BMX Gear Calculator
Calculate your BMX gear ratio, gear inches, and rollout from chainring, sprocket, and wheel size.
By Jonas Lindemann · Updated · How we build our calculators
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Calculation Method
BMX Gear Calculator: Gear Inches and Rollout Explained
BMX gearing is described by three related numbers: gear ratio, gear inches, and rollout. This calculator works them all out from your chainring, sprocket, and wheel size so you can compare setups and find the feel you want.
Gear Ratio
The gear ratio is simply your chainring teeth divided by your sprocket teeth. A 25/9 setup is 25 ÷ 9 = 2.78:1, meaning the rear wheel turns 2.78 times for every full pedal revolution.
Gear Inches
Gear inches multiply the ratio by the nominal wheel diameter in inches. On a 20-inch wheel, a 25/9 gear is about 55 gear inches. The number lets you compare gearing across different wheel sizes on one scale.
Rollout
Rollout is the distance the bike travels in one crank revolution. It is the most practical figure for BMX because it tells you exactly how far each pedal stroke pushes you. Modern street setups (25/9, 28/10) sit near 55 gear inches because shorter chainstays and lighter parts became possible with smaller chainrings.
Frequently Asked Questions
Most modern street and park BMX bikes run a 25/9 setup (about 2.78:1, ~55 gear inches), which has become the standard since it allows short chainstays and lighter parts. Racing BMX uses larger combinations like 44/16 for higher top-end speed.
Gear inches equal your gear ratio multiplied by the wheel diameter in inches. A 25/9 gear on a 20-inch wheel is about 55 gear inches, meaning it rolls the same distance per pedal stroke as a direct-drive 55-inch wheel.
Rollout is the distance the bike travels in one full crank revolution. It is gear inches converted to a real-world distance and is the most direct way to compare how far different gear setups push you per pedal stroke.