Belt Length Calculator – Calculate V-Belt Length
Calculate the required belt length for a two-pulley system. Enter pulley diameters and center distance to find the correct belt size.
Measure the small pulley diameter
Measure across the center of the smaller pulley. For V-belts, measure at the pitch diameter (approximately the belt's neutral axis), not the outer edge.
Measure the large pulley diameter and center distance
Measure the larger pulley the same way. Then measure the distance between pulley centers — the space between the shafts, not the belt edges.
Click Calculate to find belt length
Get the required belt length and the contact angle on the small pulley. Use this to select the correct standard belt size.
| Belt Designation | Top Width | Height | Common Applications |
|---|---|---|---|
| A (4L) | 1/2 inch (13mm) | 5/16 inch (8mm) | Light duty: lawn mowers, small compressors |
| B (5L) | 21/32 inch (17mm) | 13/32 inch (10mm) | Medium duty: industrial motors, pumps |
| C | 7/8 inch (22mm) | 17/32 inch (14mm) | Heavy duty: large compressors, conveyors |
| 3V | 3/8 inch (9.5mm) | 5/16 inch (8mm) | Narrow wedge: HVAC, appliances |
| 5V | 5/8 inch (16mm) | 17/32 inch (14mm) | Narrow wedge: industrial drives |
| 8V | 1 inch (25mm) | 29/32 inch (23mm) | Heavy narrow: high-power industrial |
Note: Belt numbers indicate size. A45 means A-section belt with 45 inch outside circumference. 4L460 means 4L section with 46.0 inch outside length.
The Belt Length Formula
The approximate belt length formula is: L = 2C + 1.57(D + d) + (D - d)² / 4C, where C is center distance, D is large pulley diameter, and d is small pulley diameter. This accounts for the straight sections and the arc around each pulley.
Why Contact Angle Matters
Contact angle is how much of the small pulley the belt wraps around. More wrap means more grip and less slip. Aim for at least 120° contact on the small pulley. If contact is too low, consider an idler pulley or adjusting center distance.
Center Distance Guidelines
Center distance should be 1.5 to 3 times the large pulley diameter for optimal performance. Too close reduces contact angle and belt life. Too far increases belt whip and vibration. Adjustable motor bases help fine-tune tension.
Belt Tension
Proper tension is critical. Too loose causes slip, heat, and wear. Too tight loads bearings and shortens belt life. A properly tensioned belt should deflect about 1/64 inch per inch of span when pressed with moderate thumb pressure.
Use the next size up if between sizes
Belts stretch slightly during break-in. If your calculated length falls between standard sizes, choose the longer one. You can always take up slack with an adjustable base.
Match pulley groove to belt profile
A-section belts need A-section pulleys. Using mismatched profiles causes poor contact, rapid wear, and power loss. Check both belt and pulley specifications.
Check pulley alignment
Misaligned pulleys cause uneven wear and belt tracking issues. Use a straightedge across both pulley faces. Angular misalignment should be under 1 degree.
Replace belts in matched sets
On multi-belt drives, always replace all belts together. Mixing old and new belts causes uneven load distribution. The new belt carries more load and fails prematurely.
How do I measure a worn belt for replacement?
Use a flexible tape measure around the outside of the old belt. For V-belts, this gives the outside circumference. Subtract the belt's nominal top width to estimate pitch length. Or measure pulleys and center distance, then use this calculator.
What's the difference between inside length and outside length?
Inside length is measured along the belt's inner surface. Outside length includes the belt thickness. V-belt part numbers typically use outside length. Classical belts (A, B, C) use outside length; narrow wedge (3V, 5V, 8V) also use outside length.
How much tension should a V-belt have?
Press the belt midpoint with moderate thumb pressure. Deflection should be about 1/64 inch per inch of span length. For a 32-inch span, expect about 1/2 inch deflection. Belt tension gauges provide more accurate measurement.
Can I use this calculator for timing belts?
The basic length formula works for any two-pulley system, but timing belts have different considerations. Timing belt pitch must match pulley tooth pitch. This calculator does not account for tooth engagement requirements.
Why is my belt squealing?
Squealing usually means the belt is loose and slipping. Check tension first. If tension is correct, check for worn pulleys, contaminated belts (oil or grease), or misalignment. Glazed belt surfaces also cause noise and need replacement.
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