A 6205 bearing from any manufacturer on earth is 25 mm bore, 52 mm outside diameter and 15 mm wide. That is not a coincidence, a convention or a marketing agreement - it is ISO 15, and it is the single reason a global bearing industry can exist at all. Without it, every machine would be tied to one factory's drawings and every replacement bearing would be a custom part.
This guide explains the system from the inside: how a designation encodes dimensions, how the diameter and width series work, what the standard does not fix, and how to use boundary dimensions to verify a cross-reference before you order. It is written for buyers and maintenance engineers who need to answer the question "will this fit" with a number rather than an opinion.
- ISO 15 fixes the envelope only - bore, outside diameter and width. Everything inside, including materials, clearance, cage and seals, is a manufacturer's choice.
- The bore code is the last two digits, multiplied by 5 for codes 04 and above. Codes 00, 01, 02 and 03 are exceptions that stand for 10, 12, 15 and 17 mm.
- Same bore does not mean same bearing. A 6205 and a 6305 both fit a 25 mm shaft and take completely different housings: 52 mm against 62 mm outside diameter.
- Interchangeability is dimensional, not functional. Two bearings that share an envelope can still differ in capacity, speed, clearance and life by a factor that matters.
The problem ISO 15 was written to solve
Before the 1950s, bearing dimensions were set by each manufacturer. Series, widths and even bore-to-outside relationships differed between countries and often between factories in the same country. A machine built in one country could not be repaired with parts from another, and a distributor could not carry stock that fitted more than a handful of designs.
ISO 15 standardised the dimensional and running accuracy envelope for radial bearings, and the principle was simple: fix the outside of the bearing, leave the inside to the engineers. A bore of 25 mm should always sit in one of a small number of standard housings, so that a replacement can be sourced anywhere in the world. ISO 199 does the same job for thrust bearings and ISO 104 for thrust ball bearings, while ISO 355 covers the metric tapered roller series.
How a designation encodes dimensions
Japanese and European designations build the size into the number itself, which is why a designation is a dimensional specification before it is anything else. Reading it requires only two rules.
The bore code
For bore codes 04 and above, multiply by 5 to get the bore in millimetres. Codes 00 to 03 are historical exceptions that carry fixed values.
| Last two digits | Bore (mm) | Worked example | Note |
|---|---|---|---|
| 00 | 10 | 6200 → 10 mm bore | Exception - the code does not multiply |
| 01 | 12 | 6201 → 12 mm bore | Exception |
| 02 | 15 | 6202 → 15 mm bore | Exception |
| 03 | 17 | 6203 → 17 mm bore | Exception |
| 04 | 20 | 6204 → 20 mm bore | 4 × 5 = 20 |
| 05 | 25 | 6205 → 25 mm bore | 5 × 5 = 25 |
| 08 | 40 | 6208 → 40 mm bore | 8 × 5 = 40 |
| 20 | 100 | 6220 → 100 mm bore | 20 × 5 = 100 |
Bores below 10 mm and above roughly 500 mm, plus all non-standard bores, are written explicitly: a slash introduces a bore that is not a multiple of 5, as in 6205/28 for a 28 mm bore in a 6205 envelope, or 62/22 where 22 mm is the actual bore. When you see a slash, read it as "the dimensions are this series, but the bore is not the code implied".
The series digits
The digits before the bore code carry two pieces of information: the width series and the diameter series. In 6205 the 6 is the type (deep groove ball, single row), the 2 is the diameter series and the 0 the width series. In a four-digit designation such as 3205 the type is 3, the diameter series is 2 and the width series is 0. The digit order is type, then width, then diameter - which is the opposite of how most people read it aloud.
The diameter series: same bore, different envelope
The diameter series is the lever that trades radial space against load capacity. For a given bore, each step up in the diameter series increases the outside diameter, the rolling element size and the load rating, and reduces the speed ceiling because the rolling elements are heavier and travel further per revolution.
| Designation | Series | Bore d | Outside D | Width B | Character |
|---|---|---|---|---|---|
| 6005 | Extra-light | 25 mm | 47 mm | 12 mm | Highest speed, lowest capacity, smallest radial envelope |
| 6205 | Light | 25 mm | 52 mm | 15 mm | The general-purpose default for a 25 mm shaft |
| 6305 | Medium | 25 mm | 62 mm | 17 mm | Substantially more capacity, larger housing, lower speed |
| 6405 | Heavy | 25 mm | 80 mm | 21 mm | Maximum capacity in this bore; rarely justified outside heavy duty |
Note that all four fit the same 25 mm shaft and none of them fits the same housing. This is the most common dimensional misunderstanding in bearing procurement, and it is why a cross-reference that matches only on bore is not a cross-reference at all. Our deep groove ball bearing range lists 31,521 orderable SKUs with the full boundary dimensions on every record, and the size-indexed pages group listings by bore so the envelope comparison is immediate.
