How this calculator works
Bicycle chain pitch is half an inch, so each link advances one tooth. The chain runs from the largest chainring to the largest cog and back, with two straight runs and two wraps. A standard workshop approximation adds twice the chainstay in pitches, half the chainring teeth, half the cog teeth and a one-inch allowance for the derailleur. The result is rounded up to an even number of links, because chains are joined in pairs. The shortest usable chain is the same calculation without the extra one inch.
Formula
Recommended links = round up to even of (2 x chainstay / 0.0127 + chainring / 2 + cog / 2 + 2). Shortest links = the same without the final 2. Chain length (m) = links x 0.0127. Chainstay (m) = (links - chainring / 2 - cog / 2 - 2) / 4 x 0.0254.
Worked example
A 410 mm chainstay with a 50-tooth chainring and 16-tooth cog: 0.41 / 0.0127 = 32.28 pitches, so the shortest count is 2 x 32.28 + 25 + 8 = 97.57, rounded up to 98 links. The recommended count adds two links for the derailleur, giving 100 links, which is 1,270 mm of chain. Working back from 100 links gives a chainstay of 0.41275 m.
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Frequently asked questions
Why does the recommended chain have spare links?
The derailleur cage needs room to take up slack as you move between gears. Two extra half-inch links, one inch in total, is the usual allowance. Too short a chain can jam or damage the derailleur on the big-big combination.
Why must the link count be even?
Most chains use an inner and outer plate pair, so a joined chain has an even number of links. Rounding up keeps the chain long enough; rounding down could leave it too short.
What if I have a single-speed or fixed-gear bike?
There is no derailleur to take up slack, so you set the length for the actual gear and use the rear dropout or a chain tensioner to adjust. This page assumes a derailleur drivetrain.
Does chain wear change the length?
A worn chain stretches slightly and can sit differently on the teeth, but the difference is small compared with the allowance here. Replace a chain when a wear gauge shows 0.5 percent elongation on an eleven-speed system.