Capacity is a length of chain, not a shifting spec
Take the drivetrain to its longest chain requirement — the largest ring against the largest sprocket — and then to its shortest, the smallest against the smallest. The chain does not change length, so the difference has to be absorbed by the derailleur cage swinging back. That swing is the whole of what capacity means.
The arithmetic is deliberately blunt: total wrap equals the chainring range plus the cassette range, both in teeth. A 34/50 crank has a 16 tooth range, an 11-34 cassette has 23, and the total is 39. Nothing about shift quality, cable pull or speed count enters into it.
Converting to chain makes it concrete. Each tooth of range corresponds to about a quarter inch of chain, because roughly half the teeth on a sprocket are engaged and each tooth holds half an inch of chain. A link is half an inch, so teeth divided by two gives links. That 39 tooth wrap is 19.5 links, near ten inches of chain the cage has to store when you drop into the small ring and the small sprocket.
Two limits that fail in different ways
| Limit | What it governs | Symptom when exceeded |
|---|---|---|
| Total capacity | How much slack the cage can take up | Chain sags and slaps in the small-small combination, or the cage is bottomed out at big-big |
| Largest sprocket | Whether the upper pulley clears the biggest cog | Pulley rubs or jams against the cog teeth regardless of chain length |
| Smallest sprocket | Geometry at the other end of the cassette | Poor pulley spacing on the small cogs, rarely the binding constraint |
A derailleur can sit comfortably inside its capacity and still be unusable because the cassette has a cog too large for it. The reverse also happens: a single-ring setup with a modest cassette clears the biggest cog easily while asking for more total wrap than a short cage was built for. They are separate checks and this page runs both.
Why single-ring drivetrains changed the arithmetic
With one chainring the front range is zero, so the entire capacity budget goes to the cassette. That is what made very wide cassettes practical: a 10-52 has a 42 tooth range and needs 42 teeth of capacity, which a 2x road setup with a 16 tooth front range could only match with a 26 tooth cassette range.
It also removed a hidden cost. On a double, the two ranges add, so a wide cassette and a wide crank compound into a wrap figure that pushes past what a short cage will take. Removing the front derailleur removes half the sum, and the cassette gets to spend all of it.
What the number will not tell you
It will not tell you whether a particular derailleur and cassette work together, because compatibility involves cable pull ratio, pulley offset, hanger geometry and the maker's own testing, none of which is arithmetic. It will not tell you whether a shortfall is survivable in practice; some riders run a chain sized for the combinations they actually use and simply never shift to big-big, and that is a decision with a real failure mode attached rather than a trick.
That failure mode is worth naming plainly. A chain cut short enough to keep the cage tight at small-small will run out of length if it is ever shifted into big-big, and when it does the derailleur is pulled forward into the cassette and the hanger, the cage or both are destroyed — sometimes with the wheel locking as it happens. Size the chain for the largest combination the drivetrain can physically reach, whether or not you intend to use it.
Questions people ask
How is total capacity calculated?
Largest chainring minus smallest chainring, plus largest sprocket minus smallest sprocket, all in teeth. A 34/50 crank with an 11-34 cassette gives 16 plus 23, which is 39. For a single ring the first term is zero, so the total is just the cassette range.
What happens if I exceed the capacity?
At the small ring and small sprocket the cage cannot swing back far enough to keep the chain taut, so it hangs and slaps and can drop off. At the other extreme the cage is fully extended and any further demand pulls the derailleur into the cassette. Which end bites depends on how you cut the chain, and cutting it short to fix the first end creates the second.
Is capacity the same as the maximum cog size?
No, and they can bind independently. Capacity is about storing slack chain across the range. Maximum cog is about whether the upper pulley physically clears the largest sprocket. A wide-range single-ring cassette often trips the cog limit first, while a double with two wide ranges trips capacity first.
Can I run a cassette bigger than my derailleur is rated for?
That is a question for the derailleur maker rather than for arithmetic, and the answer varies by model. What the calculation can do is show you exactly how far over you would be, in teeth and in links of chain, so you know the size of the gap before you spend anything. Hanger extenders and cage swaps exist and each carries its own caveats from whoever makes it.
Does capacity change how many links the chain needs?
Not directly, but they share an input. Chain length is set by the chainstay and the two largest sprockets, so enlarging the biggest cog lengthens the chain by about half a link per tooth and simultaneously raises the wrap by one tooth. If you are planning a cassette change, run both calculations rather than assuming the chain still fits.