How to Measure Bearing Size: A Complete Step-by-Step Guide

To measure bearing size, you need three numbers: the inner diameter (bore), the outer diameter (OD), and the width. Use a caliper to measure each dimension directly on the bearing, or read the bearing's printed part number and decode it using a standard bore chart. These three measurements, recorded in millimeters or inches, are all that is needed to identify a replacement bearing or order a matching one from a catalog.

Bearings look simple from the outside, but a mismatch of even half a millimeter in bore size can prevent a replacement part from fitting a shaft or housing at all. This guide explains exactly how to measure bearing size correctly, the tools required, how to decode bearing part numbers, and the mistakes that most often lead to ordering the wrong part.

The Three Core Dimensions of Every Bearing

Every rolling-element bearing, whether it is a ball bearing or a roller bearing, is defined by the same three boundary dimensions established under international standard ISO 15:2017 for metric ball bearings and ABMA Standard 9 in the United States. Knowing what each dimension represents is the first step in measuring bearing size correctly.

Inner Diameter (Bore Diameter)

The bore diameter is the hole through the center of the bearing where the shaft passes through. This is the single most critical dimension, because if the bore does not match the shaft diameter, the bearing cannot be installed. Bore sizes typically range from 3mm in miniature bearings to over 200mm in industrial bearings.

Outer Diameter (OD)

The outer diameter is the measurement across the outside edge of the bearing, where it sits inside the housing. The OD must match the housing bore precisely enough to seat the bearing without excessive looseness or interference.

Width (or Thickness)

Width refers to the distance from one face of the bearing to the other, measured along the axis of rotation. On thrust bearings, this dimension is sometimes called height instead of width, but it is measured the same way.

Dimension	What It Measures	Typical Tool	Common Range
Bore (Inner Diameter)	Diameter of the shaft-side hole	Inside caliper, digital caliper	3mm to 200mm+
Outer Diameter	Diameter of the housing-side surface	Outside caliper, digital caliper	8mm to 350mm+
Width	Distance between the two faces	Depth gauge, caliper jaws	2mm to 90mm+

Table 1. The three boundary dimensions defined under ISO 15:2017 and ABMA Standard 9 for radial ball bearings.

Tools Required to Measure Bearing Size Accurately

A digital caliper is the most practical tool for measuring bearing size for most users, offering readings accurate to within 0.02mm. For tighter tolerance work, a micrometer or bore gauge improves precision further.

Digital and Vernier Calipers

A digital caliper measures bore, outer diameter, and width in a single tool by switching between its outside jaws, inside jaws, and depth rod. Digital calipers typically read to a resolution of 0.01mm or 0.0005 inches, while a traditional vernier caliper is read manually to about 0.05mm resolution.

Outside and Inside Micrometers

A micrometer offers higher accuracy than a caliper, often within 0.001mm, making it the preferred tool for precision bearings used in spindles, gearboxes, or aerospace components where tolerance stacking matters.

Bearing Number Charts

If the bearing still has a legible part number stamped on its shield or ring, a bearing number chart can substitute for physical measurement entirely, since the number itself encodes the bore, OD, and width series.

Tool	Typical Accuracy	Best Use Case	Limitation
Digital Caliper	±0.02mm	General bore, OD, width checks	Battery dependent
Vernier Caliper	±0.05mm	Field measurements, no battery needed	Slower to read
Micrometer	±0.001mm	Precision tolerance verification	Limited measuring range per tool
Bearing Number Chart	Exact (if legible)	Quick identification, no tools needed	Useless if number is worn off

Table 2. Comparison of common tools used to measure bearing size and their typical accuracy ranges.

Step-by-Step Process to Measure a Bearing

Measuring a bearing correctly takes less than five minutes when the steps are followed in order. The process below applies to ball bearings, roller bearings, and most thrust bearings.

1. Clean the bearing with a dry cloth or low-pressure air to remove grease, dirt, or rust that can distort readings.
2. Check for a printed part number on the shield, inner ring, or outer ring face before measuring, since this can confirm or skip the manual process.
3. Measure the bore by opening the caliper's inside jaws inside the inner ring and taking the reading at two points 90 degrees apart, then averaging if they differ.
4. Measure the outer diameter by closing the caliper's outside jaws around the outer ring, again checking two points to confirm roundness.
5. Measure the width using the caliper's depth blade or by holding the bearing between the outside jaws on its side.
6. Round to the nearest standard size, since manufacturing tolerances mean a "17mm" bore may read as 16.98mm or 17.01mm on a caliper.
7. Cross-check with a bore chart to confirm the three measurements correspond to a real, manufactured bearing series rather than a measurement error.

