Pulley Ratio Calculator

When to use this CAD/mechanical design tool

• Use during belt-drive concept layout.
• Use before checking belt length and wrap.
• Use when replacing or resizing pulleys for a target speed.

How to use

1. Enter one mechanical-design case using typed dimensions, counts, RPM values, tolerances, or drafting selections.
2. Click the exact action button for the selected tool; default values, typing, URL-prefilled values, and worked examples do not generate results automatically.
3. Review the output cards, warnings, assumptions, visual summary, copy text, and downloads before using the result in CAD, drawing, supplier, or inspection workflow.
4. Verify final values against applicable standards, supplier catalogs, validated CAD documentation, manufacturing limits, inspection practice, and qualified engineering review.

Input guide

Pulley diameters

Effective or pitch diameters for the driver and driven pulleys.

Driver and driven RPM

Use the values required by the selected calculation mode.

Calculation mode

Choose driven RPM, driver RPM, target driven diameter, or ratio-only review.

Notation

Output interpretation

• Ratio is diameter based.
• Driven speed assumes no slip.
• Target diameter should be rounded to available pulley standards.

Supported CAD/mechanical design use cases

• simple two-pulley drives
• ratio-only checks
• target driven diameter planning

Unsupported patterns

• belt slip
• belt tension
• multi-pulley serpentine systems
• variable-speed drives
• power rating

How to interpret the output

• A larger driven pulley reduces driven speed.
• A smaller driven pulley increases driven speed.
• Use effective pulley diameter, not arbitrary outer diameter, when supplier data gives pitch diameter.

CAD and mechanical-design guidance

• Check belt length after choosing diameters.
• Confirm pulley availability and shaft/bore constraints.
• Verify torque, belt rating, guarding, and speed limits.

Worked examples

Assumptions

• Inputs are typed planning values and are not extracted from CAD files, supplier catalogs, drawings, PLM, CAM, ERP, or inspection systems.
• Metric calculations use millimetres unless the output explicitly states another unit; inch calculations use inches unless the output explicitly states another unit.
• The result is a preliminary calculator or drafting helper and not a certified standards database, CAD kernel, inspection report, or supplier approval.
• No slip and no belt stretch are assumed.
• Pulley diameters are effective pitch diameters for speed relationship.

Practical guidance

• Use Belt Length Calculator after diameter selection.
• Use Shaft Key Calculator for shaft torque-transfer review.

Common mistakes

• Reversing driver and driven diameter.
• Using outside diameter where pitch diameter is required.
• Ignoring available standard pulley sizes.

Limitations

• No belt rating, tension, slip, stretch, groove profile, tooth pitch, vibration, or guarding calculation is included.
• ScholarTool CAD and mechanical-design calculators provide general engineering calculation and planning support. They do not replace supplier requirements, certified standards, CAD software documentation, manufacturing review, inspection standards, or professional engineering judgement.
• The tool does not open, upload, parse, repair, or validate CAD files, drawings, STEP, STL, DXF, DWG, CAM, or CNC data.
• The tool does not perform strength certification, fatigue validation, finite-element analysis, supplier catalog selection, standards compliance certification, or inspection programming.

Related workflow links