Gear Inches Calculator
Express bicycle gearing as an equivalent direct-drive wheel diameter, useful for comparing setups across wheel sizes.
How to use this calculator
- Enter effective wheel-and-tire diameter rather than relying blindly on a nominal wheel label. Tire volume can noticeably change outside diameter.
- Enter the active front chainring and rear-cog tooth counts.
- Use gear inches to compare gearing across bicycles with different wheel sizes: lower values represent less distance and more leverage per crank turn; higher values represent more distance and less leverage.
- Compare the lowest gear inches when evaluating climbing range and the highest when evaluating high-speed cadence range.
- Treat gear inches as a gearing metric, not a prediction of how fast a rider can go. Speed still depends on cadence and available power.
Formula and assumptions
Gear inches = effective wheel diameter in inches × chainring teeth ÷ rear-cog teeth. It represents the diameter of an equivalent direct-drive wheel that would travel the same distance per crank revolution.
GIIRR calculates idealized geometry and kinematics. Friction, slip, compliance, tire deformation, manufacturing tolerances and control-system behavior can move real measurements away from the theoretical result.
Worked example
A 27.5-inch effective wheel with 42/21 gearing gives 27.5 × 42 ÷ 21 = 55 gear inches. The same 42/21 tooth combination on a 29-inch effective wheel gives 58 gear inches, showing why the same sprockets create slightly taller effective gearing on the larger wheel.
Frequently asked questions
Is nominal wheel size exact enough?
It is usually sufficient for rough comparison. Actual tire outside diameter gives a more precise number, especially when comparing very different tire widths or volumes.
What is a low gear-inch number?
There is no universal cutoff. Touring and off-road climbing setups often use much lower gear inches than road sprinting setups because terrain, load and cadence goals differ.
Why are gear inches useful if I already know the tooth ratio?
They include wheel diameter, which makes gearing comparisons across different wheel sizes more meaningful than tooth ratio alone.
Does tire pressure change gear inches?
Slightly. Tire deflection changes the effective rolling radius under load, although nominal calculations usually ignore that small change.
Can I convert gear inches directly to speed?
Not without cadence. Combine the gearing with crank RPM in the cadence-to-speed calculator to estimate theoretical road speed.
For safety-critical, legal or fitment decisions, verify the result against manufacturer documentation and physical measurements.
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