Work out the belt length for any two-pulley drive, or the centre distance a belt gives you. Uses the standard open-belt formula with datum diameters.
L = 2C + π(D + d)/2 + (D − d)²/4C, where C is the centre distance and D and d are the datum diameters of the large and small pulleys. The calculator does this and rounds to the nearest standard length.
An old belt has stretched and worn, so measuring it usually gives the wrong size. Measure the pulleys and centre distance on the machine, or read the size printed on the belt.