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Bike Chain Length Calculator

Enter chainstay length, chainring teeth, and largest cog to size your chain.

How it is calculated

The tool uses the classic rule for derailleur bikes with a standard half-inch-pitch chain. Chainstay length goes in inches, plus the tooth counts of the big chainring and the big rear cog:

length (in) = 2 × chainstay + chainring teeth ÷ 4 + cog teeth ÷ 4 + 1
links = length × 2, rounded to the nearest whole number and then up to an even number

Chain pitch is half an inch, so each inch of chain holds two links, and a chain needs an even link count to close with a normal master link. The final +1 inch is the allowance for the join. The chainstay must be between 5 and 40 inches and the tooth counts must be whole numbers up to 100, otherwise no result is shown. It assumes a derailleur drivetrain and does not model suspension frames whose chainstay length changes with travel.

Worked example

A bike has a chainstay of 16.5 in, a 50-tooth big chainring and a 34-tooth largest cog. The length is 2 × 16.5 + 50 ÷ 4 + 34 ÷ 4 + 1 = 33 + 12.5 + 8.5 + 1 = 55.0 in. Doubling gives 110 links, already an even number, so the tool shows 110 links. With a 32-tooth cog the length is 54.5 in, which is 109 links, an odd number, so the tool rounds up to 110 links.

Try this example →

Common mistakes

  • Entering the chainstay in millimetres or centimetres. The field is in inches; typing 420 for 420 mm is outside the accepted 5 to 40 inch range and shows no result, so convert first (420 mm is about 16.5 in).
  • Measuring the chainstay to the wrong points. The measure is from the middle of the bottom bracket to the middle of the rear axle, not along the tube's outer surface.
  • Entering the small chainring or the smallest cog. The rule is built on the largest of each, so the small ones give a chain too short for the big-big gear.
  • Cutting to an odd number of links. A standard chain closes with a master link only on an even link count, which is why the tool rounds up.
  • Treating the result as exact for a full-suspension bike. The chainstay length changes as the rear wheel moves, so check the chain at the position where it is tightest and leave extra margin.

About this calculator

Getting bicycle chain length right matters because a chain that is too short can’t reach the largest cog-and-chainring combination (or damages the derailleur), while one that’s too long sags, shifts poorly and can skip under load. The standard sizing method, popularized by bike mechanic Sheldon Brown, estimates the needed length from your chainstay length, the tooth count of your largest chainring, and the tooth count of your largest rear cog: roughly twice the chainstay length plus a quarter of the combined chainring-and-cog teeth, plus a fixed 1-inch allowance for the join (enter the chainstay in inches); there is no extra correction for the difference between chainring and cog sizes.

Chain length is usually expressed in links (each link pair, inner plus outer, adds about one inch), since chains are sized and cut in whole links — the calculator rounds to an even number — to keep the master link aligned correctly. Bike mechanics and home wrenches use this calculation when replacing a worn chain, building a new drivetrain, or swapping to a different chainring or cassette, so the derailleur cage has the right amount of slack across the full gear range without going slack or over-stretching.

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