Pulley Speed & Ratio Calculator — Two Pulley RPM and Belt Speed
Calculate pulley speed ratio, driven pulley RPM, and belt surface speed for any two-pulley system. Works for flat, V, and round belts.
Quick Answer
For a driver pulley of 100 mm at 1750 RPM with a 250 mm driven pulley: Speed Ratio = 2.5:1, Driven RPM = 700, and Belt Speed ≈ 9.2 m/s. Enter your pulley diameters above.
How Pulley Speed Calculations Work
Pulley speed is the simplest power transmission calculation — and the most commonly needed on the shop floor.
1. Pulley Speed Ratio
Ratio = D₂ / D₁. If the driven pulley is larger, output slows down (reduction). Smaller driven pulley = speed increase. This is how drill press speed changes work.
2. Driven Pulley RPM
n₂ = n₁ × (D₁ / D₂). If your motor runs 1750 RPM and you want 700 RPM at the machine, you need D₂/D₁ = 2.5. Pick pulley sizes accordingly.
3. Belt Surface Speed
v = π × D₁ × n₁ / 60000 (m/s). This must stay within the belt type’s rating — V-belts 25-30 m/s max, flat belts 30-50 m/s depending on material.
Where This Is Used
- Setting drill press, lathe, and milling machine speeds
- Sizing blower and fan pulleys for desired CFM
- Converting motor RPM to pump required RPM
- Designing line-shaft machine shop drives
- Retrofitting belt drives on vintage machinery
Common Mistakes
- Measuring pulley OD instead of pitch diameter — V-belt pitch diameter sits about 1/3 into the groove. Using outside diameter overestimates the ratio by 3-5%.
- Using wrong units — Mixing inches and mm in the same calculation destroys accuracy. Our calculator uses mm; convert inches by multiplying by 25.4.
- Forgetting belt thickness — For flat belts, effective diameter = pulley OD + belt thickness. Thin belts (<3mm) you can ignore, but thick leather belts add 6-10mm to effective diameter.
- Not checking belt speed limits — Small pulleys at high RPM can easily exceed belt speed ratings. A 50mm pulley at 3000 RPM = 7.85 m/s — fine for V-belt, but check the spec.
- Ignoring pulley crown on flat belts — Flat belt pulleys need a slight crown (1-2%) to keep the belt centered. Without it, belts walk off.
Frequently Asked Questions
What’s the difference between pulley speed and belt drive calculators?
Pulley speed focuses purely on RPM ratios. Our Belt Drive Calculator adds belt length, center distance, and tension considerations. Use pulley speed for quick “what RPM do I get?” questions, belt drive for full design.
Can I use different diameter units?
Enter both diameters in the same units and the ratio works. The calculator expects mm but if you enter both in inches, the ratio will be correct. Just don’t mix units.
How do I select pulley sizes for a target RPM?
Target ratio = desired output RPM / motor RPM. Then pick pulley diameters with that ratio. For a 2:1 reduction, a 50mm driver and 100mm driven works. For exact ratios, you may need custom pulleys.
What if I have more than two pulleys?
For compound pulley systems (multiple stages), calculate each stage separately and multiply the ratios. A 2:1 followed by 3:1 = 6:1 overall. Use our Gear Ratio Calculator for compound trains.
Why does my actual RPM differ from calculated?
Belt slip accounts for 1-3% difference (V-belt) to 0% (timing belt). Under heavy load, V-belts can slip more. For zero-slip applications, see our Timing Belt Calculator.
What pulley material for high speed?
Steel for industrial loads. Aluminum for weight-sensitive applications (drones, light machinery). Cast iron for vibration damping. Plastic for low-power (<0.5 kW). Avoid cast iron above 40 m/s — it can burst from centrifugal stress.
How does pulley wear affect the ratio?
Worn pulleys effectively change the pitch diameter — a groove worn 1mm deeper increases effective diameter, changing the ratio. Replace pulleys when groove wear exceeds 0.8mm.
Can I use this for a variable speed pulley?
Yes for the geometry — enter min and max effective diameters. The ratio range tells you your speed range. Variable pulleys can achieve 3:1 to 8:1 speed range depending on design.