Sizing a timing belt correctly comes down to three numbers: pitch length, tooth count, and width. Each one is calculated separately.
Pitch length is fixed by the centre distance and the speed ratio: L = 2C + (π/2)(D + d) + ((D − d)² / 4C), where C is centre distance, D and d are the pitch diameters of the large and small pulleys. Round to the nearest stock pitch length.
Tooth count on the small pulley comes from the speed ratio: N₂ = N₁ × (RPM₁ / RPM₂). Always pick a tooth count above 12 for HTD / RPP, above 18 for trapezoidal profiles — fewer teeth and the belt struggles to engage.
Width is fixed by the torque. Use the manufacturer's design sheet for your profile and pitch; widths typically come in standard steps like 12 / 16 / 20 / 25 / 30 / 50 / 85 mm for HTD 8M.