What Is a Cassette Ratio?
On a multi-speed bike, the cassette is the stack of toothed cogs on the rear wheel. Each cog is one gear, and shifting moves the chain between them. The cassette ratio condenses that whole stack into a single number:
[
\text{Cassette Ratio} = \frac{\text{Number of Sprockets}}{\text{Total Teeth}}
]
where total teeth means the sum of the tooth counts of all the cogs added together.
Think of it as a density measure: how many gears you get per tooth of cassette. Two cassettes can both have eleven speeds, but if one spreads those gears across far more teeth, its ratio drops — and so does the character of the shifting. That single comparison is what makes the ratio useful when choosing between cassettes.
The Cassette Ratio Formula
[
\text{Cassette Ratio} = \frac{\text{Number of Sprockets}}{\text{Total Teeth}}
]
Both inputs are simple counts, so the units cancel and the result is a plain decimal. Because every cog needs at least a handful of teeth, the ratio is always well below 1 — realistic values fall between about 0.03 and 0.10.
A handy companion figure is the average teeth per cog:
[
\text{Average Teeth per Cog} = \frac{\text{Total Teeth}}{\text{Number of Sprockets}}
]
It's just the reciprocal idea flipped around, and it makes the spacing story easier to picture: an average of 17 teeth per cog means tightly packed gears, while an average of 23 suggests big steps between them.
Worked Example: An 11-Speed Road Cassette
Take a classic road cassette with cogs sized 11-12-13-14-15-16-17-19-21-23-25. First add up the teeth:
[
11+12+13+14+15+16+17+19+21+23+25 = 186 \text{ teeth}
]
There are 11 sprockets, so:
[
\text{Cassette Ratio} = \frac{11}{186} \approx 0.059
]
The average cog carries $186 / 11 \approx 16.9$ teeth. That's a densely packed cassette — many gears per tooth — which is why road racers love it: consecutive shifts move the ratio only a little, letting a rider hold a steady cadence.
Interpreting the Number
Try it yourself: enter 12 sprockets and 253 total teeth (a 10-12-14-16-18-21-24-28-32-36-42 gravel cassette) into the calculator above. You'll get about 0.047 — lower than the road cassette, reflecting a wider range with bigger jumps between gears.
| Cassette style | Typical ratio | What it feels like |
|---|---|---|
| Road racing (e.g. 11-25, 11-speed) | ~0.059 | Tight spacing, small cadence changes per shift |
| Gravel / all-road (e.g. 10-42, 12-speed) | ~0.047 | Wider range, larger jumps between gears |
| Wide-range mountain (e.g. 10-52, 12-speed) | ~0.040 | Huge range for steep climbs, biggest jumps |
The pattern: as cassettes gain range, they either gain teeth faster than they gain sprockets, pushing the ratio down. A higher ratio isn't "better" — it's a different priority.
Quick Recap
- Cassette ratio = number of sprockets ÷ total teeth across all cogs.
- Higher ratio = more gears packed into fewer teeth = tighter shift spacing.
- Lower ratio = more teeth per gear = wider range with bigger jumps.
- Compare cassettes with it; for actual speed and cadence you still need chainring sizes and wheel circumference.
If you want to zoom in on a single chain line instead of the whole stack, the sprocket ratio calculator covers the one-chainring-to-one-cog case.