Enter your Driver Pulley Diameter, Driven Pulley Diameter, Driver RPM, and Center Distance into the Belt Drive Ratio Calculator to find your Drive Ratio, along with Driven Pulley RPM, Speed Reduction, and Approximate Belt Length — everything you need to spec out a belt drive system without the manual math.
Drive Ratio
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Driven Pulley RPM
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Speed Reduction
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Approximate Belt Length
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Belt pulley ratio is calculated by dividing the driven pulley diameter by the driver pulley diameter. For example, if the driver pulley is 6 inches and driven pulley is 12 inches, the ratio is 12÷6 = 2:1.
A 2:1 drive ratio means the driver pulley rotates 2 times for every 1 rotation of the driven pulley. This reduces speed by 50% while increasing torque proportionally.
Start with your desired output speed and motor RPM. Use the formula: Driven Diameter = (Driver Diameter × Driver RPM) ÷ Desired Driven RPM to find the required pulley size.
Drive ratio compares pulley diameters, while speed ratio compares RPMs. They are inversely related - a larger driven pulley creates a higher drive ratio but lower speed ratio.
Center distance determines the belt length needed. Use the formula: Belt Length ≈ 2 × Center Distance + 1.57 × (Driver + Driven Diameter) for approximate belt sizing.
Yes, this calculator works for any belt drive system including V-belts, timing belts, and flat belts. The ratio calculations remain the same regardless of belt type.
Reversing the pulleys inverts the ratio. A 2:1 reduction becomes a 1:2 speed increase. Always identify which pulley is connected to the motor (driver) versus the load (driven).
These calculations provide theoretical values assuming no belt slip. In real applications, expect 1-3% slip depending on belt tension, load, and pulley condition.