The formula
Bike mechanics have long used a simple formula to size a chain for a derailleur bike, based on the chainstay length C in inches, the number of teeth on the largest chainring F and on the largest rear cog R:
Chain length (in) = 2 × C + (F + R) ÷ 4 + 1
A bicycle chain has a pitch of half an inch, so each inch equals two links. The calculator rounds up to a whole inch, which gives an even number of links, the way most chains are joined with a quick link or connecting pin.
How to measure the chainstay
Measure from the center of the bottom bracket spindle to the center of the rear axle, in a straight line. It is usually between about 15.7 and 17.3 in (400 to 440 mm) on road and gravel bikes and longer on many mountain bikes. Frame geometry charts list it too.
Example
A road bike with a 16.5 in (419 mm) chainstay, a 50-tooth big ring and a 32-tooth biggest cog needs 33 + 20.5 + 1 = 54.5 in, rounded up to 55 in, or 110 links.
Checking on the bike
- Big-big check: shift to the largest chainring and largest cog (without riding in this gear). The rear derailleur must not be stretched to its limit.
- Small-small check: on the smallest ring and cog, the chain should not sag or rub the derailleur cage.
- Many mechanics instead wrap the chain around the big ring and big cog without going through the derailleur and add two links (one inch). Follow the derailleur maker’s method when it gives one.
Special cases
- Single-chainring drivetrains with very large cassettes often need a specific method, such as a chain length gauge or a sizing chart from the manufacturer.
- Full-suspension bikes: the distance between the bottom bracket and the rear axle changes as the suspension compresses. Size the chain with the suspension at the point of maximum chain growth, usually with the shock deflated or removed.
- Single-speed bikes have no derailleur to take up slack: the chain length is set with the wheel position in the dropouts.
When to replace a chain
A chain wears and lengthens. Replace it when a chain checker shows 0.5% wear for 11- and 12-speed drivetrains or 0.75% for older ones, to protect the cassette and chainrings.