Calculate pulley

Estimate two-pulley belt length, speed ratio, and driven rpm under no-slip assumptions.

freeworks offlinenothing uploaded
ToolPulley Calculator
Input
Output
—
Put this on your own site

The frame below runs the same code as this page, in the reader's own browser. Nothing is sent to us, and nothing is sent to you.

Pick a dark background and the text and panels follow it, so the frame stays readable on a dark page.

Preview

How it works

Belt length uses straight runs plus wrap arcs, and speed ratio is driven diameter ÷ driver diameter. Driven rpm is input rpm ÷ ratio for a flat belt between parallel centres.

  • Belt thickness and slip are omitted.
  • Two-pulley geometry is assumed.

Worked example

Motor pulley driving a larger idler
50mm driver, 150mm driven, 300mm apart, motor at 1800rpm
Input
											Driver pulley diameter mm: 50
Driven pulley diameter mm: 150
Centre distance mm: 300
Driver rpm: 1800
Output
												Belt length mm: 922.49
Speed ratio: 3
Driven rpm: 600

When to use this

Machine builders estimate replacement belts, drive designs check speed, and workshops compare pulley changes.

Edge cases

  • Tight or crossed belts can invalidate the straight-run approximation.
  • Slip lowers driven speed below the ideal ratio.
  • Changing diameter changes speed and wrap geometry.