Belt Length Calculator

V-belt length formula for two pulleys.

Formula: L = 2C + 1.57 x (D + d) + (D - d)squared / (4 x C)

Where:

• L = belt length
• C = center distance between pulleys
• D = large pulley diameter
• d = small pulley diameter

All measurements in the same units (inches or mm).

Speed ratio:

• Speed ratio = D / d
• Large pulley RPM = Small pulley RPM x (d / D)

Common belt cross-sections:

• A/4L: light duty (fans, pumps)
• B/5L: medium duty (HVAC, compressors)
• C: heavy duty (large industrial)
• D: very heavy duty

Tips:

• Measure old belt if possible for best accuracy.
• Belt numbers often include the length in inches (e.g., A68 = A-section, 68 inches).

The formula gives a good estimate, but for a replacement nothing beats measuring the old belt, because the calculation assumes perfect pulley geometry and ignores small effects like belt thickness and how deep it seats in the groove. If the old belt is gone, measure the center distance and pulley diameters carefully, since a small error there grows in the result.

The letter and number codes matter as much as the length. A V-belt’s cross-section (A, B, C, and so on, or the 4L/5L “fractional horsepower” series) has to match the pulley groove; a belt of the right length but wrong profile won’t seat properly and will slip or wear fast. Don’t try to force a belt that’s slightly short, either: V-belts aren’t meant to stretch, and over-tensioning to make one fit overloads the shaft bearings. Get the length and the section right, then set tension to the manufacturer’s spec rather than by feel.