⚙️ 3-Pulley Belt Length Calculator
Calculate exact open or crossed belt length for any 3-pulley drive system using industry-standard geometry formulas
| Belt Section | Top Width (in) | Height (in) | Min Sheave Dia (in) | Max Speed (ft/min) | HP Range |
|---|---|---|---|---|---|
| A / 4L | 0.50 | 0.31 | 3.0 | 4,500 | Up to 5 HP |
| B / 5L | 0.66 | 0.41 | 5.4 | 4,500 | 1–10 HP |
| C / 6L | 0.88 | 0.53 | 7.0 | 4,500 | 5–25 HP |
| D / 8L | 1.25 | 0.75 | 13.0 | 3,800 | 15–75 HP |
| E | 1.50 | 0.91 | 21.6 | 3,500 | 40–150+ HP |
| 3L (Fractional) | 0.38 | 0.23 | 2.2 | 5,000 | Up to 2 HP |
| 5L (Heavy Fract.) | 0.63 | 0.38 | 3.5 | 5,000 | 1–8 HP |
| Pulley Dia (in) | 1200 RPM | 1750 RPM | 3450 RPM | 5000 RPM | Max RPM (A-Belt) |
|---|---|---|---|---|---|
| 2 in | 628 ft/min | 916 ft/min | 1,806 ft/min | 2,618 ft/min | 8,594 |
| 3 in | 942 ft/min | 1,374 ft/min | 2,709 ft/min | 3,927 ft/min | 5,730 |
| 4 in | 1,257 ft/min | 1,833 ft/min | 3,613 ft/min | 5,236 ft/min | 4,297 |
| 6 in | 1,885 ft/min | 2,749 ft/min | 5,420 ft/min | 7,854 ft/min | 2,865 |
| 8 in | 2,513 ft/min | 3,665 ft/min | 7,226 ft/min | — | 2,149 |
| 12 in | 3,770 ft/min | 5,498 ft/min | — | — | 1,432 |
| Application | Driver Dia | Driven 1 Dia | Idler Dia | Typical C12 (in) | Belt Section |
|---|---|---|---|---|---|
| Lathe 3-Step Cone | 6 in | 4 in | 3 in | 12 | A or B |
| Bandsaw Drive | 5 in | 14 in | 4 in | 18 | B or C |
| Air Compressor | 8 in | 12 in | 3 in | 20 | B or C |
| Drill Press | 5 in | 3 in | 2 in | 8 | A or 3L |
| Conveyor System | 10 in | 10 in | 6 in | 36 | C or D |
| HVAC Fan | 6 in | 12 in | 4 in | 24 | A or B |
| Wood Splitter | 4 in | 10 in | 3 in | 16 | B or C |
| Belt Number | Outside Length (in) | Inside Length (in) | Section | Metric Equiv (mm) |
|---|---|---|---|---|
| A24 / 4L260 | 26.0 | 24.0 | A | 660 mm |
| A30 / 4L320 | 32.0 | 30.0 | A | 813 mm |
| A40 / 4L420 | 42.0 | 40.0 | A | 1067 mm |
| B35 / 5L380 | 38.0 | 35.0 | B | 965 mm |
| B48 / 5L510 | 51.0 | 48.0 | B | 1295 mm |
| B60 / 5L630 | 63.0 | 60.0 | B | 1600 mm |
| C51 | 54.0 | 51.0 | C | 1372 mm |
| C75 | 78.0 | 75.0 | C | 1905 mm |
Find the right length of Belt for Pulley does not need to be really hard. It simply comes down to some steps and a bit of basic math. Calculator for Belt length remove the need of wild guesses, you enter the diameters of all Pulley and the distance between them, and the program does the hard part instead of you.
Just do careful measure and input the numbers is quite a lot actual possibility.
How to Find the Right Belt Length
Here are formula who simplifies the task. For V-shape or flat Belts, it looks like this: L = 2C + [π(D2 + D1)]/2 + [(D2… D1)²]/4C.
This equation counts the needed Belt length for passing force between two Pulley. To use it, you need two main data: the distance between the centres of the two shafts and the named diameters of every Pulley. The named diameter measures from the centre of the Belt to the centre of the Pulley.
It stores basic diameter plus the width of the Belt, because that sits in the groove.
Sometimes the code of the Belt itself points the length clearly. For instance “4L360” shows that the Belt has 36 inches of length. The number after the prefix gives the size.
That really is a handy way to escape much trouble.
Not each enjoys to do math, and that is fair. One of the simplest ways is simply wrap piece of string or thread around the two Pulley and measure the result. It works well especially if the Pulley have same size, string or flexible tape around them helps to measure the needed length without numbers.
In other words, wrap elastic tube around the Pulley, then measure it. If needed, one can use long string, marking with thick pencil where the too finishes touch.
Here the point about units; they do not matter, if you stay with the same system through the whole process. Assume, that your Pulley diameters are 15 and 30 centimetres, with 1.5 metre between them. Simply use only centimetres or metres for everything, and you will be in order.
Old Belt, that stays almost good, can serve well as aid. Take off the beams, cut them straight so that they no more form loop, and you have handy samples. Bring straight piece to the store and compare it with the available to make everything easy.
Most stores keep V-Belts in two beams, with lengths that change in half-inch steps. Do well note, that the Belt length measure from the inside of the Belt, not from the outside. Many Belts have smallest need for Pulley diameter, so check that before buying to escape problemslater.
Adding idler Pulley settles two issues at once, it removes possible looseness and gives the Belt bigger contact with the main Pulley. That creates good help when the Belt length not exactly match.
