How it works
gear inches = (chainring teeth ÷ cog teeth) × wheel diameter (in)
Gear inches dates back to penny-farthings: it is the diameter of the equivalent direct-drive wheel you would need to roll the same distance per pedal turn. Multiply the gear ratio (chainring teeth divided by rear cog teeth) by the driven wheel’s outer diameter in inches and you get gear inches — typically 30″ for a granny-gear climb up to 120″+ for a sprinter’s top gear. Development is the real-world roll-out: gear inches × π converts the equivalent wheel’s circumference into metres travelled per crank revolution. Gain ratio, introduced by Sheldon Brown, divides the wheel radius by the crank-arm length before applying the gear ratio, so it accounts for leverage at the pedals and is dimensionless — letting you compare bikes with different crank lengths fairly. A 700×25c road wheel is about 26.4 inches in outer diameter, which this tool uses as the default.
Sources
- Gear inches and gain ratio Sheldon Brown — Bicycle Gearing: gear inches = (chainring ÷ cog) × wheel diameter; gain ratio uses wheel radius ÷ crank length. sheldonbrown.com/gears
- Development (roll-out per pedal stroke) Metres of development = gear inches × π × 0.0254 — the distance the bike travels for one full crank revolution.
- 700c wheel diameter A 700×25c tyre on an ISO 622 rim has an outer diameter of roughly 670 mm ≈ 26.4 inches, the standard road-bike reference.
FAQ
What are gear inches?
Gear inches express how far a gear pushes you per pedal stroke as the diameter of an equivalent direct-drive wheel. A low gear (around 30″) spins up easily for climbing; a tall gear (100″+) covers more ground per stroke for high speed.
How do I find my wheel diameter?
Use the outer diameter including the tyre. A 700×25c road wheel is about 26.4 inches; 700×28c is roughly 26.5; a 26-inch mountain-bike wheel is about 26.0. You can also measure the roll-out and divide by π, or enter the diameter directly in inches or millimetres.
What is development?
Development is the distance the bike actually travels for one complete turn of the cranks. It equals gear inches × π converted to metres. A 70″ gear rolls out about 5.6 metres per pedal stroke.
What is gain ratio and why use it?
Gain ratio, devised by Sheldon Brown, factors in crank-arm length as well as wheel size, so it reflects the leverage you actually feel at the pedals. It is dimensionless, which makes it the fairest way to compare bikes with different crank lengths.
How do I compare gears across bikes?
Convert each gear to gear inches or gain ratio and compare the numbers directly — they normalise away differences in chainring, cog and wheel size so a 50×17 on one bike can be lined up against a 34×11 on another.
Does this work for a single-speed or fixed gear?
Yes. Enter your single chainring and cog and your wheel size to get the one gear inches figure — handy for choosing a fixie ratio that suits your terrain and preferred cadence.
Gear inches and development assume the listed wheel diameter; real tyre size, pressure and wear shift the rolling diameter slightly. Gain ratio uses a 172.5 mm crank length — adjust your expectations if your cranks differ. General information for setup and comparison, not a fit or safety recommendation.