Bicycle Gear Calculator

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Wheel Circumference

This is the outer circumference of the tyre when fitted to the wheel. You can select from the options provided, or measure by marking one line in the ground and the tyre where it touches the ground. Roll the bike until the mark on the tyre touches the ground again. Draw another line in the ground matching the mark on the tyre and measure the distance (in millimetres) between the two marks on the ground.

Front Gears

These are the gears attached to the crankshafts. Some standard gears are shown. Rarely do stock configurations deviate from the 'standard' settings shown.

Rear Gears

These are the gears attached to the rear wheel. Some standard 9/10/11 speed configurations are listed, based on information from the internet. Your ratios may be different, you can find out by looking at the gears themselves, often the ratios are stamped into individual cogs.

Crank Length

This is only used to calculate gain ratio. Most configurations are 170mm, smaller frame sizes may use 165mm, larger frame sizes may use 175mm. It is the distance from the middle of the bottom bracket to the middle of the pedal shaft. Most have this figure stamped on the inside facing side of the crank.

Desired RPM

This is the cadence or turns per minute that (you hope) your legs will make most of the time.

Gain Ratio

This is a gauge of how much effort is required to turn the pedals. It is a ratio of the distance that the pedals must travel in one revolution relative to the distance the wheels will travel.

Gear Inches

An old fashioned measurement of how many inches the wheels will turn in one revolution.


All figures are in kilometres per hour (km/h)