Practical tools for measured work
Bicycle and BMX gearing calculator
Compare chainrings and sprockets using measured tyre rollout, cadence, gear inches and gain ratio.
Calculate the layout, check the diagram, then print the result for the shop or jobsite.
Separate tooth counts with commas; up to four chainrings.
Separate tooth counts with commas; up to sixteen sprockets.
Bicycle gear comparison
| Chainring | Sprocket | Ratio | m/rev | Gear inches | km/h |
|---|---|---|---|---|---|
| 50 | 11 | 4.545454545 | 9.545454545 | 119.6225413 | 51.54545455 |
| 50 | 13 | 3.846153846 | 8.076923077 | 101.2190734 | 43.61538462 |
| 50 | 15 | 3.333333333 | 7 | 87.72319698 | 37.8 |
| 50 | 17 | 2.941176471 | 6.176470588 | 77.40282086 | 33.35294118 |
| 50 | 19 | 2.631578947 | 5.526315789 | 69.25515551 | 29.84210526 |
| 50 | 21 | 2.380952381 | 5 | 62.65942641 | 27 |
| 50 | 25 | 2 | 4.2 | 52.63391819 | 22.68 |
| 50 | 28 | 1.785714286 | 3.75 | 46.99456981 | 20.25 |
| 34 | 11 | 3.090909091 | 6.490909091 | 81.34332811 | 35.05090909 |
| 34 | 13 | 2.615384615 | 5.492307692 | 68.82896994 | 29.65846154 |
| 34 | 15 | 2.266666667 | 4.76 | 59.65177395 | 25.704 |
| 34 | 17 | 2 | 4.2 | 52.63391819 | 22.68 |
| 34 | 19 | 1.789473684 | 3.757894737 | 47.09350575 | 20.29263158 |
| 34 | 21 | 1.619047619 | 3.4 | 42.60840996 | 18.36 |
| 34 | 25 | 1.36 | 2.856 | 35.79106437 | 15.4224 |
| 34 | 28 | 1.214285714 | 2.55 | 31.95630747 | 13.77 |
How it works
Bicycle ratio = chainring teeth ÷ rear sprocket teeth. Development = tyre rollout × ratio. Speed = development × cadence. Gear inches use the effective wheel diameter in inches; gain ratio also accounts for crank length. A 50/25 gear with 2100 mm rollout at 90 rpm gives 22.68 km/h. For BMX, a true 20-inch wheel with 44/16 gearing gives 55 gear inches.
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.
- Chainring teeth
- 50
- Rear sprocket teeth
- 25
- Measured tyre rollout
- 2100 mm
- Pedal cadence
- 90 rpm
- Crank length
- 175 mm
- Chart chainrings
- 50,34
- Chart rear sprockets
- 11,13,15,17,19,21,25,28
Primary result: Chainring-to-sprocket ratio: 2.
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
Measure one loaded wheel revolution for tyre rollout; nominal wheel labels are not exact outside diameters. The comparison table does not check chainline, derailleur capacity or component compatibility. Internal hub ratios, wheel slip, hills, wind and rider power are excluded.
Frequently asked questions
What should I verify before using this result?
Recheck the physical measurements, confirm that every input uses the displayed unit, and compare the result with the applicable drawing, product specification, code requirement or test piece.
Why might the finished result differ?
Real work can include thickness, kerf, seams, tolerances, hardware allowances, installation clearances or site conditions that are outside the calculation. The limitations section identifies the main exclusions for this tool.
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