The width series and the common trap
Width is the third dimension and the one most often overlooked. The same diameter series exists in several width series, and two bearings can share bore and outside diameter while differing in width - a 6205 is 15 mm wide, while the wide version of the same envelope is 16 mm and will not seat correctly against a shoulder cut for the narrower part. Width variation is small in absolute terms and decisive in assembly.
The 6200 family worked dimension table
The table below is the reference we use most often at the quotation desk, because the 6200 series is the most widely specified family in the world and the most frequently cross-referenced.
| Designation | d | D | B | Dynamic C (typical) | Static C0 (typical) | Approx. mass |
|---|---|---|---|---|---|---|
| 6204 | 20 mm | 47 mm | 14 mm | ≈ 13.5 kN | ≈ 6.6 kN | 0.106 kg |
| 6205 | 25 mm | 52 mm | 15 mm | ≈ 14.0 kN | ≈ 7.8 kN | 0.128 kg |
| 6206 | 30 mm | 62 mm | 16 mm | ≈ 19.5 kN | ≈ 11.2 kN | 0.200 kg |
| 6207 | 35 mm | 72 mm | 17 mm | ≈ 25.5 kN | ≈ 15.3 kN | 0.288 kg |
| 6208 | 40 mm | 80 mm | 18 mm | ≈ 29.1 kN | ≈ 17.8 kN | 0.368 kg |
| 6210 | 50 mm | 90 mm | 20 mm | ≈ 35.1 kN | ≈ 23.2 kN | 0.462 kg |
The d, D and B columns are fixed by ISO 15 and identical from every manufacturer. The C and C0 columns are not fixed by the standard and vary by a few percent between makers because ball complement, cage design, heat treatment and internal geometry differ. Treat C and C0 as typical values for first-pass selection and confirm against the chosen manufacturer's catalogue. Real orderable examples at the 25 mm bore in our own stock include 6205-2RSH, 6205-2RSR and the ANDGE 6205-2RS.
Cylindrical and tapered roller: the same principle, different numbers
The boundary dimension system is not limited to ball bearings. Cylindrical roller bearings use the same bore code logic and the same diameter series idea, which is why an NU205 and a 6205 share a 25 mm bore and 52 mm outside diameter while carrying radically different loads. Our cylindrical roller range holds 38,259 SKUs on that basis, and the NU, NJ and NUP guide explains how the rib configuration changes axial behaviour inside an identical envelope.
Tapered roller bearings follow ISO 355 for the metric series, where the designation encodes the cone and cup as a matched pair rather than a single envelope. That is why tapered roller cross-referencing needs the full designation including any prefix, and why our tapered roller listings carry 34,808 SKUs with the cone, cup and assembled width recorded separately.
Thrust bearings: ISO 199 and ISO 104
Thrust bearings are dimensioned by a separate standard because the governing dimension is the shaft washer bore and the housing washer outside diameter, with the height determined by the series. ISO 199 covers thrust ball bearings and ISO 104 covers thrust roller bearings. A thrust bearing that shares a bore with a radial bearing shares nothing else - the washers, the height and the load path are all different, which is why the thrust family is listed separately with its own 7,211 SKUs.
Where interchangeability stops
This is the part that matters commercially. The standard fixes the envelope and the running accuracy tolerances, and nothing else. Two bearings with identical d, D and B can differ in every one of the following, and each difference changes behaviour in service:
- Basic dynamic and static ratings. Different ball complements and internal geometry give different C and C0, so the calculated L10 life differs at the same load.
- Internal clearance. A CN bearing and a C3 bearing look identical on a drawing and behave differently as soon as the assembly reaches operating temperature. The numeric clearance ranges come from ISO 5753 and DIN 620.
- Tolerance class. Normal, P6, P5 and P4 control bore and outside diameter deviation, ring runout and width variation under ISO 492. A P5 replacement in a P0 housing delivers P0 accuracy at a P5 price.
- Seals and shields. 2RS, 2RZ, 2Z and open all share the envelope and differ in friction, speed ceiling, contamination resistance and whether relubrication is possible. The closure suffix guide decodes the variants.
- Cage material. Stamped steel, polyamide and machined brass change the temperature limit, the speed ceiling and the noise signature.
