Mechanical calculator
Bearing Dimensions and Size Converter
Compare a bearing’s bore, outside diameter, width, and optional flange in millimeters, decimal inches, and common inch fractions.
Sage’s guided lesson
Let’s understand the result before using it
Convert bearing bore, outside diameter, width, and flange dimensions between metric and imperial units, and learn what dimensions cannot identify. I’ll show you what the numbers mean, how to check the math yourself, and where real-world conditions can change the answer.
Start with the system
What this calculator is actually doing
Bearings are normally identified by three physical dimensions in a fixed order: bore, outside diameter, and width. Those dimensions help narrow a replacement, but they do not identify the bearing type, seal, internal clearance, load rating, speed rating, or tolerance class.
A bearing number is not enough when fit matters; confirm bore, outside diameter, width, seals, clearance, and load rating.
Now for the arithmetic
How to do the math yourself
The conversion itself is exact: inches × 25.4 = millimeters, and millimeters ÷ 25.4 = inches. The outside diameter to bore ratio is OD ÷ ID, while relative width is width ÷ OD × 100%.
Worked example
A bearing measures 8 mm bore × 22 mm outside diameter × 7 mm wide.8 ÷ 25.4 = 0.31496 in22 ÷ 25.4 = 0.86614 in7 ÷ 25.4 = 0.27559 inUse manufacturer dimensions for purchasing and fit checks, because nominal conversions can hide metric and inch series differences.
I know this is off topic, but this reminds me of a joke I heard when teaching this to one of my interns, Why did the door say sorry? It felt like it overreacted. Yeah, I don’t see the connection either, but I think it is cute!
Before you build, wire, mix, or run it
Tips, shortcuts, and mistakes to avoid
- Measure the bore and outside diameter across several positions; dirt, burrs, wear, and caliper angle can change a reading.
- Keep the dimension order consistent: ID × OD × width. Reversing the first two numbers produces an impossible bearing.
- Use the stamped part number whenever it is readable. Dimensions alone can match several bearings with very different seals, clearances, and load capacities.
Two bearings with matching dimensions can have different internal clearance, precision, sealing, speed, and load capability.
How we got here
A little history and background
Standardized bearing series developed as machinery became more interchangeable during the industrial era. Metric boundary dimensions are now common worldwide, while inch series remain important in older machines and specialized equipment.
What the result does not tell you
This calculator compares dimensions only. It cannot verify fit class, radial clearance, seal or shield type, internal geometry, load capacity, operating temperature, lubrication, or allowable speed.
Frequently asked questions
Can two bearings have the same dimensions but different ratings?
Yes. Bearings with identical boundary dimensions can use different internal designs, seals, clearances, materials, and load ratings.
How accurately should I measure?
Use clean surfaces and a suitable caliper or micrometer. Small bearings may differ by fractions of a millimeter, so rough ruler measurements are not enough.
Does the fraction identify the exact bearing?
No. Fractional inches are included for comparison, but the metric or decimal dimension and the bearing number are more reliable.
That wraps up the lesson
Class is over. It is officially cheese fries time.
You now know more about how the Bearing Dimensions and Size Converter actually works, not just which buttons to press. That earns cheese fries, Sage’s favorite way to celebrate a finished lesson.
Use the result within its limits
A dimensional match does not prove that a bearing is suitable for the machine. Confirm the manufacturer’s load, speed, fit, lubrication, and temperature requirements.