Hobbies · Cycling
Rear derailleur capacity calculator.
Enter your chainrings and your smallest and largest cogs. The calculator adds the front range to the rear range and checks the total against the derailleur's published limits. Example: 50/34 chainrings with an 11-34 cassette need (50 − 34) + (34 − 11) = 16 + 23 = 39 teeth, which sits exactly on the 39-tooth limit Mantel lists for a Shimano Tiagra 4700 long-cage derailleur on a double crankset.
Your numbers
Result
Working out the result.
How the capacity is calculated
Two sources give the same rule. Sheldon Brown's glossary says a rear derailer's capacity requirement “is equal to the front range (22 in the example above) plus the rear range”, and works a 52/42/30 crank with a 12-28 cluster as “38 teeth (22 front difference + 16 rear difference)”. Helen's Cycles writes it as chain-wrap capacity = (large chainring − small chainring) + (large rear cog − small rear cog), with 53/42 and a 12-23 cassette giving 22 teeth. They also agree on why the number matters: the derailleur has to keep the chain taut as you change gear.
The calculator then compares your figure with up to three checks that the makers print: total capacity, the largest cog, and the front chainring difference. Where a source gives a smallest-cog minimum, that is checked too. These are separate limits, so a setup can pass on total capacity and still fail on largest cog.
Worked examples with real numbers
- 50/34 with 11-34 on a Tiagra 4700 long cage (double). Front 16 plus rear 23 gives 39 teeth. Mantel lists a 16-tooth front difference limit, a 39-tooth total and a largest cog of 28 to 34 teeth, so all three checks pass with nothing to spare.
- 53/34 with 11-34 on the same derailleur. Front 19 plus rear 23 gives 42 teeth. That is 3 over the 39-tooth total and 3 over the 16-tooth front difference, so it fails two checks.
- 53/39 with 12-23 on an RD-7700. Front 14 plus rear 11 gives 25 teeth against the 26-tooth limit in Shimano's service instructions, with the front difference exactly at its 14-tooth limit. Change the cassette to 12-27 and the largest cog (27T against a 26T limit) and the total (29 against 26) both fail.
- Single chainring, 40T with 10-42. There is no front range, so the need is just the 32-tooth rear range.
Published limits used on this page
| Derailleur | Total capacity | Front difference | Largest cog | Smallest cog |
|---|---|---|---|---|
| Shimano RD-7700 (service instructions) | 26 or less | 14 or less | 26T | 11T |
| Shimano Tiagra 4700 long cage, double (Mantel) | 39T | 16 or less | 28 to 34T | 11 to 12T |
| Shimano Tiagra rear derailleur, 7-9 speed (Helen's Cycles listing) | 32 teeth | not given | 30/32T (short/mid cage) | not given |
Mantel also lists a triple-crank column for the 4700 (front difference 20 or less, total 41T, largest cog 25 to 32T). This page uses the double column because it is the common setup; use “My own limits” for a triple.
What this calculator leaves out
Capacity is a manufacturer rating, not a physical law. Sheldon Brown notes that rear derailers carry a maximum sprocket size, that the jockey pulley may rub if you exceed it, and that Shimano's 27-tooth road figure often stretches to 30 in practice. This page reports the printed limits and nothing more, so a pass is not a guarantee and a marginal fail is not proof it will not shift. Chain length, B-gap setting and shifter compatibility are outside the arithmetic.
Common mix-ups
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Often assumedA bigger cassette is the only thing that eats derailleur capacity.
What the source saysCapacity is the front range plus the rear range. In the glossary example a 52/42/30 crank needs a front derailer capable of 22 teeth by itself, and adds the 16-tooth range of a 12-28 cluster to reach 38. [1]
-
Often assumedThe maximum cog size is a hard stop.
What the source saysIt is a design figure. The same glossary entry says Shimano lists its road rear derailers for a 27-tooth maximum yet they often handle 30, and the author has never seen a case where a 28 would not work. [1]
Checked against these sources on .
Frequently asked questions
How do I calculate rear derailleur capacity?
Subtract the small chainring from the big chainring, subtract the smallest cog from the largest cog, and add the two. A 50/34 crankset with an 11-34 cassette gives 16 + 23 = 39 teeth.
What if I have a single chainring?
Set the small ring equal to the big ring. The front range is then zero and the capacity you need is just the cassette range, for example 32 teeth for a 10-42 cassette.
Is total capacity the only limit I should check?
No. Makers also print a largest cog and a maximum front chainring difference. Mantel lists all three for the Tiagra 4700, and Shimano’s RD-7700 instructions list a total, a front difference and a largest and smallest sprocket.
Can I go over the printed limit?
Sometimes, but the figures here are the printed ones. Sheldon Brown says the largest-cog rating is often exceeded in practice, for example 30 teeth on a derailer listed for 27. Treat that as an anecdote about one maker, not a rule.
Where do the limits for my derailleur come from?
They come from the maker’s spec sheet or manual for your exact model. This page includes three examples read from the sources below, and lets you type your own.
Sources
Retrieved
- Sheldon Brown: Capacity (of a derailer) — the front plus rear range rule, the 52/42/30 and 12-28 example, and the note on maximum sprocket size (read 2026-10-26)
- Helen’s Cycles: Shimano Tiagra rear derailleur — the chain-wrap capacity formula, the 53/42 and 12-23 example, and a 32-tooth capacity listing (read 2026-10-26)
- Shimano RD-7700 service instructions (ManualsLib) — total capacity 26 or less, front difference 14 or less, 26T largest and 11T smallest (read 2026-10-26)
- Mantel: Shimano Tiagra 4700 10 speed rear derailleur — limits by crankset type: total 39T/41T, front difference 16/20, largest cog 28-34T/25-32T (read 2026-10-26)
A check against printed limits, not a fitting guarantee
Not affiliated with Shimano, Mantel, Helen’s Cycles or ManualsLib. Limits were read on 26 October 2026 and makers revise them, so confirm with the spec sheet for your exact model. Nothing you type is sent or stored.