Pulley Size & RPM Calculator

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Find the driven pulley speed or the pulley size needed to reach a target RPM.

Inputs

Driver pulley diameter (mm)

Motor speed (RPM)

Driven pulley diameter (mm)

How to use this calculator: Pulley Size & RPM Calculator

Find the driven pulley speed or the pulley size needed to reach a target RPM. The example below is calculated by this page's real engine from the displayed inputs.

1Confirm that the Pulley Size & RPM Calculator matches the quantity or design check you need.
2Enter Driver pulley diameter, Motor speed, and Driven pulley diameter using the units printed beside each field.
3Check every value before calculating, especially decimal points and measurement units.
4Calculate, then follow the substituted equations in the worked example and compare the result with any stated limit.
5Read the assumptions, warnings and cited references before using the result for a financial, medical or engineering decision.

Input guide and example values

Use values from the same measurement basis and time period. Conditional fields appear only when the related option is selected.

Input	Example value	Why it matters
Driver pulley diameter	100 mm	Measured or known driver pulley diameter used by the calculation engine.
Motor speed	1440 RPM	Measured or known motor speed used by the calculation engine.
Driven pulley diameter	250 mm	Measured or known driven pulley diameter used by the calculation engine.

Formula inputs & variables for Pulley Size & RPM Calculator

These are the named quantities used by this calculator. When the source formula does not define a mathematical symbol, OneCalcApp keeps the real input label instead of inventing one.

Variable / input	Unit	Meaning in this calculation
Driver pulley diameter	mm	Measured or known driver pulley diameter used by the calculation engine.
Motor speed	RPM	Measured or known motor speed used by the calculation engine.
Driven pulley diameter	mm	Measured or known driven pulley diameter used by the calculation engine.

How the Pulley Size & RPM Calculator works

The Pulley Size & RPM Calculator uses Driver pulley diameter, Motor speed, and Driven pulley diameter to calculate Driven pulley speed, Speed ratio, Belt speed, Torque multiplication, and Pulley for 1000 RPM output. Its engine applies D1 × N1 = D2 × N2; the worked values below come from that same live calculation rather than a separately typed example.

With Driver pulley diameter 100 mm, Motor speed 1440 RPM, and Driven pulley diameter 250 mm, the main worked-example result is Driven pulley speed = 576 RPM.

How each Pulley Size & RPM Calculator input is used

Driver pulley diameter

The Pulley Size & RPM Calculator worked example uses Driver pulley diameter = 100 mm. This value is passed directly into the calculation, with an allowed minimum 1.

Motor speed

The Pulley Size & RPM Calculator worked example uses Motor speed = 1440 RPM. This value is passed directly into the calculation, with an allowed minimum 1.

Driven pulley diameter

The Pulley Size & RPM Calculator worked example uses Driven pulley diameter = 250 mm. This value is passed directly into the calculation, with an allowed minimum 1.

Pulley Size & RPM Calculator formulas and result interpretation

Formula 1: relationship used

In the Pulley Size & RPM Calculator, D1 × N1 = D2 × N2. The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Formula 2: relationship used

In the Pulley Size & RPM Calculator, N2 = D1 × N1 ÷ D2. The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Formula 3: relationship used

In the Pulley Size & RPM Calculator, Belt speed = π × D × N ÷ 60000 (m/s). The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Driven pulley speed

For the displayed Pulley Size & RPM Calculator worked example, Driven pulley speed is 576 RPM. Verify Driver pulley diameter, Motor speed, and Driven pulley diameter and their units before relying on this output.

Speed ratio

For the displayed Pulley Size & RPM Calculator worked example, Speed ratio is 2.5 : 1. Verify Driver pulley diameter, Motor speed, and Driven pulley diameter and their units before relying on this output.

Belt speed

For the displayed Pulley Size & RPM Calculator worked example, Belt speed is 7.54 m/s. safe Verify Driver pulley diameter, Motor speed, and Driven pulley diameter and their units before relying on this output.

Torque multiplication

For the displayed Pulley Size & RPM Calculator worked example, Torque multiplication is 2.5×. Verify Driver pulley diameter, Motor speed, and Driven pulley diameter and their units before relying on this output.

Pulley for 1000 RPM output

For the displayed Pulley Size & RPM Calculator worked example, Pulley for 1000 RPM output is 144 mm. Verify Driver pulley diameter, Motor speed, and Driven pulley diameter and their units before relying on this output.

Pulley Size & RPM Calculator accuracy, checks and limitations

Pulley Size & RPM Calculator units check: confirm Driver pulley diameter (mm), Motor speed (RPM), and Driven pulley diameter (mm) before calculating.
Pulley Size & RPM Calculator result check: compare Driven pulley speed, Speed ratio, Belt speed, Torque multiplication, and Pulley for 1000 RPM output with the substituted formula steps and the displayed rounding precision.
Pulley Size & RPM Calculator: Use this result for preliminary design and cross-checking. Confirm the applicable code edition, manufacturer data, site conditions and qualified-engineer approval before final design or installation.

Formula, derivation and worked example

In a belt drive the rim speed of both pulleys is the same, so diameter and speed are inversely related. A small pulley driving a big one slows the machine down and increases torque, which is exactly how most workshop machines are geared.

