Belt Drive Calculator
Calculate the driven pulley speed for a belt drive system by entering pulley diameters and the drive shaft RPM. Use it when sizing pulleys for motors, conveyor systems, or power tools.
About this calculator
In a belt drive, speed is transmitted between two pulleys by a continuous belt. The output (driven) pulley speed follows the relationship: Driven Speed = (driveSpeed × drivePulleyDiameter) / drivenPulleyDiameter. This comes from the principle that belt velocity is constant across both pulleys: v = π × d × n / 60. A larger drive pulley or a smaller driven pulley increases output speed; a smaller drive pulley or larger driven pulley reduces it. The belt length for an open drive can be estimated as: L ≈ 2C + π(D + d)/2 + (D − d)²/(4C), where C is center distance and D, d are pulley diameters. Unlike gears, belt drives allow some slip, so actual efficiency is typically 95–98%.
How to use
Example: A motor runs at 1,750 RPM and is fitted with a 4-inch drive pulley. The driven pulley has a diameter of 10 inches, and the center distance is 24 inches. Step 1 — Enter driveSpeed = 1,750 RPM. Step 2 — Enter drivePulleyDiameter = 4 in, drivenPulleyDiameter = 10 in. Step 3 — Enter centerDistance = 24 in. Calculation: Driven Speed = (1,750 × 4) / 10 = 7,000 / 10 = 700 RPM. The speed ratio is 2.5:1, reducing the motor speed significantly for the driven shaft.
Frequently asked questions
How do I calculate the speed ratio of a belt drive system?
The speed ratio is simply the drive pulley diameter divided by the driven pulley diameter: ratio = D_drive / D_driven. If the drive pulley is larger, the driven pulley spins faster; if smaller, the driven pulley spins slower. This relationship holds because the belt surface velocity must be equal at both pulleys. Unlike gear ratios expressed in tooth counts, belt drive ratios use physical diameters, making them easy to adjust by swapping pulleys.
What is the formula for belt length in an open belt drive system?
For an open belt drive, the approximate belt length is L ≈ 2C + π(D + d)/2 + (D − d)²/(4C), where C is center distance and D and d are the larger and smaller pulley diameters. This formula assumes the belt runs straight between the two pulleys without crossing. Accurate belt length is critical to ensure proper tension and prevent slipping or premature wear. Always add a small allowance for belt stretch and tensioner adjustment when ordering a belt.
Why is belt drive efficiency lower than gear drive efficiency?
Belt drives lose energy through belt flexing, creep (micro-slip between belt and pulley), and aerodynamic drag at higher speeds. Typical efficiency for V-belt drives is 93–98%, compared to 97–99% for well-maintained spur gears. Flat belts have slightly higher efficiency than V-belts at low loads but slip more easily under heavy loads. Synchronous (timing) belts nearly eliminate slip and achieve efficiencies close to gear drives, making them preferred for precise speed-ratio applications.