Belt Length Calculator

This calculator works out the length of a belt running around two pulleys, from the distance between their centres and the diameter of each pulley. Belt drives transmit power between rotating shafts throughout machinery, from engines and pumps to conveyors and workshop tools, and choosing or replacing a belt means knowing its correct length. The belt wraps part way around each pulley and runs straight between them, so its total length depends on the two pulley sizes and how far apart they sit. Calculating it by hand involves a formula with the centre distance, the pulley diameters and a correction for the difference in their sizes, which is fiddly to do accurately. This tool does it instantly. You enter the centre distance between the two shafts, the diameter of the first pulley and the diameter of the second, and the calculator returns the required belt length in metres and millimetres, along with the centre distance and the sum of the pulley diameters for reference. The results update as you type. Use it to specify or order a belt, to check a drive design, or for mechanical engineering work. The belt length is twice the centre distance, plus pi times the average of the two diameters, plus a small term for the difference in diameters squared divided by four times the centre distance. The middle term accounts for the belt wrapping around the pulleys, which for equal pulleys is simply pi times the diameter, half around each. The final term is a correction that matters only when the pulleys differ noticeably in size. For most drives the centre distance dominates, so the belt length is a little more than twice the distance between the shafts plus the wrap around the pulleys. Keep all the dimensions in the same units; the result comes out in those units.

Belt length = 2C + π(D1 + D2)/2 + (D1 - D2)² / (4C), where C is the centre distance. Keep all dimensions in the same units. Works for an open belt drive.