Measure the upper width of the belt, e.g. 17 mm (belt type 17/B)... ... or e.g. 16.3 mm (belt type SPB or XPB for belts with teeth). Measure the belt thickness, e.g. 13 mm (belt type SPB 16.3×13)... ... or e.g. 10 mm (belt type 17/B with a measurement of 17×10).