Chain Length Calculator

What the chain length calculator does

A new bike chain comes far too long and needs trimming to fit your bike, and getting that length right matters: too short can wreck your derailleur, too long and the chain sags and shifts badly. This tool works out the right length. You give it your chainstay length and the tooth counts of your largest chainring and largest rear cog, and it returns the chain length in inches and the number of links to run.

It uses the long-established formula from Park Tool, the well-known bicycle tool company, so you can size a chain confidently even without an old one to copy.

How to use it

1. Enter your chainstay length. That is the distance from the center of the bottom bracket, where the cranks turn, to the center of the rear axle. Your frame's geometry chart lists it, or you can measure it.
2. Enter the teeth on your largest front chainring and your largest rear cog. Always the largest of each, for the reason explained below.

Press Calculate for the chain length and link count, or Reset to clear the fields.

How the chain length is worked out

The formula takes twice the chainstay length, adds a quarter of each gear's teeth, and adds one inch:

Chain length (inches) = 2 × chainstay + (front teeth ÷ 4) + (rear teeth ÷ 4) + 1

The reason it uses your largest chainring and largest cog is that this big-to-big combination is where the chain is stretched the tightest, so sizing for it guarantees the chain is long enough for every gear. The extra inch on the end gives the slack the chain needs to climb over the teeth during a shift without binding. Since a chain is built from alternating inner and outer links, the final count must be an even number, so the result is rounded up to the nearest even figure.

An example with real numbers

Say your chainstay is 16.5 inches, your largest chainring is 50 teeth, and your largest cog is 28 teeth.

• Chain length = (2 × 16.5) + (50 ÷ 4) + (28 ÷ 4) + 1
• = 33 + 12.5 + 7 + 1 = 53.5 inches
• That works out to about 107 links, rounded up to an even 108

So you would size the chain to 108 links. Always round up rather than down, since a fraction too long is harmless but a fraction too short is not.

A few important exceptions

The formula is reliable for a typical setup, but a few cases need more chain than it gives, so it is worth knowing them. If you run a single-chainring drivetrain with a very large cassette, the kind with a 42-tooth cog or bigger, you will usually need to add a couple of links beyond the calculated figure. If you have a full-suspension mountain bike, add some too, because when the suspension compresses the rear axle moves back and effectively lengthens the chainstay, so the chain has to be long enough to cover that movement.

Whatever your bike, treat the calculated length as the plan and then do the final check by hand once the chain is on. Shift into the largest chainring and largest cog and make sure the chain reaches with the derailleur not yanked fully tight, then shift into the smallest of each and make sure the chain does not sag. One last thing: when replacing a worn chain, calculate fresh rather than copying the old one, because chains stretch as they wear, so an old chain is a misleadingly long template.

Questions people ask

How do you calculate bike chain length?

Take twice your chainstay length in inches, add a quarter of your largest chainring's teeth and a quarter of your largest cog's teeth, then add one inch. Round the link count up to the nearest even number.

Why use the largest chainring and cog?

Because that combination pulls the chain tightest, so sizing for it ensures the chain is long enough for every gear. The extra inch gives slack for shifting.

Why must the number of links be even?

Because a chain alternates inner and outer links, which only connect in pairs. An odd count cannot be joined, so the length is always rounded to an even number of links.

Do full-suspension or 1x bikes need a different length?

Yes, both usually need a few extra links. Full suspension lengthens the effective chainstay as it compresses, and very large single-chainring cassettes demand more chain, so add some beyond the basic formula and test-shift to confirm.

References

1. Park Tool, Chain Length Sizing (the L = 2C + F/4 + R/4 + 1 method). https://www.parktool.com/en-int/blog/repair-help/chain-length-sizing