How do I identify a V-belt size?

Measure the top width and the height and match them in the table. A belt 17 mm across the top and 11 mm tall is a classical B; 16.3 mm by 13 mm is a narrow SPB. The section letter is usually printed on the belt along with its length, so identification is normally a matter of reading it rather than measuring.

What is the difference between classical and narrow V-belts?

Narrow sections (SPZ, SPA, SPB, SPC) are taller for their width than classical sections (Z, A, B, C, D, E). The deeper wedge grips the pulley groove better, so a narrow belt carries more power for the same top width. Narrow sections have largely replaced classical ones in new designs, but classical belts remain standard for replacing existing drives.

What is the pitch width of a V-belt?

The width at the pitch line — roughly where the tensile cords run — which is narrower than the top width because the belt's sides are tapered. Pulley grooves are cut to the pitch width, so it is the dimension to compare when matching a belt to a pulley. For a B section it is 14 mm against a top width of 17 mm.

Can I replace a B belt with an SPB belt?

Only if the pulleys are also changed or are rated for both. The sections are close in top width — 17 mm against 16.3 mm — but the pitch widths differ, so a narrow belt does not seat correctly in a classical groove and will slip or wear rapidly. Drive conversions between the series normally replace the pulleys as well.

How is V-belt length specified?

By the outside length or the pitch length, depending on the standard and the manufacturer — and the two differ, which is a common source of ordering errors. Pitch length is the datum for power rating because it is measured at the cord line. Always confirm which length the supplier quotes before ordering a replacement.

Belt Sizes

V-belt cross sections in both the classical and narrow series, with top width, pitch width and height — the dimensions that determine the pulley groove and the power the belt can carry.

Data verified 2026-09-29 · based on ISO 4183:1995

Quick Answer

A V-belt is sized by its cross section, not by its length. The section letter — A, B, C, D, E for classical or SPZ, SPA, SPB, SPC for narrow — sets the top width and height, and therefore the pulley groove and the power capacity. Length is chosen separately from the standard length list.

Find a section

V-Belt Cross Sections

Section^[2]	Series^[2]	Top width mm^[2]	Pitch width mm^[2]	Height mm^[2]	Top width in^[3]	Height in^[3]	Top width in (converted)^[1]	Height in (converted)^[1]	Typical duty^[2]
Y / Z #	Classical (ISO)	6	5.3	4	0.250	0.156	0.24	0.16	Fractional horsepower drives, small appliances
A #	Classical (ISO/RMA)	13	11	8	0.500	0.313	0.51	0.31	Light drives — fans, small pumps, workshop machines
B #	Classical (ISO/RMA)	17	14	11	0.656	0.438	0.67	0.43	General-purpose drives — the most common classical section
C #	Classical (ISO/RMA)	22	19	14	0.875	0.531	0.87	0.55	Medium drives — compressors, larger pumps, conveyors
D #	Classical (ISO/RMA)	32	27	19	1.250	0.750	1.26	0.75	Heavy drives — crushers, mills, large fans
E #	Classical (ISO/RMA)	38	32	23	1.500	0.906	1.50	0.91	Very heavy drives — the largest classical section
SPZ #	Narrow (ISO)	9.7	8.5	8	0.382	0.315	0.38	0.31	Narrow-section replacement for Z, higher power in the same space
SPA #	Narrow (ISO)	12.7	11	10	0.500	0.394	0.50	0.39	Narrow-section replacement for A
SPB #	Narrow (ISO)	16.3	14	13	0.642	0.512	0.64	0.51	Narrow-section replacement for B — very widely used
SPC #	Narrow (ISO)	22	19	18	0.866	0.709	0.87	0.71	Narrow-section replacement for C

The pitch width is the dimension that locates the belt in the pulley groove, and it is measured at the pitch line — roughly where the tensile cords sit — not at the top. Pulley grooves are cut to the pitch width, not the top width, which is why a belt and a pulley of nominally matching section can still mismatch if one is dimensioned to the wrong reference. Narrow sections carry more power than classical sections of similar top width because they are taller for their width: an SPB is 16.3 mm wide against the classical B at 17 mm, but 13 mm tall against 11, and it transmits appreciably more.

Classical vs Narrow Sections

The two series overlap in size but are not interchangeable, and the narrow series has very largely displaced the classical one in new designs.

Classical sections (Z, A, B, C, D, E — or the metric equivalents Y, A, B, C, D, E) have a width-to-height ratio of about 1.6 to 1. They are the older series and are still the standard for replacement belts on existing equipment.

Narrow sections (SPZ, SPA, SPB, SPC) have a width-to-height ratio closer to 1.2 to 1 — taller for their width. Because a V-belt transmits torque through the wedging action of its flanks, a taller section grips a deeper groove and carries more load for the same top width. The practical result is that an SPB replaces a B with more capacity in the same pulley envelope, or the same capacity in a smaller one.

