Enter your driver pulley diameter, driven pulley diameter, center distance, and driver RPM to calculate driven RPM, belt speed, belt length, belt tension, and torque for a two-pulley belt drive system.
Driven Pulley RPM
--
Belt Speed
--
Belt Length
--
Speed Ratio
--
Effective Belt Tension
--
Driver Torque
--
Driven Torque
--
The driven pulley RPM is calculated using the formula: Driven RPM = (Driver Diameter × Driver RPM) / Driven Diameter. A larger driven pulley will spin slower than the driver pulley, while a smaller driven pulley will spin faster.
The belt length formula for two fixed pulleys is: L = (d1 × π / 2) + (d2 × π / 2) + (2 × D) + ((d1 − d2)² / (4 × D)), where d1 and d2 are the pulley diameters and D is the center distance between them. This accounts for both the straight runs and the arc of contact on each pulley.
The speed ratio (also called velocity ratio) is the ratio of the driver pulley diameter to the driven pulley diameter. A speed ratio of 2:1 means the driven pulley turns at half the speed of the driver. You can also express it as the inverse: the driver RPM divided by the driven RPM.
Effective belt tension is calculated from the power transmitted and the belt speed: Tension (lbf) = (Power in hp × 33,000) / Belt Speed (ft/min). This gives the net force the belt exerts on the pulleys. Actual tight-side and slack-side tensions depend additionally on the coefficient of friction and wrap angle.
Torque on a pulley equals the effective belt tension multiplied by the pulley's radius: Torque = Tension × (Diameter / 2). The driver torque and driven torque will differ when the pulleys are different sizes — a larger driven pulley produces higher output torque, which is why large sprockets are used for low-speed, high-torque applications.
A large driving chainring paired with a small driven sprocket produces a high speed ratio, meaning the rear wheel spins many times for each pedal revolution. This maximizes wheel speed at the cost of requiring more pedaling force, which is ideal on flat or downhill roads.
The driver pulley is connected to the power source (e.g., a motor) and transmits motion to the belt. The driven pulley receives that motion and delivers it to the output shaft or machine. Power flows from driver to driven; the speed and torque at the driven pulley depend on the diameter ratio between the two.
Yes — select 'Metric' from the unit system dropdown and enter diameters in millimeters, power in kilowatts, and center distance in millimeters. The outputs will be displayed in metric units including belt speed in m/s, belt length in mm, tension in Newtons, and torque in N·m.