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Chain Length Calculator

How many links a chain needs for a given chainstay, chainring and sprocket — by the Park Tool formula, rounded the way a chain actually comes apart.

Chain length is the one bicycle measurement where being wrong in one direction is an inconvenience and wrong in the other is a broken frame.

The formula, from Park Tool and echoed in Shimano's manuals:

L = 2C + (F ÷ 4) + (R ÷ 4) + 1

C is the chainstay in inches, F the largest chainring, R the largest sprocket. The result is length in inches; double it for links.

Always round up, and always to an even number

A link is one inner plate plus one outer plate. There is no such thing as an odd link on a derailleur chain — a half-link chain exists but is a different product for a different purpose.

Too long is an inconvenience. Too short is damage.

A chain that cannot reach big ring to big sprocket will pull the rear derailleur into the wheel and take the hanger with it, sometimes the frame. It happens in one pedal stroke and usually on a climb, when you are least able to react.

A chain a couple of links longer than ideal hangs slightly slack in the small–small combination you should not be riding anyway.

Why this comes out longer than the wrap method

Wrapping the chain around the big ring and big sprocket and adding two links has no allowance for the derailleur cage taking up slack. It comes out short on long-cage derailleurs, which is exactly where the failure mode above lives.

Measure to the axle center

Not to the dropout face. On a bike with sliding dropouts or an eccentric bottom bracket, measure in the position you actually ride — a chain sized at one end of the adjustment range is wrong at the other.

1 article

Chain length for a 430 mm chainstay with a 40t ring and 28t sprocket

Chain length
104links
51.9″ for a 430 mm chainstay with a 40t ring and a 28t sprocket
40t28t430 mm chainstay104links51.9″min 102
Links (round up to even)104links
Length1,317 mm51.9in
FormulaPark Tool / Shimano method2 × 16.93 + 40/4 + 28/4 + 1
Chainstay16.93″ — bottom bracket centre to rear axle430mm
Absolute minimumbig ring to big sprocket, with no slack left at all102links
From a 116-link chainlinks to take out of a new chainremove 12
Total teeth40t ring + 28t sprocket68

Notes

  • Round up to an even number, always. A link is one inner plate plus one outer plate; there is no such thing as an odd link on a derailleur chain. 103.7 rounds to 104.
  • Too long beats too short, and it is not close. A chain that cannot reach big ring to big sprocket will tear the derailleur into the wheel and take the hanger — sometimes the frame — with it. A chain a couple of links too long just hangs slightly slack in the small–small combination you should not be using anyway.
  • Measure the chainstay to the axle centre, not to the dropout face. On a bike with sliding dropouts or an eccentric bottom bracket, measure it in the position you actually ride.
  • This formula gives a slightly longer chain than wrapping it around the big ring and big sprocket and adding two links. That is deliberate: the wrap method has no allowance for the derailleur cage taking up slack, and it comes out short on long-cage rear derailleurs.
  • A new chain on a worn cassette will skip under load. If you are replacing a chain that has been on the bike a long time, check cassette wear at the same time — a chain checker under 0.75% means you probably got away with it.

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Bicycle chain length

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