Getting the wrong bearing size can mean costly downtime, equipment failure, or wasted money on parts that don't fit. Whether you're replacing a worn bearing or specifying components for a new assembly, accurate measurement is non-negotiable. This guide walks you through the standard measurement points, how to read dimension charts, and the common pitfalls that lead to ordering errors.
Understanding the Three Critical Measurements
Every bearing has three core dimensions you need to know: bore diameter (inner), outer diameter, and width. These numbers determine whether a bearing will mount correctly and handle your load requirements.
The bore diameter is the hole size—typically measured in millimeters and ranging from 8mm for tiny precision bearings to 200mm+ for heavy industrial applications. This is the most important measurement because the shaft it sits on must match exactly. A 25mm bore bearing won't fit on a 24mm shaft, and forcing it causes rapid failure.
The outer diameter sits outside the rolling elements and is equally critical for housing fit. Standard metric bearings follow ISO 286 tolerances, so a listed dimension like 52mm has a tight tolerance band (usually ±0.2mm). Measure from the outer raceway—not the shield or seal, which can add 1-2mm.
The width (or height) determines axial clearance and how much load the bearing can distribute. For deep-groove ball bearings, this typically ranges from 12mm for small sizes to 50mm+ for large ones.
How to Measure a Bearing You Already Have
If you're replacing an existing bearing and don't have the part number, direct measurement is your fastest path to the right replacement.
Use a digital caliper—mechanical ones drift too much. Measure the bore diameter by placing the caliper inside the bearing race, perpendicular to the outer edge. Take the measurement at least twice, rotating the bearing slightly, to confirm the size. Do the same for outer diameter from the outside. For width, measure from one side of the bearing housing to the other along the axis.
Write down all three measurements to the nearest 0.5mm, then cross-reference against bearing dimension tables. Most standard ball bearings, roller bearings, and needle bearings follow ISO 15:1 or similar series that make lookup straightforward—a 52mm OD and 25mm bore almost certainly points to a 6205 series bearing.
Reading Bearing Size Charts and Part Numbers
Bearing part numbers encode dimensions. A "6205" designation tells you:
- 6 = deep-groove ball bearing
- 20 = bore size in millimeters (but read as 2.5 × 8 = 20mm)
- 5 = diameter series (OD/bore ratio)
Looking at a manufacturer's size chart, you'll see columns for bore, OD, width, and basic load ratings (measured in Newtons or kilograms). For a 6205, you'd find bore 25mm, OD 52mm, width 15mm. Load ratings typically range from 4,000N for small bearings up to 100,000N+ for heavy-duty sizes.
Cross-reference your measured dimensions against multiple bearing series—not all combinations are standard. A chart from SKF, FAG, NSK, or Timken shows the full range of available sizes. Mercoly lets you compare bearing specifications and find trusted Power Transmission suppliers offering different brands in one place, saving hours on sourcing.
Common Measurement Mistakes
Measuring with the wrong tool: Rulers and tape measures introduce ±1-2mm errors. Always use a digital caliper (cost $10-25) for accuracy.
Including shields and seals in outer diameter: Metal or rubber seals can add 1-3mm to the OD. Measure only the bearing housing itself.
Confusing metric and inch sizes: Some older industrial equipment uses inch-based bearings (bore in sixteenths of an inch). A 1⅜" bore bearing is roughly 34.9mm, not 138mm. Check your equipment manual.
Ignoring tolerance grades: A standard bearing is ISO P6 (general-purpose). High-speed spindle applications need P5 or P4, which have tighter tolerances and higher cost (20-40% premium). Measure clearances if you're replacing in critical applications.
Selecting the Right Bearing Beyond Size
Once you've got dimensions locked in, consider load type (radial vs. thrust), speed rating (RPM limits), and duty cycle. A 6205 deep-groove ball bearing handles radial and some thrust load at moderate speeds (up to 10,000 RPM). Heavy radial loads over 5,000 RPM need cylindrical roller bearings. Purely axial loads require thrust bearings.
Expected bearing life depends on load—at rated load, a quality bearing lasts 10,000+ hours; overload cuts this dramatically. Factor in your application's actual conditions, not just the part number.
Frequently Asked Questions
Q: Can I order a bearing by measuring just the bore diameter? No—you need all three dimensions. Two different bearing series can share the same bore but have different outer diameters and widths, causing fit failures.
Q: What tolerance should I specify for a replacement bearing? Order the same tolerance grade as the original (usually P6 for standard applications). Tighter grades cost 15-50% more but aren't necessary unless you're operating at very high speeds or tight clearances.
Q: How do I know if a bearing is worn enough to replace? Listen for grinding or rumbling sounds, measure radial play with a dial indicator (more than 0.5mm usually means replacement), and check for discoloration or pitting on the races during inspection.
Use these measurements and cross-checks to order the exact bearing you need—get it right the first time and keep your equipment running.