What this calculator works out
Enter the number of teeth on your chainring and sprocket, choose your wheel and tyre, and this gives the gear ratio, gear inches, development in metres per pedal revolution, gain ratio, and the speed that gear produces at a given cadence.
The bare ratio is not much use on its own, because the same ratio on different wheels gives a different gear. Gear inches and development make gearing comparable between bicycles, which is what you need when deciding whether a setup will get you up a hill.
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Before you calculate
- Count the teeth, or read them off the components. Chainrings usually have the number stamped on them; cassette sprockets rarely do, so counting is often quicker than looking it up.
- Tyre size changes the answer. A wider tyre has a larger circumference and therefore a taller effective gear.
- The gear ratio on its own tells you very little. Gear inches and development turn it into something you can compare.
Gear inches dates from the penny-farthing, where the only way to change the gearing was to change the size of the front wheel. A 70-inch gear on a modern bike moves you as far per pedal revolution as a penny-farthing with a 70-inch driving wheel would. The unit survives because a century of riders have built an intuition for what the numbers feel like.
How this calculator works
Gear ratio = chainring teeth ÷ sprocket teeth
Gear inches = ratio × wheel diameter in inches
Development = ratio × wheel circumference
Speed = development × cadence × 60 ÷ 1,000
Gain ratio = (wheel radius ÷ crank length) × ratioGear ratio is how many times the wheel turns for one turn of the pedals. Gear inches multiplies that by the wheel diameter, giving a number comparable across different wheel sizes. Development is how far the bicycle travels in metres for one pedal revolution, which is the most physically meaningful of the three.
Gain ratio, devised by Sheldon Brown, also accounts for crank length: it is the distance the bicycle moves divided by the distance your foot travels. Longer cranks give more leverage, so the same gear feels easier on 175 mm cranks than on 165 mm. It is the only one of these measures that captures that.
Wheel circumference here uses standard measured values for each tyre size. Actual circumference varies with pressure, load and tread wear, so treat the speed figure as close rather than exact.
Worked example: 50 × 17 on 700 × 25c
- Gear ratio: 50 ÷ 17 = 2.94
- Wheel circumference: 2,105 mm, giving a diameter of about 26.4 inches
- Gear inches: 2.94 × 26.4 = 77.6 inches
- Development: 2.94 × 2.105 = 6.19 metres per pedal revolution
- Speed at 90 rpm: 6.19 × 90 × 60 ÷ 1,000 = 33.4 km/h
That is a typical fast-cruising gear on a road bike. Drop to a 34-tooth chainring and a 30-tooth sprocket and the same rider at the same cadence travels at 12.9 km/h — a climbing gear.
Common mistakes
- Comparing gear ratios between bikes with different wheels. A 2.5 ratio on 26-inch wheels is a much lower gear than 2.5 on 29-inch wheels. Use gear inches or development.
- Ignoring tyre width. Going from 25 mm to 32 mm tyres raises every gear by about 3%.
- Assuming more gears means a wider range. A twelve-speed cassette can have a narrower range than a nine-speed one; the range depends on the largest and smallest sprockets, not the count.
- Cross-chaining. Big ring to biggest sprocket, or small ring to smallest, duplicates gears you already have and wears the chain faster.
- Choosing gearing from flat-road riding. The gear that matters is the lowest one, on the steepest hill, at the end of a long day.
Frequently asked questions
What is a good low gear for hills?
A ratio at or below 1.0 — a chainring no larger than the largest sprocket — is a widely used benchmark for loaded touring and steep terrain. In development terms that is under about 2.1 metres per pedal revolution. Modern gravel and touring setups frequently go well below 1.0. What counts as low enough depends on the gradient, the load and your legs, not on a universal number.
What is the difference between gear inches and development?
They measure the same thing in different units. Gear inches expresses the gear as the diameter of an equivalent penny-farthing wheel; development gives the distance travelled per pedal revolution in metres. Development is more directly meaningful, gear inches is more commonly quoted. Multiply gear inches by about 0.0798 to get development in metres.
What is a gain ratio?
A measure devised by Sheldon Brown that divides how far the bike moves by how far your foot moves, which means it accounts for crank length as well as gearing. It is the only common measure that reflects the fact that longer cranks make the same gear feel easier. A gain ratio of 5.0 means the bicycle travels five times as far as your foot does.
Does tyre size really matter?
Enough to notice. Moving from 700 × 25c to 700 × 32c increases circumference by around 3%, which raises every gear by the same proportion. It will not transform the bike, but it is the same order as changing a sprocket by a tooth.
How do I count my sprocket teeth?
Mark a starting tooth with a piece of tape or a marker, then count round. For the cassette it is easiest with the wheel out. Chainrings usually have the tooth count stamped near the crank arm bolt, and cassettes are normally sold by their smallest and largest sprocket, such as 11 to 34.
What cadence should I use?
Most riders self-select somewhere between 70 and 100 revolutions per minute, and trained cyclists often sit higher. There is no single correct figure. Use whatever you actually pedal at, since the point of the speed figure is to tell you what a gear will feel like for you.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
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References
These organisations publish the guidance and figures behind this page.
- British Cycling — the national governing body, with guidance on equipment and training
- Sheldon Brown — the reference source for gain ratios and bicycle gearing theory
Sources are checked at publication and can change — how I choose and check references.