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Belt Length Calculator

Calculate the required belt length for two-pulley drive systems. Supports open and crossed belt configurations with optional center distance solving.

L = 2C + π(D+d)/2 + (D−d)²/(4C)

Calculate belt length from pulley diameters and center distance

mm
mm
Result:
C = (L − π(D+d)/2 + √[(L − π(D+d)/2)² − 2(D−d)²]) / 4

Find the required center distance for a given belt length

mm
mm
Result:
v = π × D × n

Calculate belt linear speed and driven pulley RPM

mm
RPM
%
Typical: 1–3%
Result:

How It Works

What is a Belt Drive?

A belt drive transmits rotational motion between two pulleys (sheaves) using a flexible belt. Belt drives are widely used in machinery, HVAC systems, automotive engines, and industrial equipment because they absorb vibration, allow slight misalignment, and are quieter than gear drives.

Open vs. Crossed Belts

  • Open belt: The belt wraps around both pulleys on the same side. Both pulleys rotate in the same direction. This is the most common configuration.
  • Crossed belt: The belt crosses between the pulleys, forming an X shape. The pulleys rotate in opposite directions. Provides greater wrap angle but increases belt wear.

Key Formulas

Open belt length:

L = 2C + π(D+d)/2 + (D−d)² / (4C)

Crossed belt length:

L = 2C + π(D+d)/2 + (D+d)² / (4C)

Where:

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

Belt Speed and Speed Ratio

  • Belt Speed = π × D × n (where n is the rotational speed)
  • Speed Ratio = Ddriving / Ddriven
  • ndriven = ndriving × Ddriving / Ddriven

Practical Tips

  • The center distance should generally be 1.5–2 times the sum of both pulley diameters for optimal belt life
  • Belt slip (typically 1–3%) reduces the effective speed ratio slightly
  • V-belts use the pitch diameter, not the outer diameter, for calculations
  • Standard belt sizes are available in fixed lengths — choose the nearest standard size after calculating
  • Wrap angle on the smaller pulley should be at least 120° to ensure adequate grip

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