- Grease fill and lubricant type. Often invisible in the part number and decisive for life, especially in sealed-for-life units where mixing incompatible thickeners destroys both greases.
This is why our interchange lookup presents equivalents with the differing attributes flagged rather than as a simple list of substitutes, and why the glossary defines each attribute separately.
Non-standard, inch and special bores
Three categories sit outside the main metric series and each needs its own treatment.
Inch series bearings
Inch-dimensioned bearings - common in older American and British machinery, and still standard in many automotive and aerospace positions - are not covered by ISO 15 in the same way. They use their own designation systems and are dimensioned in inches, often with fractional bore and outside diameter. Always specify inch dimensions in inches rather than a converted millimetre figure, because the conversion is not exact and the tolerance bands are defined on the original units.
Special and thin-section designs
Some applications need an envelope that no standard series offers: a very thin radial section, a very large bore relative to the outside diameter, or a non-standard width. These exist, they are orderable, and they are listed in our custom and special bearings group rather than being forced into a standard series they do not belong to.
Boundary dimensions versus mounting dimensions
The envelope is only half the drawing. A bearing seats against a shaft shoulder and a housing shoulder, and the same ISO tables that give you d, D and B also give the dimensions the shaft and housing must provide to hold it properly. Reading those columns correctly is what separates a bearing that runs for its full rating life from one that creeps, rubs or never seats at all.
Shoulders, fillets and abutment height
Each ring must be supported by a shoulder of adequate height and diameter. If the housing shoulder is too small, the outer ring is not properly supported and can creep in the housing under load, wearing the bore oval and destroying the fit. If the shoulder is too large, it fouls the seal or the cage, and the resulting rubbing shows up as heat and noise long before it shows up as a failure. The catalogue supplies a minimum shoulder diameter for the shaft side and a maximum for the housing side, both derived from the internal geometry of that specific design rather than from the envelope alone.
Chamfer and corner radius
Every bearing has a minimum chamfer dimension r at the ring corners, and every shaft has a fillet radius at the base of its shoulder. The fillet must be smaller than the bearing chamfer, otherwise the ring cannot seat flat against the shoulder and the bearing runs misaligned from the first revolution. This is a dimensional incompatibility that no amount of tightening will correct, and it is easy to miss because both numbers are small and neither appears on a simplified requisition.
| Designation | r min | da min (shaft shoulder) | Da max (housing shoulder) | ra max |
|---|---|---|---|---|
| 6204 | 1.0 mm | 25.6 mm | 41.4 mm | 1.0 mm |
| 6205 | 1.0 mm | 30.6 mm | 46.4 mm | 1.0 mm |
| 6206 | 1.0 mm | 35.6 mm | 56.4 mm | 1.0 mm |
| 6207 | 1.1 mm | 41.5 mm | 65.5 mm | 1.0 mm |
| 6208 | 1.1 mm | 46.5 mm | 73.5 mm | 1.0 mm |
Typical published mounting dimensions for the 62 series. Values vary slightly between manufacturers; confirm against the catalogue of the brand you intend to fit.
Two consequences follow. First, a cross-reference that matches the envelope but comes from a different internal design can carry a different minimum chamfer and different shoulder limits, so the mounting drawing has to be rechecked. Second, a bearing fitted without a proper shoulder is not a bearing problem - it is a housing problem, and replacing the bearing will not fix it.
Common cross-reference mistakes
These are the errors we see most often in incoming enquiries, in rough order of how much they cost.
- Matching on bore only. The single largest source of wrong orders. A 25 mm bore exists in the 6, 7 and 8 dimension series as well as in cylindrical, tapered and needle families, and no two of those fit the same housing.
- Assuming a suffix exists in every brand. A 2RS1 from one manufacturer is a 2RS from another, LLU from a third and DDU from a fourth. They are not identical articles, and treating them as equivalent without checking the seal construction is how a sealed-for-life position gets a bearing with a different friction torque.
- Ignoring the clearance class. CN and C3 bearings are dimensionally identical and behave differently as soon as the assembly reaches temperature. Substituting one for the other because "the number matched" is a documented cause of overheating and early failure.
- Treating a precision class as a drop-in. A P5 bearing in a P0 housing gives P0 running accuracy at a P5 price. The tolerance class is a system property, not a component upgrade.
- Overlooking a special variant's different width. Filled-slot, wide-series and special-clearance variants can differ in width inside the same nominal envelope, which changes whether the ring seats against the shoulder.
