Bearing Dimension Calculator: How to Read Bearing Sizes and Bore Codes

Bearings rarely fail because the steel was wrong. They fail because the fit was wrong, and the fit begins with numbers. A bearing dimension calculator exists to make those numbers quick to check: it converts a designation such as 6204 into a bore, an outside diameter and a width, or works in reverse when all you have is a shaft and a housing in front of you.

We have been manufacturing deep groove ball bearings and insert bearings since 1994, and the same three questions still arrive every week. This guide answers them, explains the arithmetic behind a dimension calculator, and points out the places where a calculator stops and experience takes over.

What a Bearing Dimension Calculator Actually Solves

Strip away the interface and almost every calculator performs one of three jobs:

• Designation to dimensions: you type 6204 and it returns a 20 mm bore, a 47 mm outside diameter and a 14 mm width.
• Dimensions to designation: you enter a 25 mm bore and a 52 mm outside diameter, and it narrows the answer to the 6205 family.
• Family comparison: you have a 6000 series bearing on the drawing and want to know whether a 6200 series part would fit the same shaft.

The third job is where confusion usually starts. Bore diameter is shared across families, but outside diameter and width are not, so a calculator that compares only bore sizes will cheerfully suggest a part that will never enter your housing.

The Four Numbers Behind Every Bearing Size

• Bore diameter (d): the hole that receives the shaft.
• Outside diameter (D): what presses into, or sits inside, the housing.
• Width (B, T or C): the axial space the bearing occupies. Tapered roller bearings call it T, and thrust bearings use a height figure instead.
• The designation: a compact code that usually contains all three, plus seals, shields and special features.

Two more numbers matter during assembly even though most calculators ignore them. The corner radius (r) decides whether the bearing sits flat against a shoulder, and the minimum shoulder diameter tells you how much of the shaft actually supports the inner ring.

How Bore Codes Turn Into Millimetres

The last two digits of a metric bearing designation normally give the bore, and the rule is simple: multiply by five. A 6204 has a 20 mm bore (4 x 5 = 20); a 6308 has a 40 mm bore (8 x 5 = 40). Three codes break that pattern, and every dimension calculator hard-codes the exceptions: 00 means 10 mm, 01 means 12 mm, 02 means 15 mm and 03 means 17 mm.

Codes containing a slash are even more direct. A 62/22 has a 22 mm bore with no multiplication involved. This shorthand appears most often on small and thin-section bearings where a standard series does not exist.

Those figures are worth remembering, not because a calculator cannot produce them, but because pattern recognition saves time when you are checking a drawing in a meeting.

Series and Type Codes Change Everything Else

Once the bore is fixed, the remaining digits decide the outside diameter and the width. In 6204, the leading 6 identifies a deep groove ball bearing, the 2 places it in the light dimension series, and 04 supplies the bore. Change the middle digit and the same 20 mm shaft suddenly needs a much larger housing: a 6304 measures 20 mm by 52 mm by 15 mm, while the 6204 measures 20 mm by 47 mm by 14 mm.

Other families use different numbering systems, and this is where a calculator earns its place:

• Cylindrical roller bearings use prefixes such as NU, NJ and N.
• Tapered roller bearings use five-digit codes like 30205 and 32205, where 205 still means a 25 mm bore.
• Insert bearings use UC205, UK205 and similar references, where 205 again means 25 mm.

The bore logic survives across metric families. The outside diameter does not.

Working Through a Real Example

Imagine a customer emails a photograph of a bearing with 6000-2RS stamped on the seal and asks for a replacement. Run the calculator logic by hand:

1. The 6000 identifies a deep groove ball bearing, and the 0 in the middle marks the extra-light dimension series.
2. The bore code 00 means 10 mm, not zero.
3. The catalogue line reads 10 mm bore, 26 mm outside diameter and 8 mm width.
4. The 2RS suffix means two contact rubber seals, which changes nothing about the dimensions but everything about the seal speed limit.

The seal is the subtle part. A sealed bearing shares boundary dimensions with an open one, yet seal lips and shield profiles differ slightly between designs, and a metal shield produces a different friction figure from a rubber seal.

Series 6000 Two Contact Seals Deep Groove Ball Bearing1. Superior Sealing Performance: Total Protection Against ContaminationView Product →

When Sizing Gets Tricky: Insert Bearings and Housed Units

Insert bearings are the classic case where a dimension calculator is necessary but not sufficient. A UC205 insert has a 25 mm bore and a spherical outside diameter that allows it to self-align inside a housing. That outside diameter is not the same as a 6205 ball bearing, and the housing adds an entirely new set of numbers: overall height, bolt hole centre distance, base width and housing material.

This is why UCP205, UCF205 and UCFL205 all share one insert yet carry completely different mounting dimensions. If you are choosing a unit rather than a bearing, start from the bolt pattern and work back toward the bore.

UC Set Screw Locking Insert Bearing SeriesUC Set Screw Locking Insert Bearing is a widely used mounted bearing unit designed for reliable shaft positioning and stable operation in various industrial applicatio...View Product →

Pillow Block Units: UCP seriesHT200 or QT450-10, QT450-10 is only for PFTD2 series and FCT2 series.View Product →

The Numbers a Calculator Will Not Give You

Bore, outside diameter and width tell you whether a bearing fits. They do not tell you whether it will work.

• Tolerance class: P0 through P4, where P0 is standard and P4 is precision grade for spindles. Two bearings can share identical boundary dimensions and behave completely differently.
• Radial internal clearance: C2, CN, C3 and C4 describe the play between rolling elements and rings. A hot-running electric motor bearing generally needs more clearance than a low-noise application.
• Shaft and housing fits: a rotating inner ring on a steel shaft is typically k5 or m5, while a stationary housing may want H7 or J7. The calculator supplies the bearing size, not the fit.
• Speed limits: seal type, lubrication and cage material set them, so a 2RS bearing and a ZZ bearing of the same size will not share a top speed.

A Quick Checklist Before You Order

1. Measure the bore with a caliper in two directions at 90 degrees and average the readings.
2. Measure the outside diameter at three points and look for ovality or wear.
3. Measure the width across the outer ring, never across the seals.
4. Read the full designation, because suffixes for seals, shields, clearance and tolerance change the part number.
5. Confirm the load and speed of the application, not just the dimensions. A farm shaft worn oval will not hold a standard bore replacement properly, and our agricultural machinery bearings cover the special round bore designs built for that exact situation.
6. When in doubt, send a photo of the marking together with the measurements; the combination of code and caliper reading almost always settles the question.

Think of a bearing dimension calculator as a translator rather than an oracle. It converts the shorthand stamped on a seal into millimetres you can verify, and it converts your measurements back into a part number you can order. Everything afterwards, including fit, clearance, lubrication and speed, is engineering, and that is where a supplier's experience matters as much as the chart.

We have been building bearings since 1994 and shipping them to more than fifty countries, and the questions have barely changed in that time. If a measurement and a designation refuse to agree, send both to us and we will work through it with you.