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Pulley RPM Calculator: Speed Ratio, Belt Length & Torque

Calculate driven pulley RPM, belt length, speed ratio, and torque for any belt drive. Imperial and metric. Supports V-belt, flat belt, and timing belt drives.

Unit:

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Input/motor-side pulley

Output/load-side pulley

Typical motors: 1750 or 3450 RPM

Real V-belt systems: 1–3%

Optional Calculations
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How to Use This Calculator

  1. 1

    Choose a calculation mode

    Select 'Driven RPM' to find how fast the output shaft spins given both pulley sizes. Select 'Required Driven Diameter' to find what size driven pulley you need to hit a target output RPM.

  2. 2

    Enter driver pulley diameter and RPM

    Type the driver (motor-side) pulley diameter and the motor's nameplate RPM. Common motor speeds are 1750 RPM (4-pole, 60 Hz) and 3450 RPM (2-pole, 60 Hz).

  3. 3

    Enter driven diameter or target RPM

    In 'Driven RPM' mode, enter the output pulley diameter. In 'Required Diameter' mode, enter the RPM you need the output shaft to run at.

  4. 4

    Set belt slip percentage

    Enter belt slip (default 0%). Real V-belt drives lose 1–3% due to elastic creep. Leave at 0 for theoretical calculations or enter 2% for a conservative real-world estimate.

  5. 5

    Add optional inputs for belt length and torque

    Enter center distance to calculate required belt length. Enter motor power (HP or kW) to calculate driver and driven shaft torque. Both are optional.

What Each Value Means

Speed Ratio (dimensionless ratio)
Driver diameter divided by driven diameter. A ratio of 2:1 means the driven shaft spins at twice the motor speed (speed increase). A ratio of 0.5:1 means the driven shaft spins at half the motor speed (speed reduction). The speed ratio also equals the inverse of the torque ratio — torque doubles when speed halves.
Belt Speed (ft/min or m/min)
The linear velocity of the belt as it travels around the pulleys. Belt speed = π × Driver diameter × Driver RPM, divided by 12 (for ft/min) or 1000 (for m/min). Exceeding the rated maximum belt speed for your belt type causes heat buildup, vibration, and premature wear.
Belt Length (inches or mm)
The total length of belt required to span both pulleys at a given center distance. Calculated using the open-belt formula. Used to select a standard belt length from manufacturer catalogs. Accounts for both the straight runs between pulleys and the arcs of contact on each pulley.
Belt Slip (%) (percent)
The percentage difference between theoretical belt speed and actual belt speed due to elastic creep and groove micro-slip under load. Standard V-belts slip 1–3%. Synchronous (timing) belts have zero slip. Higher loads and under-tensioned belts increase slip percentage.

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Frequently Asked Questions

How do you calculate the RPM of a driven pulley?
Driven pulley RPM = Driver RPM × (Driver diameter ÷ Driven diameter). For example, a 1750 RPM motor with a 6-inch driver pulley driving a 10-inch driven pulley produces: 1750 × 6/10 = 1050 RPM. To account for real V-belt slip (1–3%), multiply by (1 − slip%). A 2% slip gives 1050 × 0.98 = 1029 RPM.
What is the pulley speed ratio formula?
Speed ratio = Driver diameter ÷ Driven diameter. A ratio greater than 1 means the driven shaft spins faster than the driver (speed increase). A ratio less than 1 means speed reduction. Example: 4-inch driver and 8-inch driven pulley gives a 0.5:1 ratio — the driven shaft turns at half the motor speed.
How do you calculate belt length for a two-pulley drive?
Belt length L = 2C + (π/2)(D1 + D2) + (D1 − D2)² / (4C), where C is the center distance between pulley shafts, D1 is the driver diameter, and D2 is the driven diameter. All values in the same unit (inches or mm). This formula applies to open belt drives with both pulleys on parallel shafts.
How does belt slip affect pulley RPM?
Belt slip (1–3% for V-belts) reduces the actual driven RPM below the theoretical value. If the ideal driven RPM is 1050 and belt slip is 2%, actual RPM = 1050 × (1 − 0.02) = 1029 RPM. Slip is caused by belt stretch, load-induced micro-movement on the pulley groove, and insufficient tension. Synchronous (toothed) belts have zero slip.
What is a typical belt speed for V-belt drives?
Classical V-belts (A–E cross-sections) are rated to a maximum of 4,500 ft/min (1,372 m/min). Narrow V-belts (3V, 5V, 8V) can handle up to 6,500 ft/min (1,981 m/min). Exceeding recommended belt speed causes excessive heat, vibration, and shortened belt life. Use the belt speed result in this calculator to verify your design is within limits.
How do you find the required driven pulley diameter for a target RPM?
Rearrange the RPM formula: D2 = D1 × N1 ÷ N2, where D1 is driver diameter, N1 is driver RPM, and N2 is your target RPM. For a 1750 RPM motor with a 4-inch driver pulley targeting 875 RPM on the driven shaft: D2 = 4 × 1750 ÷ 875 = 8 inches. Use the 'Required Driven Diameter' mode in this calculator to solve directly.