Bike chain length.

Worked example

Chainstay 410 mm (16.14 in), big chainring 50 teeth, big cog 34 teeth, standard drivetrain. Length in inches = 2 × 16.14 + (50 + 34) / 4 + 1 = 54.28 in. The chain has a link every half inch, so 54.28 / 0.5 = 108.6 links; chains come in even link counts, so the nearest is 108 links (or 110 to start long and trim). For a SRAM 1x with a 32-tooth ring, 51-tooth cog and the same 410 mm stay, one extra link gives 108.1 links and the two required extra links give 110.1, so 110 links.

How it works

Inches = 2 × chainstay + (big ring teeth + big cog teeth) ÷ 4 + extra. The chain makes two straight runs of about one chainstay each and wraps half of each sprocket; each tooth is a half inch of chain, so the half-wraps add (F + R) / 2 × ½ = (F + R) / 4 inches. Divide inches by 0.5 for the count of half-inch links.

The extra. The standard method is to thread the chain on the big ring and big cog without going through the rear derailleur, with one complete link (one inch, an inner and an outer half-link) overlapping, so start with the shortest chain that connects and allow one extra complete link. SRAM 1x systems require two extra complete links. The page adds exactly that.

Use the bike as the check. The formula assumes the straight runs equal the chainstay, which is close but not exact, and the rear derailleur has to be able to take up slack in the small-small combination. Shorten a chain rather than lengthen it, and use a master link where you can.

Sources and basis (opened 2026-10-08): Sheldon Brown, Derailer Adjustment, Chain Length section (big-big method, one full link of overlap, SRAM 1x requires two extra complete links; the half-inch link and the geometric formula are standard chain arithmetic).

FAQ

How many links do I need for my bike chain?

Use 2 × chainstay + (big ring + big cog) / 4 + 1 in inches, divided by 0.5. A 410 mm stay with a 50-tooth ring and 34-tooth cog comes to 108.6, so 108 links.

Why are chain links always an even number?

The cited page notes that inner and outer half-links must alternate, so a closed loop has an even count of half-inch links.

Should I use the biggest ring and cog?

Yes. The big-big combination is the longest path the chain has to span, so sizing there keeps it from running too short. The rear derailleur takes up the slack in smaller gears.

What is different on a 1x drivetrain?

SRAM asks for two extra complete links instead of one. The 32 and 51 tooth example with a 410 mm stay goes from 108.1 links with one extra to 110.1 with two.

Is it safe to run a chain that is too short?

No. A chain that is too short can jam, and shifting into the big-big combination could damage the rear derailleur. If in doubt, leave it longer and check the big-big overlap.