Choose consistent dimensions
The belt pitch line is the reference for the geometry. A pulley outside diameter can differ from its pitch diameter, especially with timing teeth or V grooves. Obtain the correct dimensions from the component drawing and use millimeters throughout this calculator.
Exact geometry versus an approximation
For unequal pulleys the straight belt spans are inclined. The exact tangent calculation includes that inclination and unequal wrap angles. This workbench can also reverse the relationship to solve center distance for a supplied pitch length. It rejects physically overlapping pulley circles.
Allow for the real installation
The calculated length describes an ideal open belt with no stretch or slip. Actual selection still needs available stock length, installation clearance, take-up adjustment and manufacturer power ratings. The diagram is a geometric aid, not a complete drive design.
Check a reproducible open-belt example
Choose Two-pulley open belt. Enter a 40 mm driver pitch diameter, a 100 mm driven pitch diameter and a 200 mm center distance. At 1,500 input rpm, ideal output speed is 600 rpm. Solving for pitch length gives approximately 624.42 mm; the unequal diameters also produce different wrap angles on the two pulleys.
To verify the inverse calculation, switch to solving center distance and enter the unrounded calculated length from the output record. The result should return close to 200 mm. Copying a rounded length can produce a small difference, so do not mistake ordinary rounding for a geometry error.
Keep stock selection separate from the solution
The inactive length or center field is ignored according to the selected solve direction. A requested length that cannot span the non-overlapping pulley arrangement is rejected. Once you choose an available commercial belt, calculate its corresponding center distance and compare that with the adjustment range. Crossed belts, idlers, noncircular pulleys, tooth engagement and load capacity are outside this two-pulley open-belt model.