How to Read a Bearing Number to Find Its Size

Most metric bearings follow a numbering system where the last two digits of the part number indicate bore size, based on a formula set under ISO 104 and used industry-wide. For bore codes from 04 to 96, multiply the two-digit code by 5 to get the bore diameter in millimeters.

For example, a bearing numbered 6203 has a bore code of "03," which falls into the exception range below 04 rather than the multiply-by-5 rule. Bore codes 00 through 03 use fixed values instead of the formula.

Bore Code	Bore Diameter	Example Part Number
00	10mm	6200
01	12mm	6201
02	15mm	6202
03	17mm	6203
04	20mm (4 x 5)	6204
10	50mm (10 x 5)	6210

Table 3. Standard metric bore codes based on ISO 104, showing how the last two digits of a bearing number translate to bore diameter.

The complete part number 6203 also tells more than bore size: the leading "6" indicates a single-row deep groove ball bearing series, and the "2" before the bore code indicates a light duty series, meaning the OD and width follow standardized values of 40mm OD and 12mm width for that exact bore and series combination.

Metric vs Inch Bearing Measurement: Key Differences

Metric bearings are measured in millimeters and follow ISO numbering, while inch-based bearings are measured in fractions or decimal inches and follow ANSI/ABMA numbering conventions common in North American equipment built before metric standardization spread globally.

Feature	Metric Bearings	Inch Bearings
Unit of Measurement	Millimeters (mm)	Inches (in)
Governing Standard	ISO 15 / ISO 104	ABMA / ANSI
Common Region of Use	Global, predominant worldwide	Legacy North American equipment
Numbering Style	Bore code x 5 formula	Fractional bore (e.g., 1/2 inch)
Interchangeability	Not directly interchangeable with inch sizes	Not directly interchangeable with metric sizes

Table 4. Key differences between metric and inch bearing measurement systems.

Mixing the two systems is one of the most frequent sourcing errors. A bore of 19.05mm is mathematically identical to 0.75 inches, but the bearing itself will carry a different part number depending on which standard it was manufactured under, so converting the number alone is not sufficient confirmation of fit.

How to Measure a Bearing With No Visible Numbers

When a bearing's part number has worn away or it has already failed and fallen apart, measure the shaft diameter and housing bore directly instead of the bearing itself, since these two surfaces must match the bearing's bore and OD exactly.

Measure the shaft where the bearing seated using an outside caliper, and measure the housing bore using an inside caliper or telescoping gauge. The shaft diameter equals the bearing's bore size, and the housing bore equals the bearing's OD, within the manufacturer's specified fit tolerance. The width can typically be estimated from the gap left in the housing or matched against a replacement catalog using the two known dimensions.

Common Mistakes to Avoid When Measuring Bearing Size

• Measuring over dirt or grease buildup, which adds false thickness to the bore or OD reading.
• Taking only one measurement point instead of checking the bore or OD at two angles to catch out-of-round wear.
• Confusing bore size with shaft size after the shaft has already worn down from years of bearing rotation.
• Mixing metric and inch readings by rounding a 0.7874-inch reading to "20mm" without checking whether it is actually an inch-standard part.
• Ignoring the bearing's width entirely and ordering based on bore and OD alone, which can result in a part that fits the shaft and housing but leaves a gap or interferes with adjacent components.

Frequently Asked Questions

What is the easiest way to measure bearing size without a caliper?

The easiest method without a caliper is to read the part number stamped on the bearing's shield and look it up on a bore chart, since the number alone reveals bore, OD, and width without any measuring tool.

Why does my caliper reading not match a standard bearing size?

A caliper reading that falls slightly off a standard size, such as 16.98mm instead of 17mm, usually reflects normal manufacturing tolerance or minor wear, not measurement error, and should be rounded to the nearest standard bore size.

Can I use a ruler instead of a caliper to measure bearing size?

A ruler can give an approximate bearing size but is not accurate enough for ordering a replacement, since ruler readings are typically only precise to about 0.5mm, which can span the difference between two adjacent standard bore sizes.

How do I know if a bearing is metric or inch-sized?

A bearing is metric if its part number follows the ISO bore-code format such as 6203 or 6309, and it is inch-sized if the number references a fractional or decimal inch designation such as those used in tapered roller bearing series.

Does bearing width matter if the bore and OD already match?

Width still matters even when bore and OD match, because a bearing that is too narrow or too wide can shift inside the housing or fail to seat against retaining shoulders, leading to premature wear or noise.