⚙️ Bike Gear Inches Calculator

Enter your wheel diameter, chainring teeth, and rear cog teeth to calculate gear inches — a standard measure for comparing bicycle gear ratios across different bikes and wheel sizes.

Wheel Diameter (inches)

Chainring Teeth

Rear Cog Teeth

Gear inches and development

Gear inches is a Victorian idea: the wheel diameter a direct-drive penny-farthing would need to match your geared bike. Development is the same figure as ground covered per pedal stroke:

gear inches = wheel diameter × (chainring teeth / cog teeth)
development (m) = gear inches × π × 0.0254

A 50T ring with a 14T cog is a ratio of 3.57:1, and on a 27" wheel that is 27 × 3.57 = 96.4 gear inches. Multiply by π and convert to metres and each turn of the cranks carries you 7.69 m - so 90 rpm in that gear is 41.5 km/h. Drop to a 34T ring and a 28T sprocket and the same wheel gives 32.8 gear inches, a climbing gear covering 2.62 m per revolution.

Comparing bikes fairly

Ratio alone is not enough. 2:1 on a 26" mountain wheel is 52 gear inches; the same 2:1 on a 29er is 58. That is why gear inches exists.
Road riders live between about 30 and 110 gear inches; touring setups reach near 20.
Check the jumps, not just the ends. Neighbouring sprockets more than about 8 gear inches apart feel like a gap.

The wheel sizes in the dropdown are nominal. A 700c road wheel with a 25 mm tyre actually rolls at about 26.3 inches, not 27, so every figure here reads roughly 2-3% high; that is fine for comparing gears and wrong for calibrating a speedometer. Crank length is not an input either, so this cannot give Sheldon Brown's gain ratio - a rider on 165 mm cranks gets more leverage per gear inch than one on 175 mm.

Frequently asked questions

What are gear inches on a bike?

A single number combining wheel size and gear ratio, so a gear on one bike compares with a gear on another. Higher is harder to push.

What is a good low gear for steep climbs?

Around 30 gear inches for most riders, and closer to 20 if you are loaded or the gradient is above 15%. A 34T ring with a 32T sprocket on 700c is 28.7.

How do I work out speed from gear inches?

Multiply development by cadence. 7.69 m per revolution at 90 rpm is 692 m a minute, which is 41.5 km/h or 25.8 mph.