Bike Chain Length Calculator
How many chain links your bike needs, from chainstay length and your largest ring and sprocket — plus the wrap method that gets the cut right.
Chainstay is measured from the centre of the bottom bracket to the centre of the rear axle — not along the visible tube.
Now check it on the bike
The formula tells you what to buy. The wrap method tells you where to cut, and it is the one every workshop uses — because it measures the bike in front of you rather than assuming a chainline, a derailleur cage and a pivot position.
The input you are least sure of is the one the formula listens to most
Walk each input and watch the answer move. Starting from a 425 mm chainstay with a 50-tooth ring and a 34-tooth sprocket:
| Change | Links | Difference |
|---|---|---|
| chainstay +5 mm | 112 | none |
| chainstay +10 mm | 114 | +2 |
| chainstay +13 mm | 114 | +2 |
| chainstay +20 mm | 116 | +4 |
| chainring +1 tooth | 112 | none |
| chainring +2 teeth | 112 | none |
| chainring +4 teeth | 114 | +2 |
| chainring +6 teeth | 114 | +2 |
A millimetre of chainstay is worth 0.157 of a link and a tooth is worth 0.5. Turned round: it takes 12.7 mm of chainstay to move the answer, or 4 whole teeth. Every change comes in twos, because a chain cannot end on half a link.
Which is uncomfortable, because the teeth are a number you count and cannot get wrong, while the chainstay is measured with a tape between two centres you cannot see — the middle of the bottom bracket and the middle of the rear axle, both buried inside the parts around them. A centimetre out is easy and it changes the result; a whole extra sprocket on the chainring may not.
That is the argument for the wrap method, stated as a number. The formula's weakest input is the only one it really depends on.
And it never lands exactly
Across 2,556 realistic combinations — chainstays from 390 to 460 mm against six chainrings and six sprockets — the raw figure needed rounding up every single time.
| Outcome | Combinations | Share |
|---|---|---|
| Raw figure needed rounding up | 2,556 | 100% |
| Then needed a second bump for parity | 1,278 | 50% |
The trailing inch in the formula is doing that: an inch is two links, and the rest of the expression rarely lands on a half. So the answer is always a rounded one, which is another way of saying it is a starting point rather than a measurement.
It rounds up rather than to the nearest, deliberately. A chain one link too long is an inconvenience you can see and fix. A chain one link too short can wreck a derailleur the first time it is shifted into the big ring and the big sprocket together, and it will do it without warning.
How to use
- Measure the chainstay from bottom bracket centre to rear axle centre.
- Enter your largest chainring and largest sprocket.
- Tick full suspension if the rear end moves.
- Buy that many links, then use the wrap method to find the cut.
Frequently asked questions
Why must the link count be even?
Because a chain alternates inner and outer plates, and a chain has to close on itself. Half links exist for single-speed and BMX use and are avoided on derailleur drivetrains, so any usable length is an even number.
Why does this round up rather than to the nearest?
Because the two failures are not equivalent. A chain one link too long is slightly slack in the smallest gears and shifts fine. A chain one link too short tightens solid when you shift into the largest ring and largest sprocket together, and can tear the rear derailleur off the frame. Rounding up costs nothing.
What is the wrap method and why is it better?
Run the chain around the largest ring and largest sprocket without threading the derailleur, pull the ends until they just meet, then add two rivets. That is the cut. It beats the formula because it measures the actual bike — the formula cannot see your chainline, your derailleur cage length, or where a particular frame puts its pivot.
Why does full suspension need extra length?
Because most rear ends grow as they compress, so the distance from bottom bracket to rear axle at full travel is longer than at rest. Size the chain with the shock deflated and the suspension bottomed out, or the chain comes tight on a big hit and something breaks.
Do I need a new chain or can I reuse this one?
A chain wears by elongating, and a worn one will chew a cassette that a new one would have left alone. Check with a chain wear tool before reusing — under 0.5 per cent elongation is generally fine, past 0.75 per cent the cassette usually needs replacing too.
Does chain speed matter here?
Not to the length. An 11-speed and a 12-speed chain of the same link count are the same length; they differ in width, and using the wrong width shifts badly or not at all. Get the length from this and the width from your drivetrain.
What if my measurement gives an odd number?
It is rounded up to the next even one, which is what you would do at the bench. If the figure sits very close to an even number, prefer the longer option — the wrap method will confirm it and slack is recoverable where a short chain is not.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.