Bike Chain Length Calculator: Links for 1x, 2x, MTB and Single Speed

The one everyone quotes is Sheldon Brown's, also used by Park Tool:

L = 2C + F÷4 + R÷4 + 1

L is chain length in inches, C the chainstay in inches, F the largest chainring and R the largest cog. The +1 is slack for the derailleur cage to take up. Divide L by 0.5 for links, since every bicycle chain has a half-inch pitch, then round up to an even number — a chain joins in inner-and-outer pairs.

It is an approximation and a good one. Against the exact loop geometry it lands about 0.8–1.0 inch longer than a bare big-big wrap, which is the same allowance the shop method tells you to add by hand. On a single speed that approximation is no use, which is why this calculator switches to solving the loop exactly when you pick that mode.

Every manufacturer's official procedure is the same idea: wrap the chain round the largest chainring and largest cog with the derailleur bypassed, then add a fixed amount. What differs is how much.

Cassettes up to 36T — add 2 rivets (one inch) past the point where an inner plate meets an outer. This covers most road and older MTB setups.

Big 1x cassettes, 40T and up — add 4 rivets (two inches). The cage swings much further on an Eagle or 11-51, and two rivets leaves it bottomed out.

Full suspension is the one people get wrong. Size the chain with the shock fully compressed — let the air out or take it off and bottom the linkage. The chainstay effectively lengthens as the suspension moves, and a chain sized on an extended bike goes tight over a big hit. SRAM's own instruction for Eagle is to add one inner and one outer link once bottomed out, which is fewer than the hardtail figure precisely because the compression is already in the measurement.

Then check the other end: in the small-small gear the cage should still hold tension. If the chain sags there, it is a link too long.

With no derailleur there is nothing to take up slack, so the chain has to come out at very nearly the exact length — and the exact length is almost never a whole number of links. That is what the axle position is for.

The calculator solves the loop properly and then works backwards: for each whole link count either side of exact, it tells you where the axle has to sit. If one of them lands within about 3 mm of your nominal chainstay, that pairing is a magic gear — it tensions up without a tensioner, which is what you need on vertical dropouts.

Otherwise you need horizontal dropouts, track ends, a sliding dropout or an eccentric bottom bracket to make up the difference — or a half link, which buys you half an inch of adjustment and lets you use an odd link count. Tick the box above to allow that.