D1 × N1 = D2 × N2

N2 = D1 × N1 ÷ D2

Belt speed = π × D × N ÷ 60000 (m/s)

Substitution steps

1. Output speed
D1 × N1 ÷ D2
= 576 RPM
2. Belt speed
π D N / 60
= 7.54 m/s

Computed example results

Driven pulley speed: 576 RPM
Speed ratio: 2.5 : 1
Belt speed: 7.54 m/s; safe
Torque multiplication: 2.5×
Pulley for 1000 RPM output: 144 mm

Understanding the result

Read the main result together with supporting checks, assumptions, limits and intermediate values.

For a manual check, repeat the first equation, confirm the units and change one input at a time.

Assumptions and calculation scope

Driver pulley diameter is assumed to be entered in mm; converting from another unit before entry avoids changing the numerical meaning of the calculation.
Pulley Size & RPM Calculator applies the displayed D1 × N1 = D2 × N2 relationship to the entered engineering inputs. Factors that are not exposed as inputs or stated assumptions are outside this calculator's calculation scope.

Common mistakes when using Pulley Size & RPM Calculator

Do not mix units for Driver pulley diameter (mm), Motor speed (RPM), Driven pulley diameter (mm). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
Do not replace the displayed D1 × N1 = D2 × N2 relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
Do not treat Driven pulley speed = 576 RPM from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.

When the Pulley Size & RPM Calculator is useful

Pulley Size & RPM Calculator is designed for cases where Driver pulley diameter, Motor speed, Driven pulley diameter are known and you need Driven pulley speed, Speed ratio, Belt speed. The page keeps the live calculator, calculation method and worked example together so the result can be checked instead of treated as a black-box number.

Use the calculator for the scope described by its inputs and notes. The displayed method is D1 × N1 = D2 × N2. If the real project or decision needs factors that are not represented here, treat the result as an estimate and add the missing checks separately.

Driver pulley diameter and Motor speed: what changes the answer

The worked example uses Driver pulley diameter = 100 mm, Motor speed = 1440 RPM, Driven pulley diameter = 250 mm. With those values, Driven pulley speed is 576 RPM. Changing an input should be interpreted according to that field's unit, range, option and hint rather than by the number alone.

For this calculator, the main input roles are: Driver pulley diameter (mm): Measured or known driver pulley diameter used by the calculation engine. Motor speed (RPM): Measured or known motor speed used by the calculation engine. Driven pulley diameter (mm): Measured or known driven pulley diameter used by the calculation engine.

How to sanity-check a Pulley Size & RPM Calculator result

Start by confirming the entered values and units, then compare the substituted working with the displayed formula or calculation steps. Pay particular attention to Driven pulley speed, because it is the first worked-example output shown by the live engine.

Finally, compare the result with the assumptions, warnings and related calculators on this page. A nearby calculator can be useful as a cross-check when it measures the same workflow from a different input or output direction.

Next logical calculator

Continue with Belt Length Calculator

Useful next check because both tools use Driver pulley diameter and Driven pulley diameter, while Belt Length Calculator answers a different part of the same workflow.

Open Belt Length Calculator

Formula

D1 × N1 = D2 × N2
N2 = D1 × N1 ÷ D2
Belt speed = π × D × N ÷ 60000 (m/s)

Worked example

1A 100 mm pulley on a 1,440 RPM motor driving a 250 mm pulley.
2N2 = 100 × 1440 ÷ 250 = 576 RPM.
3Belt speed = π × 0.1 × 1440 ÷ 60 = 7.54 m/s.

Tips

Keep belt speed below 25 m/s for standard V-belts.
A ratio above 6:1 on a single belt drive is unreliable — use two stages.

Frequently asked questions

How do I slow a machine down?

Fit a larger driven pulley or a smaller motor pulley — speed falls in proportion to the diameter ratio.

Does the belt affect the speed?

Only slightly; V-belts slip about 1–2% under load.

What about torque?

Torque changes inversely with speed — halving the speed roughly doubles the torque available.

What inputs does the Pulley Size & RPM Calculator use?

It uses Driver pulley diameter, Motor speed, and Driven pulley diameter. Follow the unit printed for each field and choose any selectable option to match the real scenario.

What does the Pulley Size & RPM Calculator calculate?

It calculates Driven pulley speed, Speed ratio, Belt speed, Torque multiplication, and Pulley for 1000 RPM output. With the displayed default inputs, Driven pulley speed is 576 RPM.

Which formula does the Pulley Size & RPM Calculator use?

The primary relationship is D1 × N1 = D2 × N2. The page also shows substituted values and calculation steps so the result can be checked independently.

How can I verify a Pulley Size & RPM Calculator result?

First verify the units for Driver pulley diameter, Motor speed, and Driven pulley diameter. Then compare the substituted formula steps with Driven pulley speed and its displayed precision.

What are the limitations of the Pulley Size & RPM Calculator?

Pulley Size & RPM Calculator: Use this result for preliminary design and cross-checking. Confirm the applicable code edition, manufacturer data, site conditions and qualified-engineer approval before final design or installation.

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