Belt Drive Ratio Calculator

A belt drive system uses two pulleys of different sizes connected by a belt to transfer power and change rotational speed — the Belt Drive Ratio Calculator tells you exactly how much speed is gained or lost in that transfer. Enter your Driver Pulley Diameter, Driven Pulley Diameter, and Driver RPM to get the Drive Ratio, Driven Pulley RPM, and Speed Reduction. Optionally add the Center Distance between pulleys to also calculate the Approximate Belt Length you'll need.

inches

Diameter of the driving pulley (motor side)

inches

Diameter of the driven pulley (load side)

RPM

Rotational speed of the driving pulley

inches

Distance between pulley centers (optional)

Results

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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Frequently Asked Questions

How do you calculate belt pulley ratio?

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.

What does a 2:1 drive ratio mean?

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.

How do I determine the right pulley sizes for my application?

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.

What's the difference between speed ratio and drive ratio?

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.

How does center distance affect belt selection?

Center distance determines the belt length needed. Use the formula: Belt Length ≈ 2 × Center Distance + 1.57 × (Driver + Driven Diameter) for approximate belt sizing.

Can I use this calculator for timing belts and V-belts?

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.

What happens if I reverse the driver and driven pulleys?

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).

How accurate are these belt drive calculations?

These calculations provide theoretical values assuming no belt slip. In real applications, expect 1-3% slip depending on belt tension, load, and pulley condition.