Bike chain length calculator
A new chain always has to be shortened to the drivetrain. A typical road bike (chainstay 410 mm, 50 tooth ring, cassette up to 32) needs 108 links. Enter your own numbers for an exact answer from the Park Tool equation, with a correction for big mountain bike cassettes.
Result from the rigorous equation (Park Tool). Verify after shortening: on the largest chainring with the largest cog the derailleur cage must not be stretched to its limit, and on the smallest with the smallest the chain must not sag.
How is chain length calculated?
The calculator uses what Park Tool calls the rigorous equation:
L = 2C + F/4 + R/4 + 1 [inches] C - chainstay length in inches (mm / 25.4) F - teeth on the largest chainring R - teeth on the largest cog round the result UP to an even number of links (1 link = 1/2 inch) 1x MTB with a 42t cog or larger: add 1 inch for the long cage
Where does the formula come from? Two full runs between the axles (2C), half the circumference of the ring and the cog (teeth times the half-inch pitch, divided by two), plus an inch of slack for the derailleur. Worked example: a 410 mm chainstay (16.1 in), a 50 tooth ring and a 32 tooth cog give L = 32.3 + 12.5 + 8 + 1 = 53.8 inches, which rounds to 108 links (137 cm). The workshop method of wrapping big-big and adding two links gives the same answer, but requires physically fitting the chain first.
Replacing a worn chain rather than building a new drivetrain? Measure wear with a gauge: plan the swap at 0.5% and replace immediately at 0.75%, or at 0.5% for 11 and 12 speed systems, where manufacturers set the tighter threshold. If you are changing the cassette at the same time, check what the new ratios give you in the gear ratio calculator first.
Frequently asked questions
- How do you size a chain with the big-big method?
- Wrap the chain around the largest chainring and the largest cog, bypassing the rear derailleur entirely, pull the ends together until they meet, and add two links (one inch). It is the classic workshop method and it is fast. The equation used here gives the same result without taking the chain off the bike, as long as you know your chainstay length.
- Where do I find my chainstay length?
- In the frame geometry chart on the manufacturer's website, usually listed as chainstay length and labelled C or CS. Typical values: 405-415 mm for road, 420-445 mm for gravel and mountain bikes. You can also measure it directly from the centre of the bottom bracket to the centre of the rear axle.
- What happens if the chain is too short or too long?
- Too short, and the big-big combination stretches the derailleur to its limit, which can tear off the derailleur hanger or break the chain. Too long, and it sags in the small-small combination, slaps the frame and drops off the chainring over bumps. Too short is the more dangerous failure, so when in doubt, leave one extra link.
- Do clutch derailleurs need extra chain?
- Modern 1x mountain drivetrains with clutch derailleurs and very large cassettes need more chain wrap than the base equation assumes, which is why this calculator adds an inch once the largest cog reaches 42 teeth. Always verify the fit afterwards on both extreme combinations before riding.
- When should a bike chain be replaced?
- Measure it with a chain checker: plan a replacement at 0.5% elongation and replace at once at 0.75%, with 11 and 12 speed drivetrains warranting the tighter 0.5% threshold. A worn chain destroys the cassette and chainrings, which cost several times what the chain does, so replacing on time is the cheapest maintenance there is.