Belt & Pulley Speed Calculator

Multi-pulley RPM · belt length & centre distance · wrap angle · slip check

Drive Setup

Belt Type

Belt Pitch (mm)

Driver Pulley Ø (mm)

Driver RPM

Driven Pulleys

Belt Geometry (driver ↔ driven #1)

Mode

Centre Distance (mm)

Belt Length (mm)

Result

Set up your driver and driven pulleys, then calculate to see RPMs, belt length, belt speed, wrap angle and the pulley diagram

Open-belt-drive geometry with kinematic (no-slip) speed relations. Real V/flat belts creep 1–2%; belt length should be matched to a standard belt size and tensioned per the manufacturer. Wrap-angle and speed limits shown are common rules of thumb.

Belt & Pulley Speed Calculator: Multi-Pulley RPM, Belt Length, Wrap Angle and Slip Check

Sizing a belt drive means more than just pulley speed – you also need the correct belt length for your centre distance (or the reverse, if you’re stuck with a fixed belt) and a check on wrap angle so the belt does not slip on the smaller pulley. This tool handles V-belt, flat belt and timing belt (teeth + pitch) drives, supports multiple driven pulleys off one driver and flags slip risk automatically when wrap angle or belt speed falls outside safe limits – all with a visual pulley diagram.

How to Use

Select belt type – V-belt, Flat belt or Timing belt (enter pitch in mm for timing belts).
Enter the driver pulley diameter (or teeth for timing belts) and driver RPM.
Click Add Driven Pulley for each pulley on the drive – supports multiple driven pulleys in one calculation.
Choose the geometry mode: “Centre distance → belt length” (forward) or “Belt length → centre distance” (reverse) and enter the known value.
Click Calculate Belt Drive to see driven RPM for each pulley, belt length or centre distance, belt speed vs. the safe limit, wrap angle and a slip-risk warning if applicable.

Key Features

Supports V-belt, flat belt and timing belt (teeth + pitch) drives in one tool
Multiple driven pulleys off a single driver, each with its own RPM and ratio
Reverse mode – solve for centre distance when you already know the belt length
Belt speed checked against common safe-speed limits (V-belt ~30 m/s, flat ~20 m/s, timing ~50 m/s)
Wrap angle calculation with automatic slip-risk warning below 120°
Proportional SVG pulley-and-belt diagram
Handles the “pulleys overlap / geometry impossible” edge case cleanly
Export as PDF or copy the result .

Formula / Logic Used

Driven Pulley Speed $RPM_{driven} = RPM_{driver} \times \frac{D_{driver}}{D_{driven}}$

For timing belts, pulley diameter is derived from teeth count first: $D = \dfrac{Teeth \times Pitch}{\pi}$

Open-Belt Length (Centre Distance → Length) $L = 2C + \frac{\pi}{2}(D_1 + D_2) + \frac{(D_2 – D_1)^2}{4C}$

Reverse Mode (Length → Centre Distance)

Solved from the quadratic: $8C^2 + C\left[2\pi(D_1+D_2) – 4L\right] + (D_2-D_1)^2 = 0$

Wrap Angle (on the smaller pulley) $\theta = 180 – 2\sin^{-1}\left(\frac{|D_2 – D_1|}{2C}\right)$

A wrap angle below 120° is flagged as a slip risk, especially for V-belts.

Belt Speed $v = \pi \times D_{driver}(m) \times \frac{RPM_{driver}}{60}$

This is checked against the safe speed limit for your selected belt type.

Who Should Use This Tool

Design engineers and technicians sizing belt drives for motors, conveyors and machine tools. Also useful for diploma and B.Tech Mechanical Engineering students learning belt drive kinematics, wrap angle and slip concepts.

Frequently Asked Questions (FAQs)

1. How do I calculate the RPM of a driven pulley?

Multiply the driver’s RPM by the ratio of driver diameter to driven diameter. This tool calculates it instantly for one or multiple driven pulleys off the same driver.

2. How do I find the belt length for a given centre distance?

Use the open-belt length formula, which combines twice the centre distance with a term for the pulley diameters and a small correction for the difference between them. This tool calculates it directly or works in reverse if you already know the belt length.

3. What is wrap angle and why does it matter?

Wrap angle is how much of the belt’s circumference actually contacts the smaller pulley – a wrap angle below about 120° increases the risk of the belt slipping, especially on V-belts. This tool calculates the exact wrap angle and flags it automatically if it’s too low.

4. What’s a safe belt speed for V-belt, flat belt and timing belt drives?

As common rules of thumb, V-belts are typically kept under about 30 m/s, flat belts under 20 m/s, and timing belts can run up to about 50 m/s. This tool checks your calculated belt speed against the correct limit for your selected belt type.

5. Can I calculate the centre distance if I already have a fixed-length belt?

Yes, switch to “Belt length → centre distance” mode, enter your belt length and the tool solves for the centre distance that fits, then checks the resulting wrap angle for slip risk.

Related Tools

Gear Ratio Calculator – for drive systems using gears instead of belts and pulleys
Shaft Diameter Calculator – size the shaft carrying torque from this belt drive

Belt & Pulley Speed Calculator: Length, Wrap Angle & Slip Check