Belt Length Calculator
This belt length calculator finds the length of a V-belt or timing belt from the two pulley diameters and the center distance. Enter them and the belt length calculator returns the belt length in millimeters and inches. As a drive belt length calculator it covers V-belts, timing belts, and any two pulley drive.
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The Belt Length Calculator is part of our Machinery collection. Browse more machinery to size, convert, and check your work alongside this tool.
What the belt length calculator shows
A belt wraps two pulleys and spans the gap between them, so its length depends on both diameters and the distance apart. This belt length calculator adds the straight runs and the curved wraps to give the total belt length. Whether you run it as a V belt length calculator, a timing belt length calculator, or a pulley and belt calculator, it sizes the belt for a two pulley drive.
How the drive belt length calculator works
The drive belt length calculator adds twice the center distance, the average wrap, and a correction for unequal pulleys:
C is the center distance, D and d are the large and small diameters. The first term is the two straight runs, the second is the belt wrapping half of each pulley, and the last corrects for the diameters differing. This is the standard two pulley belt length formula.
Belt length calculator example
Pulleys of 100 mm and 50 mm with a 500 mm center distance. The belt length calculator gives:
So the belt is about 1237 mm long. Change the center distance and the belt length calculator updates the length at once.
How to use the belt length calculator
Enter the small pulley diameter, the large pulley diameter, and the center distance in millimeters, then the belt length calculator returns the belt length in millimeters and inches. Measure the pulleys at their pitch diameter, where the belt sits, for the most accurate result. As a belt size calculator it gives the length you match to a stock belt.
Reading the belt size calculator result
The belt length is the size to match to a stock belt; V-belts and timing belts come in standard lengths, so pick the nearest and set the center distance to suit. Most drives have an adjustable center distance or a tensioner to take up the small difference and to allow for stretch and wear. For a timing belt, the length must match a whole number of teeth, so a timing belt calculator rounds to the nearest standard tooth count. Measure at the pitch diameter, not the outer edge, for the belt size calculator to be accurate.
When to use the pulley and belt calculator
- Sizing a V-belt with the pulley and belt calculator
- Finding a timing belt length with a timing belt length calculator
- Setting the center distance with a pulley center distance calculator
- Matching a replacement belt to a stock size
Common belt length calculator mistakes
- Using the outer diameter. Measure the pitch diameter where the belt rides, not the outer edge.
- Ignoring tooth count for timing belts. A timing belt must have a whole number of teeth, so round to a standard length.
- No tension adjustment. Leave an adjustable center or a tensioner for the gap to a stock belt.
Tips for belt sizing
- Measure pulleys at the pitch diameter.
- Match to the nearest stock belt and adjust the center distance.
- Keep enough wrap on the small pulley to avoid slip.
V belt and timing belt length calculator
A V belt calculator and a timing belt length calculator use the same geometry but round differently at the end. A V belt grips by wedging into the pulley groove, so its length can be matched to the nearest stock size and the center distance adjusted to suit. A timing belt has teeth that must mesh with the pulley teeth, so a timing belt calculator has to land on a length that is a whole number of teeth at the belt pitch, such as 2 mm for a GT2 belt. The formula gives the raw length either way; the difference is only in how you round it. For both, an adjustable center distance or a tensioner absorbs the small gap between the calculated length and the nearest standard belt.
Pulley belt calculator and center distance
A pulley belt calculator and a pulley center distance calculator are two views of the same drive. Given the pulleys and the center distance, you get the belt length; given the pulleys and a chosen stock belt, you can work backward to the center distance that fits it, which is what a tensioner or slotted motor mount then fine tunes. The wrap on the smaller pulley shrinks as the diameters differ more or the center distance closes, and too little wrap makes a V belt slip, so keep the pulleys reasonably far apart. The two pulley sizes also set the speed ratio, which the gear ratio calculator handles the same way gears do.
V belt length calculator reference
Belt length by center distance for 100 mm and 50 mm pulleys, from the V belt length calculator:
| Center distance | Belt length (mm) | Belt length (in) |
|---|---|---|
| 300 mm | 837.6 mm | 32.98 in |
| 400 mm | 1037.2 mm | 40.83 in |
| 500 mm | 1236.9 mm | 48.70 in |
| 600 mm | 1436.7 mm | 56.56 in |
Belt Length Calculator FAQ
What is the belt length formula?
L = 2C + pi (D + d) / 2 + (D - d) squared / (4C), where C is the center distance and D and d are the pulley diameters. The belt length calculator applies it.
Should I measure the pulley pitch or outer diameter?
Use the pitch diameter, where the belt rides. Measuring the outer edge overstates the length slightly, so a belt size calculator wants the pitch diameter.
How do I size a timing belt?
A timing belt must have a whole number of teeth, so a timing belt calculator rounds the length to the nearest standard tooth count and pitch.
Does the center distance need to be exact?
No. Most drives have an adjustable center or a tensioner to take up the difference to a stock belt and to allow for stretch.