Ask what a gear "is" and you get a ratio — 2.9:1. Ask whether that is good and the ratio alone fails: a 2.9 on a 29er differs from a 2.9 on a folding 16-incher. Gear inches solved this in the 1890s by comparing every gear to a penny-farthing wheel, and the standard still wins arguments because it is wheel-agnostic.
The definition
Gear inches = (chainring ÷ cog) × wheel diameter (inches)
It answers: how big would the front wheel of a direct-drive penny-farthing need to be, to travel this far per pedal turn? A 46/16 on 700c is 77.5 inches — as if you pedaled a penny-farthing with a 77.5-inch wheel once per revolution. High number = harder gear = more distance per pedal.
The reference table
| Gear inches | Feels like | Typical use |
|---|---|---|
| 20–30 | Spinning up walls | MTB climbing bailouts, loaded touring |
| 40–55 | Comfortable hills | All-round riding |
| 55–75 | Fast flats | Commuting, fixed-gear street |
| 75–100 | Power flats, downhills | Road racing |
| 100–120 | Sprints only | Track racing |
Range: the number that matters for cassettes
Single gearing decisions compare inches; cassette decisions compare ranges — lowest to highest, and the gaps between. A 1×12 MTB spans 20–96 inches (huge range, big jumps); a road 2×12 spans 35–110 (tighter steps). Your terrain picks the range; your knees pick the gaps.
The modern cousin: development
Development (meters per pedal revolution) is gear inches in metric — the fixed-gear and touring standard because it converts directly to cadence: speed ÷ development ÷ 60 = rpm. The development calculator lives exactly on that bridge; the ratio calculator prints both numbers. Learn one, and the other is a unit conversion.
Gear inches are convention, not physics — but conventions that survive 130 years have earned their keep. Run your drivetrain through both calculators and the adjectives become numbers.