- Assuming the ratings are identical. Because C and C0 are not fixed by the standard, the same envelope from two makers can differ enough in capacity to move the calculated life from acceptable to marginal on a duty that is close to the limit.
Checking a drawing against ISO 15
A quick verification sequence before releasing any bearing order:
- Confirm the bore from the code, and check whether a slash indicates a non-standard bore.
- Confirm the series by looking up the outside diameter, not by assuming the designation family. A 6205 and a 6305 differ by 10 mm of outside diameter.
- Confirm the width including any width series variant, since this is the dimension most often dropped from a requisition.
- Confirm the fit intended for the chosen envelope, using the ISO 286 tables rather than habit - the shaft and housing fits guide covers the selection.
- Confirm the load and speed case against the actual ratings of the intended manufacturer, not against a generic table. The deep groove selection guide and the bearing type selection guide work through both.
If you only have a drawing and no part number, the bearing selector filters the whole catalogue by bore, outside diameter, width and brand, so an envelope can be turned into a list of orderable numbers in one step. To see the system applied across a whole brand range instead of a single part, compare the dimensions on any SKF, FAG or ANDGE listing against the ISO 15 table above - the envelope will match and the ratings will not, which is exactly the point. Our brand directory covers all eleven brands we stock.
Frequently asked questions
Are all bearings with the same number the same size?
Yes for the boundary dimensions. ISO 15 fixes the bore, outside diameter and width for each designation, so a 6205 from any manufacturer is 25 mm by 52 mm by 15 mm. The internal geometry, load ratings, clearance, cage, seal design and tolerances are not fixed and can differ between brands.
How do I work out the bore from a bearing number?
Take the last two digits. For codes 04 and above, multiply by 5 to get the bore in millimetres, so a 6205 has a 25 mm bore. Codes 00, 01, 02 and 03 are exceptions meaning 10, 12, 15 and 17 mm. A slash anywhere in the number indicates a bore that is not a multiple of 5, such as 6205/28 for a 28 mm bore.
What is the difference between a 6205 and a 6305?
They share a 25 mm bore and therefore fit the same shaft, but the 6305 belongs to the medium diameter series with a 62 mm outside diameter and 17 mm width against the 6205's 52 mm and 15 mm. The 6305 carries substantially more load and needs a different housing. Bore alone never establishes a fit.
Does ISO 15 guarantee that two brands are interchangeable?
It guarantees that they are dimensionally interchangeable, which is not the same as functionally interchangeable. Two bearings with the same envelope can still differ in basic dynamic and static ratings, internal clearance class, tolerance class, cage material, seal type and grease fill. Each of those changes life or behaviour in service, so a cross-reference still needs verification.
What does the width series do?
The width series controls the third dimension, and it is the one most often omitted from a requisition. Two bearings can share bore and outside diameter but differ in width, so a replacement can seat incorrectly against a shoulder machined for the narrower part. Always confirm bore, outside diameter and width together.
Which standard covers thrust bearings?
ISO 199 covers thrust ball bearings and ISO 104 covers thrust roller bearings. They dimension by shaft washer bore and housing washer outside diameter with a height determined by the series, which is separate from the radial ISO 15 system. A thrust bearing that shares a bore with a radial bearing shares no other dimension.
Which standard covers tapered roller bearings?
The metric tapered roller series is covered by ISO 355. A tapered roller bearing is supplied as a matched cone and cup rather than a single interchangeable envelope, which is why the full designation including any prefix is needed for a cross-reference and why cone and cup dimensions are recorded separately in our listings.
What do C and C0 mean and why do they differ between brands?
C is the basic dynamic load rating used for fatigue life calculations and C0 is the basic static load rating used to check for permanent deformation under shock or static load. Neither is fixed by ISO 15, because both depend on ball or roller complement, internal geometry, cage design and heat treatment. Published values typically vary by a few percent between manufacturers for the same envelope.
What does a slash in a bearing number mean?
A slash introduces a dimension that follows the designation series but is not the value the code implies. The most common use is a non-standard bore, such as 6205/28, which is a 6205 envelope with a 28 mm bore instead of 25 mm. Slashes can also introduce special widths or clearances, so read the full designation rather than the leading digits.
How do I find a bearing from a drawing with no part number?
Measure bore, outside diameter and width accurately, then search the catalogue by those dimensions rather than by number. Our selector filters the whole range by bore, outside diameter, width and brand, and the size-indexed pages group listings by bore so the envelope comparison is immediate. Send the measured dimensions with the load and speed case if you would like us to propose the family as well.