Bearing Size Calculator in mm: How to Find Yours

A bearing size calculator in mm works out the key dimensions of a rolling-element bearing — bore (inner diameter), outer diameter, and width — from a part number or from direct measurements. You enter either a standard bearing code or the three diameters in millimeters, and it returns the matching bearing size and often the standard designation. This article explains how bearing sizing works, how to read the numbers stamped on a bearing, and how to use a calculator correctly.

Formula

The core relationship for metric bearings is simple: the bore diameter in millimeters is the first two digits of the bearing code multiplied by 5, for bore sizes from 20 mm up to 480 mm. The formula for the code is:

Code = (d / 5) × 10 + (D − d) / 5

Where:

• d = bore diameter in mm (the shaft diameter)
• D = outer diameter in mm
• B = width in mm

The first part (d/5) gives the two-digit size series (e.g., 20 mm bore → 04). The second part (D − d)/5 gives the diameter series number (e.g., 47 − 20 = 27, divided by 5 ≈ 5.4, which rounds to 5, giving a 6205 bearing). This works for the common 6xxx deep-groove ball bearings. For other types (angular contact, roller, thrust), the same logic applies but with different prefix letters (e.g., 7xxx for angular contact, 3xxx for cylindrical roller).

Worked Example

Using the default values from the calculator: bore = 20 mm, outer = 47 mm, width = 14 mm.

1. Bore code: 20 / 5 = 4 → the last two digits of the code are “04�.
2. Diameter series: (47 − 20) / 5 = 27 / 5 = 5.4 → rounds to 5.
3. Combine: the code is 6205 (the “6� is the type prefix for deep-groove ball bearings).
4. Width check: a 6205 bearing has a width of 15 mm, but your measured width is 14 mm — this suggests you might have a 6205-2RS (sealed) or a different variant with a slightly narrower width. Always verify with the manufacturer’s table.

The calculator shows “Likely bearing code� as 6205, bore in inches = 0.787, outer in inches = 1.850, width in inches = 0.551. This matches a standard 6205 bearing.

How Bearing Sizes Are Standardized

Most metric bearings follow ISO 15, which defines the boundary dimensions (bore, outer diameter, width) for each size series. The key is the bore code:

• Bore < 20 mm: the code is the bore in mm (e.g., 608 bearing has 8 mm bore).
• Bore 20–480 mm: the code is bore / 5 (e.g., 20 mm → 04, 25 mm → 05, 30 mm → 06).
• Bore ≥ 500 mm: the code is the bore in mm directly (e.g., 500 mm → 500).

The diameter series (the second digit in a 4-digit code) indicates how heavy the cross-section is. For example, a 6205 (medium series) has a larger outer diameter than a 6005 (light series) for the same bore. The width series is the third digit, but for the most common 62 and 63 series, it’s fixed.

How to Use the Calculator Correctly

To get an accurate result, you need to measure the bearing precisely:

• Use a caliper (digital or dial) for all three dimensions.
• Measure the bore at the widest point (the inner ring’s hole).
• Measure the outer diameter at the widest point (the outer ring’s outside).
• Measure the width across the full ring, not just the exposed part.
• Take three readings and average them to avoid errors from dirt or wear.

If you have a bearing with a code stamped on it (e.g., 6204, 6308), you don’t need to measure — just look up the code in a bearing table. The calculator is most useful when the code is worn off or you have a loose bearing and need to identify it.

Common Mistakes and Misconceptions

One common mistake is mixing up the bore code with the actual bore. For example, a “6204� bearing does not have a 4 mm bore — it has a 20 mm bore (4 × 5). This trips up many beginners. Another error is ignoring the width: two bearings can have the same bore and outer diameter but different widths (e.g., 6205 is 15 mm wide, while 6305 is 17 mm wide). Always check the width.

Another misconception is that the calculator can identify the bearing type (ball, roller, needle) from dimensions alone. It can’t — you need to look at the rolling elements. A 6205 and a 3205 (angular contact) have the same dimensions but different internal designs. Use the calculator for size, then confirm the type visually.

When to Use a Bearing Size Calculator

Use this calculator when you need to replace a bearing and only have the shaft and housing dimensions. For example, if you have a shaft that is 20 mm in diameter and a housing that is 47 mm, the calculator tells you a 6205 is the likely fit. You can then cross-reference with a bearing catalog to find the exact type and any suffixes (e.g., 2RS for rubber seals, ZZ for metal shields, C3 for internal clearance).

It’s also useful when you have a bearing in hand but the markings are unreadable. Measure the three dimensions, run the calculator, and you’ll get a shortlist of possible codes. Then compare with the width and any other features (flanges, snap rings) to narrow it down.

Limitations of the Calculator

This calculator works for standard metric bearings that follow the 5-mm increment rule. It does not handle:

• Inch bearings (e.g., 6205 is metric, but a 6205-2RS is still metric; inch bearings like 6205-2RS are not standard).
• Very small bearings (< 20 mm bore) where the code equals the bore directly.
• Special bearings with non-standard dimensions (e.g., thin-section, custom).
• Bearing types with different dimension series (e.g., 4-point contact, tapered roller).

For those, you need a full bearing catalog or a manufacturer’s cross-reference. The calculator gives a good starting point but should not be the final word for critical applications.

Frequently Asked Questions

What does the number on a bearing mean?

The first digit is the type (6 = deep groove ball, 7 = angular contact, 3 = cylindrical roller). The next two digits are the bore code (multiply by 5 for bores 20–480 mm). The last digit is the diameter series (e.g., 2 = light, 3 = medium). Suffixes like 2RS or ZZ indicate seals or shields.

How do I measure a bearing without a caliper?

If you don’t have a caliper, use a ruler with millimeter markings. Place the bearing on a flat surface, measure the inner hole (bore) and outer diameter at the widest points. For the width, stand the bearing on edge and measure the height. A caliper is more accurate, but a ruler gives you a rough estimate that can still help identify the size.

Can I use the bore size in inches instead of mm?

Yes, but you must convert to millimeters first. Multiply inches by 25.4 to get millimeters. For example, a 0.787-inch bore is 20 mm. The calculator works in mm, so always convert.

Why does my bearing have a different width than the standard?

Some bearings have non-standard widths due to seals or shields (e.g., a 6205-2RS is often 15 mm, but a 6205-ZZ is also 15 mm — they are usually the same). However, some manufacturers make “narrow� versions for specific applications. Check the bearing code and the manufacturer’s data sheet.

What if the calculator gives a code that doesn’t exist?

This can happen if your measurements are off or if the bearing is a non-standard type. Double-check your measurements, and try rounding to the nearest standard size. If it still doesn’t match, you may have a special bearing that requires a catalog search.

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in d | Bore diameter | mm | 20 | 0 | 1000
in D | Outer diameter | mm | 47 | 1 | 2000
in B | Width | mm | 14 | 1 | 500
out code | Likely bearing code | - | round((d/5)*10 + (D-d)/5) | 0
out bore_in | Bore in inches | in | d / 25.4 | 3
out od_in | Outer diameter in inches | in | D / 25.4 | 3
out width_in | Width in inches | in | B / 25.4 | 3