Bearing Size Calculator in mm: Find the Right Dimensions

A bearing size calculator in millimetres uses a bearing’s inside diameter, outside diameter and width to identify or compare its basic dimensions. Enter those three measurements to get a standard size description; the dimensions alone can narrow the search, but they do not confirm a bearing’s load capacity, fit or suitability.

Formula

This calculator reports the three measurements in the conventional order: bore diameter × outside diameter × width. The symbolically expressed dimension set is d × D × B, where d is the bore (the diameter of the shaft opening) in millimetres, D is the bearing’s outside diameter in millimetres, and B is its width in millimetres. The first result displays the bore value; use the three entered dimensions together as the actual size.

For inch equivalents, divide each millimetre measurement by 25.4, because one inch equals exactly 25.4 mm. The ratio result is D ÷ d; it is a simple comparison, not a bearing standard or a selection rule. A complete size set should have positive dimensions, and the outside diameter and width normally exceed zero. This simple calculator checks the permitted non-negative input range, but it does not validate whether a combination corresponds to a manufactured bearing.

Worked Example

Using the calculator’s default inputs—bore 20 mm, outside diameter 47 mm and width 14 mm—the dimension set is:

• Bore: d = 20 mm
• Outside diameter: D = 47 mm
• Width: B = 14 mm

Write the dimensions in the usual order as 20 × 47 × 14 mm. For the inch equivalents, calculate each value separately: 20 ÷ 25.4 = 0.787 in; 47 ÷ 25.4 = 1.850 in; and 14 ÷ 25.4 = 0.551 in, rounded to three decimal places. The ratio is 47 ÷ 20 = 2.35. These conversions do not mean that the bearing should be ordered by rounded inch measurements; retain the original metric dimensions when matching a metric bearing.

How to Measure a Bearing

Measure a clean, unmounted bearing with a suitable measuring instrument. A vernier or digital caliper is often useful for an initial check; a micrometer or other appropriate instrument may be needed when tighter dimensional tolerances matter. Take care to measure the bearing itself rather than a seal lip, chamfer, shoulder or surrounding housing feature.

1. Measure the bore: measure across the central opening, through its axis. Avoid measuring at a chamfered edge.
2. Measure the outside diameter: measure across the outer ring at its widest cylindrical surface, not across a seal or protruding feature.
3. Measure the width: measure axially from one side face to the other. Note any flanges or unusual faces separately.
4. Repeat and record: take readings at more than one orientation where possible. Record all three in mm, and note the bearing type and any markings.

For a bearing still installed in a machine, direct access may be obstructed. Measuring the shaft or housing can give clues, but those are not automatically the bearing’s actual bore or outside diameter: shoulders, sleeves, clearances and fits affect what can be measured in place.

What the Dimensions Tell You

The three numbers describe the bearing’s basic envelope, not every property that determines whether it will work. Bearings with the same nominal bore, outside diameter and width can differ in rolling-element design, seals, shields, internal clearance, precision class, cage construction and allowable operating conditions. A size match is a useful first filter, not proof of interchangeability.

Common radial bearing types include deep-groove ball bearings, angular-contact ball bearings and cylindrical roller bearings. Their shapes and intended applications differ, even where their outer dimensions coincide. Thrust bearings and special designs may not be described adequately by the same three measurements. The relevant manufacturer’s drawing or catalogue may list additional dimensions, such as a flange diameter, shoulder dimensions or corner radii.

Many metric bearing designations encode a series and bore information, but the code is not a universal substitute for checking a catalogue. Some bore-code conventions have exceptions, and different bearing families may use different designation systems. Use a legible marking, a reliable dimensional reference or supplier documentation to verify a candidate part.

Choosing and Matching a Replacement

Start with the existing bearing’s full designation if it is readable. Confirm the type and all suffixes against a current product reference; suffixes can distinguish features that are not visible in the three basic dimensions. If the code is missing, use the measured dimensions to shortlist candidates and then verify the application requirements before ordering.

• Check the required bore, outside diameter and width, including any tolerance or fit requirement.
• Confirm the bearing type and whether seals, shields or an open construction are needed.
• Check relevant load direction, rotational speed, alignment, temperature and lubrication conditions using the equipment documentation and bearing manufacturer’s data.
• Verify that the shaft and housing interfaces, shoulders and retaining features match the replacement design.
• Use the original equipment documentation or obtain qualified technical advice where a wrong selection could cause equipment damage or injury.

A bearing is not made suitable simply by fitting into the available space. Do not select a bearing for a safety-critical or high-load application from dimensions alone; this calculator does not perform load, life, speed, fit or safety calculations.

Metric and Inch Measurements

In South Africa, bearing dimensions are commonly specified in millimetres in metric catalogues and technical documents. Imported equipment or older references may show inch values, so converting accurately can help identify a likely size. Keep the original units visible throughout the calculation: 1 mm is approximately 0.03937 in, while 1 in is exactly 25.4 mm.

Do not round a measurement too early. A displayed value of 20 mm could represent a nominal size, while a caliper reading may vary slightly because of measurement technique, wear, dirt or manufacturing tolerance. The calculator’s inch output is rounded for readability; use unrounded measurements and the bearing’s nominal specification when checking a catalogue or discussing fit with a supplier.

Common Measurement Mistakes

• Mixing units: entering one measurement in inches and the others in millimetres produces a misleading set. Convert all inputs to mm first.
• Measuring a seal: a seal or shield can extend beyond the ring and distort an outside-diameter reading.
• Confusing width with total assembly length: spacers, washers and collars are not part of the bearing width unless the drawing specifies an integrated feature.
• Assuming equal dimensions mean equal performance: internal design and application requirements still need checking.
• Using worn or contaminated parts as a reference: corrosion, damage and debris can affect readings. Clean the part carefully and compare against its designation or drawing.

Frequently Asked Questions

How do I find bearing size from measurements?
Measure the bore, outside diameter and width, then record them in that order in millimetres. Compare the resulting set with a catalogue or verified part drawing; dimensions alone do not establish type or performance.

What does 20 × 47 × 14 mm mean?
It means a nominal 20 mm bore, 47 mm outside diameter and 14 mm width. It describes the basic envelope, not the bearing’s seal arrangement, internal clearance or load rating.

Can I identify a bearing by its outside diameter only?
No. Many bearings can share an outside diameter while having different bores, widths and designs. Measure all three dimensions and check any visible designation.

How do I convert bearing dimensions from mm to inches?
Divide each millimetre measurement by 25.4. Keep enough precision for comparison, and use the nominal catalogue dimensions rather than rounded conversions when ordering.

Does this calculator tell me which bearing to buy?
No. It formats and converts basic dimensions. To choose a replacement, also confirm bearing type, seals, fits, load and operating conditions against reliable technical documentation.

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in id | Bore diameter | mm | 20 | 0 | 10000
in od | Outside diameter | mm | 47 | 0 | 10000
in w | Width | mm | 14 | 0 | 10000
out dimensions | Bearing dimensions (bore × outside diameter × width) | mm | id | 0
out inch | Equivalent dimensions (bore × outside diameter × width) | in | id / 25.4 | 3
out ratio | Outside diameter to bore ratio | - | od / id | 2