How to Measure a Bearing: Inner Diameter, Outer Diameter, and Width with a Caliper

You pulled a bearing off a motor spindle and the part number is gone. Before you order a replacement, you need three numbers: the inner diameter (ID), the outer diameter (OD), and the width (W). Measure those three straight with a good caliper, and you can match almost any standard bearing on the market.

Tools You Need Before You Start

Measuring a bearing is quick if you use the right tool. A digital caliper is the best all-round choice because it measures inside, outside, and depth with the same instrument.

• Digital caliper (0–150 mm capacity covers most small and medium bearings)
• Clean, lint-free cloth
• Magnifying glass or bright light to read any remaining markings
• Flat work surface so the bearing does not roll away

Clean the bearing first. Grease, dirt, or paint on the rings will throw off your reading. Wipe the bore, outside surface, and faces, then let the bearing sit at room temperature for a few minutes so thermal expansion does not distort your numbers.

The Three Critical Dimensions

Every standard single-row bearing is defined by three dimensions. Once you have these, you can identify the size series and order a replacement.

Step 1: Measure Inner Diameter (ID)

The inner diameter is the hole where the shaft fits. Use the smaller jaws on top of the caliper. Open them until they touch the bore surface, then read the value. Take two readings at right angles and average them, because the ring may be slightly out of round after years of service.

Do not place the jaws across the seal or shield. You want the actual machined bore of the inner ring. If the bearing has a rubber seal, position the caliper jaws on the exposed bore surface.

Step 2: Measure Outer Diameter (OD)

The outer diameter is the outside width of the outer ring. Use the large lower jaws and close them around the outside surface. Again, measure in two or three places and take the largest value that is consistent. A worn or damaged OD can affect the housing fit, so if your readings vary by more than a few hundredths of a millimetre, the bearing may have been running loose in its housing.

Step 3: Measure Width (W)

Width is the distance from one side of the outer ring to the other. Hold the bearing flat and place the outer jaws against both faces. Measure at a few points around the circumference because one face may be chamfered or slightly uneven. For sealed bearings, the nominal width includes the seals, but measure the metal ring faces if the seals protrude too far to give a reliable reference.

Read the Bearing Marking to Confirm Your Measurements

Most bearings have a code stamped on one face. This code is worth reading even if the bearing is worn, because it tells you exactly what the bearing was when it left the factory. A typical code looks like 6203-2RS. Here is what it means:

• The first digit, 6, is the bearing type: deep groove ball bearing.
• The digits 2 and 0 together, 62, represent the dimension series, which governs the outer diameter and width for a given bore.
• The last two digits, 03, are the bore code. For bores up to 20 mm, 00 means 10 mm, 01 means 12 mm, 02 means 15 mm, and 03 means 17 mm. For codes 04 and higher, multiply by 5 to get the bore in millimetres.
• The suffix 2RS means two rubber seals. Other common suffixes include ZZ for metal shields.

Use this code as a cross-check. If the numbers you measured do not line up with the bore code and dimension series, you might have measured the wrong face or the bearing might be a non-standard type.

Avoid These Common Measurement Mistakes

Most bad measurements come from a few repeated mistakes. Knowing them will save you from ordering the wrong bearing.

1. Not zeroing the caliper: close the jaws fully and press the zero button before measuring.
2. Mixing units: if your caliper is set to inches, convert to millimetres or switch back. Write the unit down next to each number.
3. Measuring over the seal: ID should be the machined bore, not the seal opening.
4. Measuring a worn bearing: if the rolling track is visibly pitted, the OD and width may still be accurate, but the bore can wear oval. In that case, trust the stamped code over the measured ID.
5. Squeezing the caliper too hard: the rings are thin on small bearings, and heavy pressure flexes them.

If your measurements are consistent within 0.02–0.05 mm, you have taken them well. Standard bearing tolerances for normal applications typically fall in that range for metric series, so a reading inside that window is reliable.

Choosing the Right Replacement Bearing

Once you have the ID, OD, and width, you know the size series. The next step is ensuring that the bearing you order is rated for the conditions it will see: load, speed, noise requirement, and environment.

For general industrial applications, standard deep groove ball bearings cover the vast majority of shafts and housings. They handle radial loads, moderate axial loads, and high speeds, and they are available with seals by default. A full size series, such as the 6200 range, is usually enough to replace a worn bearing in a motor, pump, or gearbox.

6200 Series Deep Groove Ball Bearings with SealsStandard chrome steel bearings in the 6200 size range for general industrial use, suitable for motors, pumps, and gearboxes where radial and moderate axial loads are expected.View Product →

If the original bearing failed because of moisture, chemicals, or frequent washdowns, switch to a stainless steel bearing. They use the same ID, OD, and width as standard steel versions, so your measurements carry straight over. A stainless bearing with rubber seals resists corrosion far better than a chrome steel one, which is why it is the first choice for chemical and electroplating environments.

SS 6200 Stainless Steel Deep Groove Ball BearingsCorrosion-resistant stainless steel bearings with rubber seals, matching standard 6200 dimensions, ideal for washdowns, chemical exposure, and food or agricultural machinery applications.View Product →

Before you buy, compare your three dimensions against the size table of the series you are ordering. The table will show a row such as 6203 with ID 17 mm, OD 40 mm, W 12 mm. If your measurements match, the bearing is the right one. For a broader explanation of how deep groove bearings are sized and selected, see this guide to deep groove ball bearing sizing and material selection.

Measuring a bearing is a simple three-step check: record the ID, OD, and width, verify them against the stamped code, and then choose the right material and seal arrangement for the job. Take your time, measure clean, and when a dimension seems off, measure again before you order. When in doubt, send your measurements to your bearing supplier—most will confirm the size and recommend a replacement in minutes.