Enter chainring, cog and wheel size — get gear inches, development, gain ratio, and a full speed-at-cadence table. A modern, complete take on the classic bicycle gear calculator.
Your drivetrain
Gear ratio
–
chainring ÷ cog
Gear inches
–
inches
Development
–
m / pedal rev
Gain ratio
–
Speed at your cadence
–
km/h
Rollout per wheel rev
–
m
Speed at cadence
How fast this gear goes as you pedal faster or slower.
Cadence (rpm)
km/h
mph
Compare cogs (your cassette)
See every gear your current chainring gives across the cassette. Edit the cog list.
Cog
Ratio
Gear in
Dev (m)
Gain
km/h @90
Compare chainrings
Same rear cog, different front rings — see the jump between your rings.
Ring
Ratio
Gear in
Dev (m)
Gain
km/h @90
Try a classic setup
What the numbers mean
Gear inches
The classic measure: (chainring ÷ cog) × wheel diameter in inches. A 90-inch gear on a 27″ wheel behaves like a direct-drive penny-farthing with a 90-inch wheel. Example: 50÷15×27 = 90.0 gear inches.
Development (meters)
The distance your bike travels per full crank revolution: gear inches × π × 0.0254. Our example: 90.0 × π × 0.0254 = 7.18 m per pedal stroke.
Gain ratio (the fairest comparison)
Sheldon Brown's metric — it also accounts for crank length: (chainring ÷ cog) × (wheel radius ÷ crank length). With a 172.5 mm crank on a 27″ wheel, our example gives 6.63: the wheel moves 6.63 times as far as your foot pushes. Use gain ratio when comparing bikes with different wheel sizes or crank lengths.
Speed at cadence
Speed = development × rpm × 60 ÷ 1000 (km/h). Our example at 90 rpm: 7.18 × 90 × 60 ÷ 1000 = 38.78 km/h.
Roll-out tip: the most accurate wheel size is a measured roll-out — mark your tire, roll the loaded bike one full revolution, measure in meters, and enter it as circumference.
Related tool: our bike cadence and speed calculator works the other direction — it takes a known gear (development) and your cadence as inputs to find speed. This page instead computes the gear numbers (inches, development, gain ratio) from chainring, cog and wheel size.
Frequently asked questions
What are gear inches?
Gear inches is the classic way to express a bicycle gear: (chainring ÷ cog) × wheel diameter in inches. For example, a 50-tooth chainring with a 15-tooth cog on a 27-inch wheel gives 90.0 gear inches. A higher number means a harder, faster gear.
What is development in bike gearing?
Development is the distance your bike travels with one full turn of the cranks, usually given in meters. It equals gear inches × π × 0.0254. Our 90.0-inch example develops 7.18 meters per pedal revolution.
What is gain ratio?
Gain ratio, popularized by Sheldon Brown, is the fairest way to compare gears: (chainring ÷ cog) × (wheel radius ÷ crank length). Unlike gear inches, it accounts for crank length. A 50×15 gear on a 27-inch wheel with 172.5 mm cranks has a gain ratio of 6.63 — the wheel travels 6.63 times as far as your foot pushes.
Which number should I compare: gear inches, development or gain ratio?
Use gain ratio when comparing bikes with different wheel sizes or crank lengths — it is the only one of the three that accounts for crank length. Gear inches and development are proportional to each other, so either works fine for bikes with the same wheel size.
How fast will I go at a given cadence?
Multiply development by cadence: speed (km/h) = development in meters × rpm × 60 ÷ 1000. A 7.18 m development at 90 rpm gives 38.78 km/h.
How do I get an easier climbing gear?
Fit a smaller chainring or a larger rear cog — either lowers the ratio, the gear inches, and the gain ratio, so each pedal stroke moves the bike a shorter distance with less effort.
What cadence should I pedal at?
Most riders are most efficient between 80 and 100 rpm. Use the speed-at-cadence table above to see what each gear gives you across 60 to 120 rpm.
How do I measure my wheel circumference accurately?
Do a roll-out: mark the tire, roll the loaded bike (you sitting on it) exactly one wheel revolution on flat ground, and measure the distance in meters. Enter that number as a custom circumference — it beats any published table.
Does crank length change my gears?
It changes gain ratio but not gear inches or development. Longer cranks give more leverage on the same chainring and cog, so the gain ratio is lower. That is why gain ratio is the better comparison across different bikes.
What is a good gear for a singlespeed bike?
A common all-round choice is around 65 to 75 gear inches — for example 42×16 on 700×28c tires gives about 69.6 inches and 5.55 m of development. Flat cities can go higher; hilly areas should go lower.
What is the difference between this and a cadence/speed calculator?
This calculator works out the gear itself — gear inches, development and gain ratio — from your chainring, cog and wheel size. A cadence-and-speed calculator works the other way: you enter an already-known gear (development) plus your cadence to get speed.
Are these formulas the same as Sheldon Brown's gear calculator?
Yes — the underlying math (gear inches, development, gain ratio) is identical to the classic calculator; this tool adds a modern interface, a speed-at-cadence table, cassette and chainring comparison tables, and explanations of what each number means.
Keywords covered: bike gear ratio calculator, bicycle gear calculator, gear inches calculator, bike development calculator, gain ratio calculator, chainring cog calculator, bike gear chart, speed at cadence table, rollout calculator.
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