RPM Calculator: Pulley Speed, Belt Length and Cutting Speed
For pulleys, enter both diameters, motor rpm and the centre distance. For machining, enter tool or job diameter and cutting speed to get the spindle rpm.
- Nothing you type is stored
- Free
- No signup
Driven pulley speed
720rpm
- Speed ratio
- 2 : 1
- Belt speed
- 7.54 m/s
- Belt length (open belt)
- 1,476 mm
Belts slip about 1–3%, so real speed is slightly lower.
Show working
- N2 = 1,440 × 100 ÷ 200 = 720 rpm
- v = π × 100 × 1,440 ÷ 60000 = 7.54 m/s
How to use
- Choose pulley drive or cutting speed.
- Pulley: enter driver and driven diameters, motor rpm and centre distance.
- Cutting: enter diameter and cutting speed (m/min).
Good to know
Pulley: driven rpm = driver rpm × driver diameter ÷ driven diameter. Belt speed (m/s) = π × driver diameter (mm) × rpm ÷ 60,000. Open-belt length = 2C + π(D + d)/2 + (D − d)²/(4C), where C is centre distance. Belts slip about 1–3%, so real speed is a little lower.
Cutting: spindle rpm = 1000 × cutting speed (m/min) ÷ (π × diameter in mm). Typical HSS cutting speeds: mild steel about 20–30 m/min, aluminium 60–100 m/min, cast iron 15–25 m/min; carbide tools allow several times more.
Common Indian motors run at 1,440 rpm (4-pole) or 2,880 rpm (2-pole). For gears, use the Gear ratio calculator; for flour mills, pumps and machines, check the maker’s recommended speed.
Questions
What is the speed of a 200 mm pulley driven by a 100 mm pulley at 1,440 rpm?
1,440 × 100 ÷ 200 = 720 rpm.
How do I find belt length?
L = 2C + π(D + d)/2 + (D − d)²/(4C). For 100 and 200 mm pulleys 500 mm apart: about 1,476 mm.
What rpm should I drill a 20 mm hole in mild steel?
At 25 m/min with HSS: 1000 × 25 ÷ (π × 20) ≈ 400 rpm.
Why is the machine slower than calculated?
Belt slip and motor slip under load reduce speed by a few percent.
What belt speed is safe?
V-belts usually work best below about 25–30 m/s; check the belt maker’s limit.