Belt Length & Ratio Calculator

Belt length calculator: work through an ideal two-pulley open-belt drive using your reference diameters. Enter the driver and driven reference diameters, then calculate belt length from center distance or solve center distance from a known belt length. Use millimeters or inches and add driver rpm when you want speed results.

CALCULATE THE DRIVE
• See exact geometric and approximate open-belt lengths, with their difference clearly shown.
• Find center distance using your choice of the exact or approximate model.
• Read the output/input speed ratio with the driver and driven roles labeled.
• Calculate ideal driven rpm and belt speed in meters per second or feet per minute.
• Leave rpm blank when you only need geometry and ratio.
• Review the formulas, reference-diameter assumptions and input limits in the app.

CORE CALCULATIONS ARE FREE
Forward belt length, reverse center-distance solving, ratio, ideal speed, unit switching and formula reference are included without a purchase. Calculations work offline. Choose a light, dark or system appearance.

KEEP A MACHINE NOTEBOOK WITH PRO
The optional one-time Lifetime Pro purchase adds new named machine records and notes, calculation history, two-machine comparison, and calculation reports. Copy or share a text report, save a CSV or JSON result report, or export a comparison as CSV. CSV reports use millimeters and meters per second.

Existing saved work remains available without Pro: open records, edit saved machines and their notes, or export and import recovery JSON. Creating new saved machines and history, making comparisons, and generating reports require Pro. Recovery files are separate from result reports and do not transfer purchase ownership.

LOCAL RECORDS, EXPLICIT EXPORTS
No app account is required. Measurements, drafts, saved records, notes and preferences are stored on your device. The app has no ads or first-party analytics. You choose when to copy, share or save a file, and where it goes. Google Play services handle the optional purchase and ownership checks; those services may need internet access. Export a recovery file before uninstalling or clearing Android app storage.

KNOW THE MODEL
Results describe two separate, uncrossed pulleys in the same plane. Use consistent effective or pitch reference diameters and a belt length on that same reference line. “Exact” means exact geometry within this ideal model, not a physical-fit guarantee.

The calculation does not model slip, stretch, belt thickness, tension, load or power. It does not select a stock belt, convert catalog dimensions, assess guarding, or guarantee safety or manufacturer suitability. Check the applicable equipment and belt specifications before acting on a result.

Support: contact@patch-media.com

For workshop students considering Pulley & Belt Calculator, use this open-belt length worksheet to explore an ideal drive in a training exercise. Treat the calculated pulley speed as discussion material for a bench sketch, keeping physical-fit and equipment decisions outside the arithmetic.

Recent changes:
Added German and French interfaces with a language selector. Existing calculations, machine data and file formats are preserved. Localization and compatibility checks improved.