Practical tools for measured work
V-Belt Length & Pulley Center Distance Calculator
Calculate V-belt pitch length from pulley diameters and center distance, or solve center distance from a selected belt length.
Use the effective pitch diameter, not the outside flange diameter.
Use the effective pitch diameter, not the outside flange diameter.
Enter center distance in length mode, or the selected belt pitch length in center mode.
Results describe an open belt at the pitch line. Confirm pulley pitch diameters and the selected belt's published pitch length before setting the drive.
How it works
Direct answer. Enter both pulley pitch diameters and either the shaft center distance or a belt pitch length. The calculator returns the corresponding open-belt geometry, pulley ratio and belt wrap.
Method. Pitch length is approximated by L = 2C + pi(D + d) / 2 + (D - d)^2 / (4C), where C is center distance and D and d are the large and small pitch diameters. Center distance is found by repeatedly bisecting the valid range until that same equation matches the entered pitch length.
Worked example. With 100 mm and 250 mm pitch diameters at 400 mm centers, the estimated pitch length is about 1,363.84 mm. The diameter ratio is 2.5:1 and the smaller pulley has about 158.4 degrees of wrap.
Inputs. Pitch diameter is the belt's effective running diameter. It is not necessarily the pulley's outside diameter. For inch inputs, all calculations are converted to millimetres internally and converted back for display.
Current-input example
The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.
- Solve for
- Belt pitch length
- Input units
- Millimetres
- Small pulley pitch diameter
- 100
- Large pulley pitch diameter
- 250
- Center distance / belt pitch length
- 400
Primary result: Belt pitch length: 1363.84 mm.
Before using the result
- Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
- Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
- When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.
Limitations
This is a geometric planning calculator for an open, uncrossed two-pulley drive. It does not select a belt section, standard catalog length, tension, power rating, service factor, groove count or take-up allowance. Real drives require the pulley and belt manufacturer's pitch-diameter, installation and rating data.
Frequently asked questions
Why does outside pulley diameter give a different answer?
The belt's neutral or pitch line runs below the outside edge. Use the effective pitch diameter provided for the pulley/belt system.
Why can an entered belt length be invalid?
The requested pitch length must be long enough to pass around both pulleys while keeping their pitch circles geometrically separate.
Does the ratio include belt slip?
No. The displayed diameter ratio is the ideal geometric speed ratio. Actual speed can differ because of belt slip and operating deflection.
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