Bike Gear Ratio & Cadence Calculator

Speed = cadence × (chainring teeth ÷ cog teeth) × wheel circumference. At 90 rpm in a 50×15 gear on a 700×25c wheel (2,105 mm), that is 90 × 3.33 × 2.105 m = 632 m per minute, or 37.9 km/h (23.5 mph).

Gear inches = wheel diameter in inches × chainring ÷ cog. It describes how hard a gear is to turn, and comes from the diameter of the equivalent front wheel on an old high-wheel bicycle.

Sheldon Brown’s gain ratio also allows for crank length: (wheel radius ÷ crank length) × chainring ÷ cog. It is the distance the bike travels for each unit of distance the pedal moves.

It depends on rim and tyre: about 2,105 mm for 700×25c and 2,136 mm for 700×28c (Sheldon Brown’s chart), and about 2,286 mm for a 29 × 2.1 in tyre (Wahoo’s chart). Charts differ by a few millimetres. For accuracy, roll the bike one full turn and measure.