How many links does my chain need?
Enough to wrap the biggest chainring and the biggest cog with some spare for the derailleur. Park Tool's method: run the chain around the big ring and the big cog, not through the derailleur, and add two links (one inch). On a 1x drivetrain with a 42-tooth or bigger cog, add four. The calculator does that arithmetic exactly and rounds up to an even number, because a chain joins outer plate to inner plate. For the usual chainstays:
| Drivetrain | 405 mm stay | 420 mm stay | 435 mm stay | 450 mm stay |
|---|---|---|---|---|
| Road compact 50/34 × 11-34 | 108 links | 112 links | 114 links | 116 links |
| Road 53/39 × 11-30 | 108 links | 112 links | 114 links | 116 links |
| Gravel 1x 40 × 10-44 | 110 links | 114 links | 116 links | 118 links |
| MTB 1x 32 × 10-52 | 112 links | 114 links | 116 links | 118 links |
Exact big-big wrap plus Park Tool's allowance (2 links; 4 on 1x with a 42T+ cog), rounded up to even. Road chainstays run about 405–415 mm, gravel 420–435, trail bikes 430–450.
The chain length formula, and the shortcut most calculators use
Most chain calculators print the same shortcut: L = 2C + F/4 + R/4 + 1, with the chainstay C in inches, F and R the biggest ring and cog, and L in inches. That's two chainstays, half the teeth on each sprocket (half an inch per tooth, half the sprocket wrapped), plus one inch of allowance. The exact geometry for two sprockets, the standard used for industrial roller chain, adds one more term for the angle of the chain between different-sized sprockets: ((F − R)/2π)² ÷ (C in pitches). On bikes that term is tiny, at most 0.49 links in the cases below.
| Setup | Exact big-big wrap | The extra term | This calculator | Bare shortcut |
|---|---|---|---|---|
| 53/39 × 11-28, 410 mm | 105.6 links | 0.49 | 108 | 108 |
| 50/34 × 11-34, 415 mm | 107.6 links | 0.20 | 110 | 110 |
| 1x 42 × 11-42, 425 mm | 108.9 links | 0.00 | 114 | 112 |
| 1x 32 × 10-52, 435 mm | 110.8 links | 0.30 | 116 | 114 |
"This calculator" = exact wrap + Park Tool's allowance, rounded up to even. "Bare shortcut" = 2C + F/4 + R/4 + 1 inches, rounded up, with no 1x adjustment.
The shortcut's geometry is fine: on a road double it lands on the same 108 links as the exact formula. It fails only where its built-in "+1 inch" is the wrong allowance: on 1x 42 × 11-42 and 1x 32 × 10-52 it comes out 2 links short, because Park Tool asks for two inches of spare with a 42-tooth or bigger cog. A chain two links short on a 52-tooth cog can over-stretch the derailleur the first time you shift into big-big.
What about full suspension bikes?
On most full suspension bikes the distance from the crank to the rear axle changes as the suspension moves, and the chain has to be long enough at its longest point. Let the air out of the shock (or remove the spring), cycle the rear wheel through its travel in the big-big gear, and size the chain where it's tightest, which varies with the suspension design. Frame makers often publish the right chain length for their bike; if yours does, use it.
Shift to the big ring and big cog. The derailleur cage should be stretched forward but not straight, with room to move. Then shift to the small ring and small cog: the chain shouldn't sag or touch itself, and the cage shouldn't fold back. If both can't be true, the derailleur doesn't have the capacity for the drivetrain: capacity needed = (big ring − small ring) + (big cog − small cog). Enter your derailleur's rated capacity above and the calculator checks it.
How to size a singlespeed or fixed-gear chain
With no derailleur, the chain has to fit the ring, cog and chainstay exactly, and a whole number of links rarely does. Horizontal dropouts or track ends solve it: fit the next even length and slide the wheel back until it's tight. The calculator works out how far. For a 405 mm chainstay:
| Gear | Exact links | Chain | Slide the axle back |
|---|---|---|---|
| 42 × 16 | 93.3 | 94 links | 4.4 mm |
| 42 × 17 | 93.8 | 94 links | 1.4 mm |
| 44 × 16 | 94.4 | 96 links | 10.2 mm |
| 46 × 17 | 95.9 | 96 links | 0.3 mm |
| 48 × 18 | 97.5 | 98 links | 3.2 mm |
Same exact formula, rounded up to even. A half link allows odd counts and halves the worst-case slide. Vertical dropouts need a "magic gear" that happens to fit, or a chain tensioner.
Still choosing the gear? The gear inches calculator lists every ring and cog that makes nearly the same gear, so you can pick the one whose chain fits.
Which chain should I buy, and when do I replace it?
Buy the shortest chain that's long enough, in the right speed for your drivetrain. Common lengths are 114, 116, 118, 120, 126 and 138 links, and the calculator tells you how many to remove. Keep the removed links for a trailside repair. A new chain starts wearing from day one: measure it with a chain checker and replace it at 0.5% wear on 11- and 12-speed (0.75% on 10-speed and below), before it wears the cassette. The maintenance schedule works out how often to check for the miles you ride. When you replace it, count the old chain's links before you cut the new one, unless you've changed the cassette, the chainrings or the frame.