Size a synchronous (timing) belt drive: pick the profile and pulley teeth, and get the belt in teeth, the exact centre distance and the teeth in mesh.
Pitch diameter = pitch × teeth ÷ π for each pulley; the open-belt formula then gives the pitch length, which is rounded to a whole number of belt teeth × pitch. The calculator then works back to the exact centre distance.
At least 6 teeth on the small pulley, or the belt can jump teeth under load. Use a bigger small pulley or a longer centre distance if you have fewer.