Ratio is not distance
One turn of the chainring pulls its own tooth count of chain past, and the sprocket takes that up at its own tooth count. A 50T ring against a 25T sprocket turns the wheel exactly twice. That two is the gear ratio, and it is the whole of the mechanical story.
It is also not enough to know anything useful. A ratio of 2 covers 13.8 feet per pedal stroke on a 700 x 25c wheel and 9.9 feet on a 20 inch folding bike. Multiplying by the wheel circumference gives development, which is what the legs actually experience.
Four measures, and when each earns its place
| Measure | Calculation | Use |
|---|---|---|
| Gear ratio | front / rear | Comparing combinations on one bike. Simplest, and blind to wheel size. |
| Development | ratio x tyre circumference | Comparing bikes with different wheels. The basis for any speed figure. |
| Gear inches | ratio x wheel diameter in inches | Talking to other riders and reading English-language sources. The conventional unit for climbing gears. |
| Gain ratio | wheel radius / crank length x ratio | Comparing bikes with different crank lengths. Closest to the load the legs feel. |
Gear inches is a fossil. On a penny-farthing the pedals drove the front wheel directly, so wheel diameter was the gearing, and after the chain drive arrived the convention of describing a gear as "equivalent to an N inch wheel" stuck. The numbers mean nothing in isolation but the habits around them are firm: below 20 is very low, above 100 is very high.
Gain ratio is Sheldon Brown's contribution and it fixes the one thing the others miss. A longer crank is a longer lever, so the same ratio is easier to turn. A rider on 165 mm cranks and one on 175 mm feel about 6% different loads at the same gear ratio, and only gain ratio shows it.
Range against spacing
Range is the top ratio divided by the bottom. A 50/34 with an 11-28 cassette gives (50/11) ÷ (34/28), which is 3.74, usually quoted as 374%. Wide range means you can climb steeply and still pedal on a descent.
The constraint is that the number of sprockets is fixed. Widening the range spreads the gears further apart, so holding an exact cadence gets harder; tightening the spacing narrows the range. A road racing 11-25 and a mountain 10-51 are the two ends of that trade, and neither is correct in general — terrain decides.
Why 22 speeds is not 22 gears
Two chainrings by eleven sprockets is sold as 22 speeds, and the distinct ratios number rather fewer. A 50/19 gives 2.63 and a 34/13 gives 2.62; those are the same gear reached two ways. The calculator counts pairs landing within 5% of each other and reports them.
Subtract those, then subtract the extreme cross-chained combinations that wear the drivetrain and are avoided in practice, and a 2x11 delivers something like 16 to 18 usable ratios. Single-ring drivetrains eliminate both problems at once, which is why mountain and gravel bikes have largely moved to them, accepting bigger jumps between gears as the price.
Choosing a climbing gear
On a climb only the bottom gear matters. Once the gradient forces cadence below about 60 rpm the effort shifts from aerobic to muscular, fatigue arrives quickly and knee loading rises. The practical test is whether the lowest gear lets you hold 70 rpm or more on the steepest grade you actually ride.
As orientation: unloaded riding on ordinary road climbs is generally comfortable with a bottom gear around 25 gear inches, while loaded touring or sustained grades past 15% want something under 20. Moving a cassette from 28 to 32 teeth lowers the bottom gear by 12.5%, which is roughly 5 to 10 rpm of extra cadence on an 8 to 10% grade — usually within what a rear derailleur will accept, but worth checking against its stated maximum sprocket and the chain length before ordering.
Measuring your own rollout
Every circumference in the selector is a published nominal figure, and real tyres depart from them by around 2% depending on pressure, rider weight and how worn the tread is. For comparing gears that error is irrelevant, because it scales every combination identically and the ordering never changes. For anything absolute — matching a speed sensor, calibrating a trainer, checking a development figure against a manufacturer's chart — measure it.
The method takes two minutes. Inflate to riding pressure, sit on the bike, line the valve stem up with a mark on the ground, roll forward exactly one wheel revolution while a second person steadies you, and mark the ground again. The distance between the marks in millimetres is your rolling circumference. Being on the bike is the part people skip, and it is the part that matters, since an unloaded tyre reads several millimetres large.
Questions people ask
Is a higher gear ratio better?
It depends entirely on the terrain. A high ratio covers more ground per pedal stroke and costs more force to turn; a low one does the opposite. Flat riding rewards the top end and climbing rewards the bottom. The question worth asking is not which number is bigger but whether the range contains a gear that lets you hold 80 to 95 rpm on the terrain you actually ride. Constantly sitting on the largest or smallest sprocket is the signal that it does not.
How much easier is an 11-32 cassette than an 11-28?
The bottom gear drops by a factor of 28/32, which is 12.5%. With a 34T chainring that moves the lowest gear from about 32 gear inches to about 28. In riding terms it buys roughly 5 to 10 rpm of cadence on an 8 to 10% grade, which is the difference between spinning and grinding on a long climb. Before buying, check the rear derailleur maximum sprocket size and whether the chain needs to be longer — exceeding the derailleur capacity means replacing that too.
What is gain ratio for, if gear inches already exists?
Crank length. Gear inches, ratio and development all describe what happens at the wheel and ignore the lever the rider is pushing on. A 175 mm crank is about 6% longer than a 165 mm one, so at an identical gear ratio the longer crank feels easier for the same road speed. If you are comparing two bikes with different cranks, or considering a crank length change, gain ratio is the only one of the four that shows the difference. Within a single bike it just rescales the ratios and adds nothing.
Why do the tables warn about big ring with big sprocket?
Chain line. In that combination the chain runs at a pronounced sideways angle between ring and sprocket, which increases friction and noise and wears the chain, rings and sprockets faster. The same applies to small ring with smallest sprocket. Neither is going to break anything immediately, but both are avoidable, because a double chainring nearly always offers a comparable ratio on the other ring with a straighter chain line. That redundancy is the same crossover the calculator counts as duplicated gears.
Does tyre pressure change the calculation?
It changes the rolling circumference by around 2% between a hard and a soft tyre, and speed and development move proportionally with it. Gear ratios and the comparison between combinations are untouched, since the same factor applies to every cell in the table. If you need the absolute figures to be right — matching a speed sensor or a trainer — measure the rollout yourself at riding pressure with your weight on the bike and enter that instead of the nominal value.