaft fit. Fit a 6205 to a 1-inch shaft and it simply will not go on; fit it to a worn shaft that “measures about an inch” and you are likely looking at the wrong bearing.

So the inch figures on a metric bearing page are there to help people who measure in inches — not to suggest the bearing has an inch equivalent.

An inch example: the R-series

Small inch ball bearings are often found in the R-series. For example, the R8 has a ½ inch bore, a 1⅛ inch outside diameter and a 5⁄16 inch width — 12.700 × 28.575 × 7.938 mm. In millimetres those numbers look awkward; in inches they are clean fractions. That is the giveaway.

How to tell which one you have

1. Measure carefully with calipers: bore, outside diameter, width. Our guide on how to measure a bearing walks through it.
2. Check millimetres first. If all three readings are within a few hundredths of whole millimetres (25.00, 52.00, 15.00), it is metric.
3. Then check inches. Divide by 25.4. If the results land on common fractions (0.500, 0.750, 1.125, 0.3125), it is probably an inch bearing.
4. Look at the marking. Metric standard numbers follow the type–series–bore pattern explained in how to read a bearing number. Inch numbers follow different schemes (R-series, numeric inch series, or paired cone and cup numbers for tapered roller bearings).
5. Check the equipment. Machinery designed in inch units usually uses inch bearings throughout.

Watch out for near-misses

Some metric and inch sizes are close enough to cause confusion at a glance but not close enough to work:

Any of those differences is enough to ruin a fit. If your reading sits halfway between a metric and an inch value, re-measure, clean the part, and consider wear before deciding.

Inch bearings in catalogs

For catalog teams, the most common problems are:

• Unit-less dimension columns. A value of “1” might mean 1 inch or a data error. Always store the unit with the value.
• Converted values stored as originals. If a supplier converted inch sizes to millimetres with rounding, you can lose the exact fraction. Keep the original value and unit, and compute conversions for display.
• Mixed filters. A filter for “25 mm bore” should not quietly include 1-inch bearings because they round to 25.

Our guide to normalizing width fields describes how to keep raw values and units next to normalized ones.

Coverage on MyBearings

The bearing size reference currently covers standard metric series. Inch series are on our list. Until then, use the size chart to rule metric sizes in or out, and check inch bearings against the manufacturer’s inch catalog.

Apply it

Measure first, then search the bearing reference with the dimensions (for example 25x52). If you need to clean up unit handling across a catalog, see normalization or contact us.