Gear Inches Calculator (Cycling)

Gear Inches

Gear inches is the traditional way cyclists compare gearing across bikes with different wheel sizes, chainrings, and cogs. The number tells you the diameter (in inches) of a directly-driven wheel that would travel the same distance per crank revolution.

Formula

gear inches = (chainring teeth ÷ cog teeth) × wheel diameter (inches)

Higher gear inches = harder to push, faster top speed. Lower gear inches = easier to push, better for climbing and slow-speed control.

Worked Example: 50/12 with 700×25c Wheel

A 700c rim is ISO 622 mm and a 25 mm tyre adds 25 at the top and 25 at the bottom, so the wheel is 672 mm across. That is 26.5 inches.

• Gear inches = (50 / 12) × 26.5 = 110.4 in

That is a fast top gear suited to descending or pacelines.

Common Reference Ranges

Wheel Diameter Cheat Sheet

These are rim plus twice the tyre, which is the diameter you actually roll on. The name of the wheel is not the same thing: a 29er is 28.7 inches across with a 2.1 inch tyre, and a 27.5 is 27.2. Both names come from the era when tyres were fatter than the ones people fit now.

Why Not Just Use Ratio?

Two bikes with identical 50/12 ratios but different wheel sizes (a 26" mountain bike vs a 700c road bike) feel completely different. Gear inches normalizes that: a 100 gear-inch top is the same effort and speed regardless of wheel size.

Modern Alternative: Development

European cyclists often use development (meters per crank revolution) instead of gear inches. Development = π × wheel diameter (m) × ratio. Both convey the same information; gear inches remain dominant in U.S. and U.K. cycling culture.