Gear inches vs gain ratio and development
Three ways cyclists measure a gear: gear inches, metres of development and gain ratio. What each one means and when to reach for it.
Why one number beats a gear chart
A drivetrain has a lot of moving parts: chainrings of different sizes, a cassette with many cogs, and wheels that vary from bike to bike. Comparing two setups tooth by tooth is slow and error prone. Gear inches collapse all of that into a single figure, so 90 gear inches always means the same tallness of gear whether it comes from a 50x15 on a big wheel or a different combination on a smaller one. That is why racers and bike fitters lean on it.
How gear inches are calculated
The formula is simple: divide the chainring teeth by the cog teeth to get the ratio, then multiply by the wheel diameter in inches. A 50-tooth chainring with a 15-tooth cog gives a ratio of 3.33, and on a 27 inch wheel that is 90.0 gear inches. The name is historical: it is roughly the diameter of the single drive wheel a penny-farthing would need to travel the same distance per pedal turn.
Metres of development, the metric cousin
Gear inches answer how tall the gear is; development answers how far you actually roll. To get it, take the gear inches, multiply by pi to turn a diameter into a rolling circumference, then convert inches to metres. Ninety gear inches becomes about 7.18 metres of development per crank revolution. Riders who think in metric distance, or who want to reason about speed at a given cadence, often prefer this figure.
Gain ratio and its trade-off
A third measure, gain ratio, divides the rolling wheel radius by the crank length before applying the drivetrain ratio, so it factors in the leverage your legs feel. It is arguably the most complete measure of effort, but it needs your crank length and is less widely quoted, so shop talk and gear charts still default to gear inches. This calculator reports gear inches and development because they need only the parts you can read off the bike.