A parent-friendly BMX gear calculator — gear ratio, gear inches, and rollout for kids ages 5–12. Built to use at the track on a phone. Starting points, not final setups.
Three common questions for BMX families: what gear ratio is my rider actually running, does changing a chainring or cog make the bike easier or harder to pedal, and what combination is close to a target rollout. Pick a tool below, plug numbers in, read the plain-language interpretation.
Records are saved in this browser only — not synced to your child's profile. Sign in to save setups to your child's profile
Plug in a chainring, rear cog, and tire/wheel. Get the gear numbers and a plain-language read.
Tap a class to load a common starting point. Then fine-tune with chainring, cog, and tire below.
Good for most 20" BMX Racing estimates. For better accuracy, choose a specific tire model or enter a custom measurement.
Most riders in this class start around this rollout range. Fine-tune by rider strength, track, cadence, and coach feedback.
Right on the historical 44/16 baseline. Not universally best — just a familiar reference.
Send a coach or family member this setup — no account needed.
Snapshot the current gear with date and an optional label.
Pick a rider and a bike to save this setup as a snapshot.
Current setup vs. a new setup. See whether the change is lighter, heavier, or too small to feel.
Good for most 20" BMX Racing estimates. For better accuracy, choose a specific tire model or enter a custom measurement.
Easier out of the gate, quicker to accelerate, may spin out at top speed sooner. Often a good first adjustment for young or new riders.
Pick a target gear inches or rollout. Get the closest chainring / cog combinations within a range.
Good for most 20" BMX Racing estimates. For better accuracy, choose a specific tire model or enter a custom measurement.
Rollout is how far a BMX bike moves in one full pedal stroke (one full crank turn), from a dead stop or at any speed. It accounts for chainring, rear cog, and real wheel/tire diameter — so two bikes with the same gear ratio can have different rollouts if the tires are different. BMX coaches and race families use rollout to compare setups in a language that's independent of specific chainring/cog numbers.
A BMX gear calculator converts your chainring and rear cog into gear ratio, gear inches, and rollout — numbers that describe how heavy or light the gear feels. It lets parents compare setups (the bike you have vs. a setup someone recommends), find combinations close to a target rollout, and understand why the same gear feels different with different tires. It is a starting point — real race setup also depends on rider strength, crank length, tire model, and coaching.
Real BMX setup depends on rider size, strength, crank length, cadence, tire pressure, tire model, rim width, track layout, coaching, and riding style. Use this as a reasonable place to begin the conversation — not a recommendation for your specific child.
Short answers in plain language.
Gear ratio is front chainring teeth divided by rear cog teeth — e.g., 44 ÷ 16 = 2.75. Higher = harder to pedal but more top speed; lower = easier to pedal but less top speed.
Gear inches is gear ratio × effective wheel diameter. It describes the effective size of the 'drive wheel' — a cross-bike comparison that folds wheel size into the number. Useful when comparing a 20" setup to a 24" setup.
Rollout is how far the bike moves in one pedal stroke — gear ratio × wheel circumference. Coaches like it because it describes the actual distance the bike covers per crank revolution on the rider's own tire.
Not automatically. Heavier gears give more top speed but make gate starts harder and can bog down young riders. Most kids race better on a gear that lets them spin cleanly — confidence and cadence beat raw leverage.
Tire size changes the effective wheel diameter, which changes gear inches and rollout even if chainring and cog numbers stay the same. A larger-diameter tire = slightly heavier gear, a smaller one = slightly lighter. It's a small but real effect.
Yes. Fit Lab is a technical starting point; Bike Service & Fit Check handles the in-person fit, gear, and race-prep tune. Contact us to book a session before your next event.
