Pulley Speed Calculator
Calculates the output shaft speed of a belt-and-pulley system based on pulley diameters and input speed. Use it when designing drive systems to achieve a target RPM ratio between driver and driven shafts.
About this calculator
In a belt-and-pulley drive, the belt travels at the same linear speed across both pulleys (assuming no slip). Linear belt speed equals the circumferential speed of each pulley: v = π × D × N / 60, where D is the pulley diameter and N is its rotational speed in rpm. Setting the linear speeds equal for both pulleys gives the fundamental pulley ratio equation: N_out = N_in × (D_in / D_out), where N_in and N_out are the input and output speeds in rpm, and D_in and D_out are the respective pulley diameters (in any consistent unit — the ratio is dimensionless). A smaller output pulley than input pulley increases speed (speed-up drive), while a larger output pulley reduces speed (speed-reduction drive). This relationship is mathematically identical to the gear ratio formula and is the cornerstone of all belt-drive and chain-drive design.
How to use
A motor drives a pump via a belt. The motor pulley has a diameter of 150 mm and spins at 1,450 rpm. The pump pulley has a diameter of 300 mm. Step 1 – Apply the formula: N_out = 1,450 × (150 / 300) = 1,450 × 0.5 = 725 rpm. Enter 1,450 in Input Speed, 150 in Input Pulley Diameter, and 300 in Output Pulley Diameter. The calculator returns 725 rpm — exactly half the motor speed, as expected from the 2:1 pulley size ratio.
Frequently asked questions
How do I calculate the pulley diameter needed to achieve a specific output speed?
Rearrange the pulley speed formula: D_out = D_in × (N_in / N_out). For example, if your motor runs at 1,800 rpm, your input pulley is 100 mm, and you need 600 rpm at the output, then D_out = 100 × (1,800 / 600) = 300 mm. This target diameter then guides your selection from standard pulley sizes, and you may need to adjust the input pulley or accept a slightly different output speed based on what is commercially available.
What is the effect of belt slip on the calculated output pulley speed?
The pulley speed formula assumes a perfectly inextensible belt with no slip between belt and pulley. In practice, elastic creep and slip in V-belts or flat belts can reduce the actual output speed by 1–3% compared to the theoretical value. For most industrial applications this is acceptable, but precision drives (such as timing systems) use toothed synchronous belts or chain drives that eliminate slip entirely. Always factor in an expected slip loss when designing systems where exact speed ratios are critical.
When should I use a stepped pulley or variable-speed pulley drive instead of fixed-diameter pulleys?
Fixed-diameter pulleys deliver a single, constant speed ratio and are ideal when the driven machine always operates at one speed. Stepped (cone) pulleys offer two to four discrete speed ratios by manually shifting the belt between pulley steps — common on older drill presses and lathes. Variable-speed drives using adjustable sheaves allow continuous speed variation over a range, which is useful for conveyors, fans, and mixers where process conditions change frequently. The choice depends on how often speed changes are needed and the required precision of each speed setting.