Conversion between the series is not one-to-one. SPZ is close to Z, SPA is close to A, and SPB is close to B, but the pitch widths differ and the pulleys are not the same. A narrow-section belt in a classical pulley will not seat correctly.

Section, Length and Power Rating Are Three Separate Choices

Selecting a V-belt drive means choosing three things independently, and the catalogues treat them separately.

Section comes from the power and the speed — the faster and more powerful the drive, the larger the section. Manufacturers publish section selection charts with shaft speed on one axis and design power on the other.

Length comes from the geometry — pulley diameters and centre distance, from the belt length calculator. Belt lengths are then rounded to the nearest standard length in that section, and the centre distance adjusted to suit. This is why a drive is designed around a catalogue length rather than the length being specified to an arbitrary figure.

Power rating per belt comes from the manufacturer's tables and depends on the section, the small pulley diameter and the speed. The number of belts is then the design power divided by the rating per belt, adjusted for the arc of contact on the small pulley and for belt length. A drive that is right on section and length can still be wrong on belt count.

Frequently Asked Questions

How do I identify a V-belt size?: Measure the top width and the height and match them in the table. A belt 17 mm across the top and 11 mm tall is a classical B; 16.3 mm by 13 mm is a narrow SPB. The section letter is usually printed on the belt along with its length, so identification is normally a matter of reading it rather than measuring.
What is the difference between classical and narrow V-belts?: Narrow sections (SPZ, SPA, SPB, SPC) are taller for their width than classical sections (Z, A, B, C, D, E). The deeper wedge grips the pulley groove better, so a narrow belt carries more power for the same top width. Narrow sections have largely replaced classical ones in new designs, but classical belts remain standard for replacing existing drives.
What is the pitch width of a V-belt?: The width at the pitch line — roughly where the tensile cords run — which is narrower than the top width because the belt's sides are tapered. Pulley grooves are cut to the pitch width, so it is the dimension to compare when matching a belt to a pulley. For a B section it is 14 mm against a top width of 17 mm.
Can I replace a B belt with an SPB belt?: Only if the pulleys are also changed or are rated for both. The sections are close in top width — 17 mm against 16.3 mm — but the pitch widths differ, so a narrow belt does not seat correctly in a classical groove and will slip or wear rapidly. Drive conversions between the series normally replace the pulleys as well.
How is V-belt length specified?: By the outside length or the pitch length, depending on the standard and the manufacturer — and the two differ, which is a common source of ordering errors. Pitch length is the datum for power rating because it is measured at the cord line. Always confirm which length the supplier quotes before ordering a replacement.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	Value computed from the standard's defining relationship	derived	Computed at build time from the defining formula stated on the page, then verified against every row and anchored by known standard values.
[2]	ISO 4183 — Belt drives, classical and narrow V-belts, grooves	standard	ISO 4183:1995
[3]	RMA/IP-20 — Classical and narrow V-belt cross sections	standard	RMA/IP-20 (current edition)

Data Sources

Standard	Revision	What it covers on this page
ISO 4183 — Belt drives, classical and narrow V-belts, grooves	ISO 4183:1995	the metric cross-section dimensions and the pulley grooves
RMA/IP-20 — Classical and narrow V-belt cross sections	RMA/IP-20 (current edition)	the inch cross-section dimensions and the classical section letters
ISO 4184 — Belt drives, classical and narrow V-belts, lengths	ISO 4184:1992	the standard belt length series

Cross-checked against:

Top widths and heights checked against both the ISO metric and the RMA inch standards
The overlap between the two standards used as a cross-check — the sections agree where both define them
Six sections held as known-value checks spanning classical and narrow series
Metric-to-inch conversions recomputed at build time for every row

Derived values — the following values on this page are calculated, not taken directly from the standard:

Value	How it is derived
Converted inch dimensions	Millimetres × 0.0393701, exact. Columns 6 and 7 are the standards' own inch values, given alongside so the two can be compared — the small differences reflect rounding in the respective standards, not error.

Cross-section dimensions are standard and exact. Power rating per belt, standard available lengths and the pulley groove dimensions are not given here and come from the manufacturer's catalogue, since they depend on the belt construction as well as its section.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "Belt Sizes." https://refmanual.com/charts/belt-sizes/ (accessed 2026-09-29). Data from ISO 4183 — Belt drives, classical and narrow V-belts, grooves, ISO 4183:1995.

BibTeX

@misc{belt-sizes,
  title        = {Belt Sizes},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/belt-sizes/}},
  note         = {Accessed: 2026-09-29},
  source       = {ISO 4183 — Belt drives, classical and narrow V-belts, grooves, ISO 4183:1995}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - Belt Sizes
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/belt-sizes/
PY  - 2026
DA  - 2026-09-29
N1  - Source: ISO 4183 — Belt drives, classical and narrow V-belts, grooves, ISO 4183:1995
ER  -

Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.