Pulley Speed Calculator (RPM, Pulley Size and Belt Length)

Shop machines such as drill presses, lathes, grinders, band saws and line shafts get their speed from a pair of pulleys and a belt. This calculator gives the speed of the driven shaft from the motor speed and both pulley sizes, or works backwards to the pulley you need for a target speed. With the centre distance it also gives the belt length and the angle the belt wraps round the small pulley, which tells you whether it is likely to slip.

Pulley speed calculator

How it works

The belt moves at the same speed over both pulleys, so speed times diameter is the same on each:

driven RPM = motor RPM × driver diameter ÷ driven diameter
driven pulley for a target speed = driver diameter × motor RPM ÷ target RPM
belt length (open belt) = 2C sin(θ/2) + r₁(2π − θ) + r₂θ, with θ = 2 arccos((r₁ − r₂) ÷ C) the wrap angle on the small pulley
belt speed = π × driver diameter × motor RPM ÷ 12 ft/min

Real belts slip and stretch a little, so the driven shaft runs a few per cent slower than the ratio. For V-belts use the pitch diameter, where the belt’s load-carrying cords run, which is a little smaller than the outside diameter of the pulley.

Worked example

A 1,725 RPM motor with a 2 in pulley driving a 4 in pulley on a shaft 12 in away:

Units: Inches
Work out: Driven speed from two pulley sizes
Motor (driver) speed: 1725 RPM
Motor (driver) pulley diameter: 2 in
Driven pulley diameter: 4 in
Distance between shaft centres: 12 in

The calculator gives:

Driven speed: 863 RPM
Speed ratio: 2 : 1 (slows down)
Belt speed: 903 ft/min
Belt length (pitch line): 33.51 in (851 mm)
Wrap on the small pulley: 170 degrees

The driven shaft turns at 863 RPM, half the motor speed. The belt runs at about 900 ft/min and needs to be about 33.5 in long on the pitch line; it wraps 170 degrees round the small pulley, plenty for grip. To reach 500 RPM from the same motor and 2 in pulley you would need a 6.9 in driven pulley, or two stages of smaller ratio.

Driven speed with a 1,725 RPM motor (RPM)

Motor pulley	2 in driven	3 in driven	4 in driven	5 in driven	6 in driven	8 in driven	10 in driven
1.5 in	1,294	863	647	518	431	323	259
2 in	1,725	1,150	863	690	575	431	345
2.5 in	2,156	1,438	1,078	863	719	539	431
3 in	2,588	1,725	1,294	1,035	863	647	518
4 in	3,450	2,300	1,725	1,380	1,150	863	690
5 in	4,313	2,875	2,156	1,725	1,438	1,078	863

Driven RPM = 1,725 × motor pulley ÷ driven pulley, before belt slip.

Driven speed with a 3,450 RPM motor (RPM)

Motor pulley	2 in driven	3 in driven	4 in driven	5 in driven	6 in driven	8 in driven	10 in driven
1.5 in	2,588	1,725	1,294	1,035	863	647	518
2 in	3,450	2,300	1,725	1,380	1,150	863	690
2.5 in	4,313	2,875	2,156	1,725	1,438	1,078	863
3 in	5,175	3,450	2,588	2,070	1,725	1,294	1,035
4 in	6,900	4,600	3,450	2,760	2,300	1,725	1,380
5 in	8,625	5,750	4,313	3,450	2,875	2,156	1,725

Driven RPM = 3,450 × motor pulley ÷ driven pulley, before belt slip.

Notes and limits

Check the motor plate for its speed rather than assuming one; the plate figure already allows for the motor’s slip under load.
The less the belt wraps round the small pulley, the more easily it slips. Large ratios on close centres need either a longer centre distance or two stages.
Check the speed limit of whatever the driven shaft turns: a grinding wheel, a saw blade or a lathe chuck each has a maximum safe RPM.
Belt makers state lengths in different ways (inside, pitch or outside length). Check which one your belt size refers to before ordering.
Lathe speeds by workpiece size are in the wood lathe speed calculator; drill speeds in the drill speed calculator.

Frequently asked questions

How do you calculate pulley RPM?

Driven RPM = motor RPM × motor pulley diameter ÷ driven pulley diameter. A 1,725 RPM motor with a 2 in pulley turns a 4 in pulley at about 863 RPM.

What size pulley do I need to slow a motor down?

Driven pulley diameter = motor pulley diameter × motor RPM ÷ target RPM. To get 500 RPM from 1,725 RPM with a 2 in motor pulley you need a 6.9 in driven pulley.

How do I calculate belt length for two pulleys?

A close approximation is 2C + π(D + d) ÷ 2 + (D − d)² ÷ 4C, with C the centre distance. The calculator uses the exact form, which agrees to within a few thousandths for normal drives.

Does pulley size change torque?

Yes. Slowing a shaft down by a ratio multiplies its torque by about the same ratio, less friction; speeding it up divides it.

Sources

Last reviewed: 25 October 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.