A breakdown at 9 pm, an old belt with its printing rubbed away, and a dealer asking "which size?" Measuring a V-belt takes five minutes with a vernier caliper and a tape. The only trick is knowing which length you measured, because belts have three of them.
First identify the section by its top width (A 13 mm, B 17 mm, C 22 mm, D 32 mm, E 38 mm). Then measure the outside length with a flexible tape and subtract about 2π × the belt height to get the inside length: roughly 50 mm for A, 69 mm for B, 88 mm for C, 119 mm for D and 145 mm for E. Divide by 25.4 for inches, and round to the nearest standard size.
Measure the top width and height with a vernier caliper. Old belts wear, so take the closest match:
| Section | Top width (mm) | Height (mm) |
|---|---|---|
| A | 13 | 8 |
| B | 17 | 11 |
| C | 22 | 14 |
| D | 32 | 19 |
| E | 38 | 23 |
| SPZ (wedge) | about 10 | 8 |
| SPA (wedge) | about 13 | 10 |
| SPB (wedge) | about 16 | 13 |
| SPC (wedge) | 22 | 18 |
Note that A and SPA are both about 13 mm wide; the height (8 mm against 10 mm) tells them apart. Our V-belt size chart has the full dimensions.
The outside of a belt loop is longer than the inside by about 2π × belt height:
| Section | Height (mm) | Outside minus inside (approx.) |
|---|---|---|
| A | 8 | 50 mm (about 2 in) |
| B | 11 | 69 mm (about 2.7 in) |
| C | 14 | 88 mm (about 3.5 in) |
| D | 19 | 119 mm (about 4.7 in) |
| E | 23 | 145 mm (about 5.7 in) |
Example: a B-section belt measures 1,600 mm outside. Inside length ≈ 1,600 − 69 = 1,531 mm. 1,531 ÷ 25.4 = 60.3 inches. The belt is a B-60.
Belts stretch with use, so an old belt may measure slightly long. Round to the nearest standard size and, if possible, confirm against the drive.
If the old belt is lost or badly stretched, measure the pulley diameters and the centre distance between the shafts, with the motor in its mid-adjustment position. The belt's datum length is approximately:
L ≈ 2C + 1.57 (D + d) + (D − d)² ÷ 4C
where C is the centre distance, D the large pulley diameter and d the small pulley diameter, all in mm and measured at the datum diameter.
Example: C = 600 mm, D = 250 mm, d = 125 mm gives L ≈ 1,200 + 589 + 7 ≈ 1,796 mm datum length. Send us these three measurements and we will convert them into the right belt code for your section.
For SPZ, SPA, SPB and SPC belts, the code is the datum length in millimetres (for example SPB 1600). The difference between outside and datum length is smaller than between outside and inside, so ask us to convert if you only have an outside measurement.
How do I measure a V-belt without a code?Measure the top width and height to find the section, then the outside length with a flexible tape. Subtract about 2π × height to get the inside length.
Are V-belt sizes inside or outside length?In India, classical belts (A to E) are usually coded by nominal inside length in inches, and wedge belts (SPZ to SPC) by datum length in mm.
What size is a B-60 belt in mm?About 1,524 mm inside length (60 × 25.4), and roughly 1,593 mm outside.
Can I calculate belt length from the pulleys?Yes. Use L ≈ 2C + 1.57 (D + d) + (D − d)² ÷ 4C with the centre distance and pulley datum diameters, then convert to the standard size for your section.
Measured your belt but not sure of the size? WhatsApp us the top width, the outside length and a photo of the old belt, and our team will identify it.