# How a bicycle chain is sized
A bicycle chain is a loop of 12.7 mm pitch roller links. The shortest loop that can sit on the largest chainring and the largest rear cog is two chainstay lengths plus half the tooth count of each sprocket. That wrap is not yet a working derailleur chain. The rear cage needs extra rivets so the pulleys can bend the lower run without going slack in the smallest gears or binding in the largest ones. 12.7 mm Modern chain pitch
Even count Joinable rivets
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# The wrap formula in plain numbers
Convert the chainstay to inches, multiply by four, then add half the largest chainring teeth and half the largest cog teeth. That number is the theoretical wrap in half links, also called rivets. A derailleur bike then adds two rivets for cassettes up to 36 teeth, or four rivets when the largest cog is 42 teeth or more. The calculator rounds up to the next even number so inner plates can meet outer plates, including a master link.| Input | What it changes | Workshop meaning |
|---|---|---|
| Chainstay | The two straight runs | Measure centre to centre, not the tube outside |
| Largest chainring | Front wrap | Use the big ring even on a 2x that you rarely shift |
| Largest cog | Rear wrap and extra rivets | 42T and up follow the wide range method |
| Drivetrain | Extra rivets and growth | Singlespeed and fixed add no derailleur slack |
# Largest to largest check on the bike
The formula is a planning tool. Park Tool sizes a new chain by wrapping it around the largest ring and largest cog while bypassing the derailleur, finding the closest rivet where inner plates meet outer plates, then adding two rivets, or four rivets on a dedicated 1x with a 42T or larger cassette. Do that check on the real bike before the chain tool bites. A chain that is a little long can often be lived with. A chain that is one inch short can break a derailleur.Cut after the bench wrap
If the snug wrap lands outer plate on outer plate, that rivet cannot join. Step one rivet to a joinable reference, then add the extra rivets. Install half a master link first if the chain uses one, because that plate is already half an inch of length.# Wide cassettes and full suspension
Compact derailleur
Largest cog 36T or smaller. Add two rivets after the wrap so the cage can clear the pulleys in the largest pairing.
Wide range 1x
Largest cog 42T or more. Add four rivets. The extra length keeps the parallelogram from folding flat.
Moving rear end
Compress the shock to bottom out, or add measured chainstay growth, then size. Sag is the wrong length.
Do not skip the bike check
Chain guides, clutch derailleurs, aftermarket pulleys, and unusual chainstay shapes can move the cut. This page does not inspect your frame. Treat the even link count as a starting length, then confirm largest to largest.
# Singlespeed and fixed gear chains
Without a derailleur there is no cage to swallow extra links. The calculator therefore adds zero rivets and still reports an even count. Final tension comes from horizontal dropouts, an eccentric bottom bracket, an idler, or a half link. A fixed gear cannot use a sprung tensioner. If the even length is a few millimetres tight or slack, that is a frame adjustment problem, not a reason to guess an odd rivet count.- Half link / rivet
- One 12.7 mm pitch step. Chains are joined on even counts so inner plates meet outer plates.
- Chainstay
- Centre of the bottom bracket to centre of the rear axle. This is the CS length in the wrap formula.
- Master link
- A reusable outer pair that adds half an inch. Fit one half before you mark the reference rivet.
Workshop rules that keep derailleurs alive
Plan from the largest ring and the largest cog, never from the gear you ride most.
Add two rivets for compact cassettes and four rivets for 42T and wider 1x drums.
Size full suspension at the longest chainstay, not at sag.
Round to an even joinable count, then prove it with a largest to largest wrap before cutting.