BMX Gear Ratio Calculator

This page turns a BMX sprocket pair into the numbers riders actually compare: roll-out in inches per crank revolution, gain ratio, gear inches and the plain ratio. It uses a real wheel diameter from the rim standard and the tyre width rather than a nominal twenty inches, and it takes the crank length, which is the only thing that separates gain ratio from gear inches. A second action puts two setups side by side and gives the change in roll-out, the speed at a cadence and the cadence needed to match. The wheel diameter is the weak link, so a measured roll-out can be entered and will be used instead of the estimate.

Calculator Type
Roll-Out Measured In
Only affects the measured roll-out box above. Tyre width and crank length are stamped on the part in inches and millimetres wherever you are.

Sprockets alone give the ratio. Adding the real wheel and the crank length turns that into roll-out and gain ratio, which are the numbers that say how the bike rides.

Teeth on the front sprocket. BMX runs 23 to 44; 25 and 28 are the common ones.
Teeth on the rear driver or cog. BMX cassette drivers are usually 8, 9 or 10.
From the sidewall: 20 x 2.125 means 2.125 here. A tyre is taken as tall as it is wide, which is close enough for a BMX tyre on a matching rim.
Stamped on the back of the arm. BMX runs 155 to 180; 175 is standard on a 20 inch bike.
Mark the tyre, sit on the bike, roll it one full wheel turn and measure the ground. If you give this, it is used instead of the rim and tyre estimate.
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Two sprocket pairs on the same bike. You get the change in roll-out and gain ratio, and what each does to speed and cadence.

Teeth on the front sprocket. BMX runs 23 to 44; 25 and 28 are the common ones.
Teeth on the rear driver or cog. BMX cassette drivers are usually 8, 9 or 10.
Teeth on the front sprocket. BMX runs 23 to 44; 25 and 28 are the common ones.
Teeth on the rear driver or cog. BMX cassette drivers are usually 8, 9 or 10.
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