Pulley Calculator
Calculate pulley speed ratios, output RPM, and belt speed. Enter driver and driven pulley diameters and input RPM — get output RPM, belt speed, and mechanical advantage.
mm
mm
RPM
Enter your values above to see the results.
Tips & Notes
- ✓Speed ratio = driver diameter / driven diameter. A 200 mm driver turning a 50 mm driven pulley produces 4:1 speed increase (output = 4× input RPM). Larger driven pulley = slower output, more torque.
- ✓Belt speed (m/s) = π × driver diameter (m) × driver RPM / 60. At 1,500 RPM with a 150 mm (0.15 m) driver: belt speed = π × 0.15 × 1,500 / 60 = 11.78 m/s. Exceeding 30 m/s causes belt wear and vibration.
- ✓When speed increases, torque decreases proportionally (power = torque × angular velocity is constant, ignoring losses). A 4:1 speed-up means output torque = input torque / 4.
- ✓Always check belt sag and tension after installation — proper tension prevents slipping without overloading bearings. Rule of thumb: deflect the belt 1% of center distance with moderate thumb pressure.
- ✓Multiple-pulley compound systems multiply speed ratios. Three stages of 2:1 give 8:1 overall ratio. Calculate each stage separately then multiply all ratios together for total system ratio.
Common Mistakes
- ✗Confusing driver and driven pulley — the driver is connected to the motor (input), the driven is the output shaft. Swapping them inverts the ratio and gives the wrong RPM direction.
- ✗Entering diameter in different units for each pulley — both diameters must be in the same unit (both mm or both inches). Mixing units produces wrong ratios.
- ✗Ignoring belt slip — real belt drives slip 1-3% under load. The calculated output RPM is theoretical; actual RPM is slightly lower. Use V-belts for higher torque and less slip than flat belts.
- ✗Forgetting that mechanical advantage equals the inverse of speed ratio — a 3:1 speed reduction gives 3:1 torque multiplication (before losses). Engineers often confuse which shaft is input vs output.
- ✗Exceeding maximum belt speed ratings — most V-belts are rated for 20-30 m/s. Running faster causes vibration, heat buildup, and rapid wear. Check belt manufacturer specifications for your drive system.
Pulley Calculator Overview
Pulley and belt drive systems are among the most widely used power transmission mechanisms in industrial machinery, HVAC equipment, automotive accessories, and agricultural equipment. They allow speed reduction or increase, torque multiplication, and flexible shaft orientation — often with no lubrication required.
Pulley speed ratio formula:
Speed Ratio = Driver Diameter / Driven Diameter | Output RPM = Input RPM × (D_driver / D_driven)
EX: Motor at 1,750 RPM, driver pulley 80 mm, driven pulley 320 mm → Output RPM = 1,750 × (80/320) = 437.5 RPM. Belt speed = π × 0.08 × 1,750 / 60 = 7.33 m/sTorque and power in pulley systems:
Output Torque = Input Torque × (D_driven / D_driver) × efficiency | Power = Torque × ω (constant, minus losses)
EX: Input torque 15 N·m, driver 100 mm, driven 300 mm, efficiency 0.95 → Output torque = 15 × (300/100) × 0.95 = 42.75 N·m at 1/3 the input RPMPulley ratio design reference:
| Application | Typical Ratio | Effect | Example |
|---|---|---|---|
| Speed increase (e.g. fan) | 2:1 to 5:1 | Higher RPM, lower torque | Motor 1,450 RPM → Fan 4,000 RPM |
| Speed reduction (e.g. conveyor) | 3:1 to 10:1 | Lower RPM, higher torque | Motor 1,750 RPM → Conveyor 175 RPM |
| 1:1 (same speed) | 1:1 | Direction change only | Remote shaft positioning |
| Automotive alternator | 2:1 to 3:1 | Alternator runs faster than engine | Engine 800 RPM → Alternator 2,000 RPM |
| Washing machine drum | 10:1 to 20:1 | Slow drum, high torque | Motor 1,400 RPM → Drum 70 RPM |
| Belt Type | Max Speed | Power Range | Best Use |
|---|---|---|---|
| Flat belt | 50 m/s | Low-medium | Long center distances, clean environments |
| V-belt (A, B, C, D) | 30 m/s | Up to 500 kW | Industrial drives, most common |
| Synchronous (timing) | 80 m/s | Medium-high | No slip required, camshafts, printers |
| Poly-V belt | 50 m/s | High | Automotive serpentine, compact drives |
Frequently Asked Questions
Output RPM = Input RPM × (Driver Diameter / Driven Diameter). Example: motor at 1,750 RPM, driver pulley 100 mm, driven pulley 250 mm → Output RPM = 1,750 × (100/250) = 1,750 × 0.4 = 700 RPM. The output shaft rotates slower because the driven pulley is larger. To increase output RPM (speed up), make the driven pulley smaller than the driver. To decrease RPM (and increase torque), make the driven pulley larger.
Speed ratio = Driver diameter / Driven diameter = Output RPM / Input RPM. A 4:1 speed ratio means the output shaft spins 4 times faster than the input (or 4 times slower, depending on which is driver). Speed ratio also equals the inverse of the torque ratio: a 4:1 speed increase produces a 1:4 torque decrease. Example: 80 mm driver, 320 mm driven → speed ratio = 80/320 = 0.25 (output is ¼ input RPM, but torque is 4× input torque).
Belt speed (m/s) = π × Driver Diameter (m) × Driver RPM / 60. Example: 150 mm (0.15 m) driver pulley at 1,450 RPM → Belt speed = π × 0.15 × 1,450 / 60 = π × 0.15 × 24.17 = 11.39 m/s. In imperial: Belt speed (ft/min) = π × Driver Diameter (in) × RPM / 12. Optimal belt speed for V-belts is typically 10-25 m/s (2,000-5,000 ft/min) for maximum power transmission efficiency.
Mechanical advantage (MA) = Driven diameter / Driver diameter = Input torque / Output torque (before losses). A large driven pulley provides torque multiplication at the cost of reduced speed. Example: 60 mm driver, 240 mm driven → MA = 240/60 = 4. If input torque is 10 N·m, output torque = 10 × 4 = 40 N·m (at ¼ the input RPM). This is how gearboxes and variable speed drives trade speed for torque — the same power with different speed-torque combinations.
For a multi-stage system, total speed ratio = product of all individual stage ratios. Example: 3-stage system to achieve 12:1 overall speed reduction. Stage 1: 2:1 (200 mm driver, 400 mm driven). Stage 2: 2:1 (200 mm driver, 400 mm driven). Stage 3: 3:1 (100 mm driver, 300 mm driven). Total ratio = 2 × 2 × 3 = 12:1. If input is 1,440 RPM: output = 1,440 / 12 = 120 RPM. Multi-stage allows achieving large ratios with practical pulley sizes — a single-stage 12:1 would require an impractically large driven pulley.
An open belt drive has both pulleys rotating in the same direction — the standard configuration for most machinery. A crossed belt drive has the belt twisted 180° between pulleys, causing them to rotate in opposite directions. Crossed belts are used when counter-rotation is needed without adding an idler pulley, but the belt wears faster and cannot transmit as much power. Most industrial drives use open belts. The speed ratio formula is the same for both configurations: Output RPM = Input RPM × (Driver / Driven diameter).