How to Measure a Pulley Size

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Pulleys, like the ropes you run through them to control sails and rigging on your boat, are sized by their diameter. Pulleys are sized by their diameter because of the effect of "bends," or changes in direction in a rope when it's riven (installed over a pulley), as in a block an tackle, where the rope makes a complete U-turn over the pulleys. The size of a pulley---or more properly, the sheave of a block---should equal at least eight times the diameter of the rope running over the pulley to avoid creating internal stress on the rope's strands.

Measure the distance across the shaft hole of the pulley with a measuring tape. Divide this by two with your calculator. If the shaft hole is 1 inch wide, then: 1 divided by 2 equals 1/2.

Add the distance from the edge of the shaft hole to 1/2 the distance across the shaft hole: 6 plus 1/2 equals 6 1/2.

Multiply the result by 2: 6 1/2 times 2 equals 13, the size of the pulley. A 13-inch pulley is suitable for use with ropes 1.6 inches in diameter, or line commonly sized as 1 1/2-inch line.


  • Using a block that's more than eight times the diameter of the rope increases the efficiency of the block.


  • "The American Merchant Seaman's Manual;" Wm. Hayler (Editor); Cornell Maritime Press; 1981

About the Author

Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.

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