What this calculator does
Chain length comes from the chainstay length and the largest chainring and sprocket. The standard formula doubles the chainstay, adds a quarter of each tooth count, adds one inch, and rounds up to an even number of half-inch links.
A 17 inch chainstay with a 42 tooth chainring and a 32 tooth sprocket gives 53.5 inches, which is 108 links. The links must be even because a chain alternates inner and outer plates, so an odd count cannot join to itself without a half link.
The formula
The calculated length in inches is twice the chainstay, plus the front teeth divided by four, plus the rear teeth divided by four, plus one. That is divided by 0.5 inches per link and rounded up to an even number. The formula sizes for the largest chainring and largest sprocket, which is the combination that needs the most chain.
| Term | Meaning |
|---|---|
| Link | Half an inch of chain. Links must be counted in even numbers because inner and outer plates alternate. |
| Chainstay | Distance from the bottom bracket centre to the rear axle. |
| Big-big | Largest chainring with largest sprocket, the combination chain length is sized against. |
| Quick link | A removable connecting link, which affects how the chain is joined rather than how long it needs to be. |
The inputs explained
| Field | What to enter |
|---|---|
| Chainstay length (center-to-center) (in) | Chainstay length in inches, measured from the bottom bracket centre to the rear axle. |
| Largest chainring teeth | Teeth on the largest chainring. |
| Largest rear cog teeth | Teeth on the largest rear sprocket. |
When to use it
Fitting a new chain
New chains come longer than needed and must be shortened. Calculating first avoids the common error of cutting too short, which cannot be undone.
Changing cassette or chainring size
A larger sprocket or chainring needs more chain, and fitting the old length after an upgrade can damage the derailleur in the big-big combination.
Checking an unfamiliar bike
A chain that is too long shifts poorly and slaps; one too short risks tearing off the derailleur. The calculation gives a figure to check against.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much chain does each chainstay length need?
A range of chainstay lengths with the same drivetrain.
| Chainstay length | Chain length | Calculated length | In centimetres |
|---|---|---|---|
| 15 in | 100 links | 49.50 in | 125.7 cm |
| 17 in | 108 links | 53.50 in | 135.9 cm |
| 19 in | 116 links | 57.50 in | 146.1 cm |
Questions
Why must chain links be an even number?
Because a chain alternates inner and outer plate pairs, and joining an odd count would put two inner or two outer plates together, which does not fit. Half links exist to work around this but are uncommon on derailleur systems.
What happens if the chain is too short?
Shifting into the largest chainring and largest sprocket at the same time will over-extend the derailleur, and at best it will not shift while at worst it tears the derailleur off and can destroy the frame hanger. Sizing for big-big is what prevents this.
What happens if it is too long?
Poor shifting, chain slap on the chainstay, and the derailleur cage hanging slack enough that the chain can drop off in the small-small combination. Too long is less dangerous than too short but still worth avoiding.
Does this work for a single speed?
Not directly. Single speed and fixed gear chain length depends on the exact chainstay length achievable with the wheel in the dropouts, since tension is set by moving the wheel. The formula here is for derailleur drivetrains, where the derailleur takes up the slack.
For gear ratios on the same drivetrain, see the bike gear calculator. For frame size, see the bike frame size calculator.