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Gear ratio calculator

Gives ratio, gear inches, development and the speed a given cadence produces — the three numbers cyclists use interchangeably and which mean different things.

The question this answers: “Is this gear tall enough for the flat, and low enough for that climb?”

Measure it: mark the tyre, roll one revolution with your weight on the bike.

Result

Gear ratio
2.94:1
Chainring ÷ sprocket. Ignores wheel size, so it cannot compare across wheel sizes.
Gear inches
77.6"
The penny-farthing wheel diameter that would give the same gearing. The usual comparison figure.
Development
6.19m
Distance travelled per pedal revolution.
Speed at 85rpm
31.6 km/h
Arithmetic, not a prediction of what you can sustain.

Worth knowing about this answer

  • Circumference is the variable people get wrong. It depends on the tyre, not just the rim, and the difference between a 25mm and a 32mm tyre is real. Measure by marking the tyre and rolling one revolution with your weight on the bike rather than using a table.
Show the working
  1. Ratio50 ÷ 172.941
  2. Wheel diameter2105mm ÷ π ÷ 25.426.38"
  3. Gear inches2.941 × 26.3877.6"
  4. Development2.941 × 2105mm6.19m
  5. Speed6.19m × 85 × 6031.6 km/h

Gear ratio: 2.94:1. Gear inches: 77.6". Development: 6.19m. Speed at 85rpm: 31.6 km/h

How it works

Ratio is chainring over sprocket. Gear inches multiplies that by the wheel diameter derived from circumference. Development multiplies the ratio by circumference. Speed is development times cadence.

What it assumes

  • Wheel circumference includes the tyre and is measured under load, which is not the same as a published rim figure.
  • A perfectly circular, non-deforming wheel and no drivetrain loss. Real tyres squash and chains lose a few per cent to friction; neither is worth modelling for a comparison between gears, where both apply equally.

Where it stops being reliable

  • The speed figure is arithmetic. It says what that cadence in that gear produces, not what you can sustain, and it ignores wind, gradient and drivetrain losses.
  • It compares one gear at a time, not a whole transmission. Whether a groupset has usable steps between gears, or a gap that leaves you between two cadences on a climb, is a question about the whole cassette that a single ratio cannot answer.
  • Nothing here says whether a combination is mechanically sensible. Cross-chaining, chain length and rear derailleur capacity all constrain which pairs you should actually use, and the arithmetic works just as happily on ones you should not.

Runs entirely in your browser — nothing you type is sent anywhere. Last reviewed 2026-08-14. More on Cycling.