How to use the calculator
- Pick your drivetrain. The number of links you add differs between road, Shimano mountain and SRAM.
- Enter your chainstay length. It is the distance from the center of the bottom bracket to the center of the rear axle, listed as chainstay in the manufacturer’s geometry chart. If you do not have it, measure center to center with a tape measure; a 5 mm error barely changes the result by one link.
- Enter your biggest chainring and cog. On a double, the big ring; on the cassette, the largest cog.
- If your bike has full suspension, turn it on and measure the chainstay at the point where the rear axle is farthest from the bottom bracket (with the shock deflated and the wheel moved up to that point).
- Read the result and confirm it on the bike before cutting: it is easier to take links out than to put them back.
Below, you can check the wear on the chain you have on now.
How we calculate it
The big-big method (Shimano and SRAM)
This is how the manufacturers size a chain: wrap it around the biggest chainring and biggest cog without going through the derailleur, bring the ends together where they overlap and add a few links of slack. The calculator does the same with geometry: the chain is a belt wrapped around two sprockets.
Wrap (mm) = 2 × √(C² − (R − r)²) + π × (R + r) + 2 × (R − r) × arcsin((R − r) ÷ C)
R and r = chainring and cog radius = 12.7 ÷ (2 × sin(π ÷ teeth))
C is the chainstay and 12.7 mm (half an inch) is the chain pitch. We divide by 12.7 to get links, round up to the next even number (an inner link always goes with an outer link) and add what each manufacturer calls for:
| Drivetrain | Links to add | Source |
|---|---|---|
| Road and gravel 2x (Shimano, SRAM AXS 2x) | +2 | Shimano DM-CN0001; SRAM |
| Shimano 12-speed 1x, hardtail | +4 to 5 (with QUICK-LINK) | Shimano DM-MARD001 |
| Shimano 12-speed 1x, full suspension | +5 to 6, with the suspension extended | Shimano DM-MARD001 |
| SRAM 1x (Eagle AXS, XPLR), hardtail | +4 (two inner and two outer) | SRAM |
| SRAM Eagle AXS, full suspension | +2, with the suspension compressed to its longest point | SRAM |
| SRAM Eagle Transmission | Its own chart by chainstay and chainring | SRAM |
Where Shimano gives a range (4-5 or 5-6), we use the even value, which is the one that lets you close the chain with a quick link.
SRAM Eagle Transmission is different
With Transmission (T-Type), SRAM does not use big-big. It publishes a chart that crosses chainstay length, measured with the suspension fully extended, with chainring size (30 to 48 teeth). Values run from 112 to 136 links, and above 126 you need two chains. The SRAM AXS app also calculates it for many frames. The calculator shows you the classic formula as a guide, but use SRAM’s chart.
The classic formula
Before giant cassettes, bike shops used this approximation, in inches:
L = 2 × C + (chainring + cog) ÷ 4 + 1
Every inch is 2 links. At heart it is a simplified big-big plus 2 links, and on road bikes it gives the same result as the Shimano method. With cogs over 42 teeth it stops matching, because the chain no longer runs nearly straight.
Reference chain lengths
Calculator results for typical bikes. Your frame may differ by 2 links either way. More chainstays and combinations in the chain link chart.
| Bike | Chainstay | Big ring × big cog | Links |
|---|---|---|---|
| Road, 52/36 with 11-30 | 405 mm | 52 × 30 | 108 |
| Road, 50/34 with 11-34 | 410 mm | 50 × 34 | 110 |
| Road, 48/35 with 10-36 | 415 mm | 48 × 36 | 110 |
| Gravel 1x SRAM, 40 with 10-44 | 425 mm | 40 × 44 | 114 |
| Gravel 1x Shimano, 40 with 10-51 | 430 mm | 40 × 51 | 118 |
| Shimano hardtail mountain bike, 32 with 10-51 | 430 mm | 32 × 51 | 114 |
| Shimano full-suspension mountain bike, 32 with 10-51 | 435 mm | 32 × 51 | 118 |
| SRAM Eagle hardtail mountain bike, 32 with 10-52 | 435 mm | 32 × 52 | 116 |
When a chain is worn out
A chain does not “stretch” the metal: the pins and rollers wear, and the distance between links grows. A worn chain eats your cassette and chainrings, which cost a lot more. When you buy the new one, make sure it matches your number of speeds in the chain width by speed chart.
All you need to measure it is a ruler: 24 links of a new chain measure 12 inches (304.8 mm) from pin center to pin center. Park Tool recommends replacing it at:
| Speeds | Replace at | Length over 24 links |
|---|---|---|
| 11, 12 or 13 | 0.5% | 12 1/16 in (306.3 mm) |
| 5 to 10 | 0.75% | 12.09 in (307.1 mm) |
| Single speed | 1% | 12.12 in (307.8 mm) |
A chain checker does the same thing faster: if the 0.5 tab drops all the way in, it is time to replace the chain.
Common mistakes
- Copying the length of the old chain. If it was ever cut wrong or you changed cassettes, you carry the error over.
- Measuring the chainstay on a full-suspension bike without compressing it. The distance grows as the suspension sinks; with a chain that is just long enough, the derailleur can break on a bump.
- Running the chain through the derailleur when measuring big-big. The method is without the derailleur; if you thread it through, you end up with extra links.
- Cutting to an odd number with a quick link: it will not close. If you get an odd number, go up to the next even one.
- Fitting a bigger cassette without checking the chain. Four more teeth on the big cog usually need 2 more links.
- Waiting until the chain skips. By then the cassette is usually worn too.
If you are changing your cassette or chainrings, first see how your gears will look in the gear ratio calculator.
Frequently asked questions
How many links are in a bike chain?
New chains usually come with 114 to 126 links. A road bike typically ends up at 106-112 and a mountain bike with a 50-52 tooth cassette at 114-120. It is always an even number if you use a quick link.
How do you figure out bike chain length?
The most reliable way is the manufacturers' method: wrap the chain around the big chainring and the big cog without going through the derailleur and add links. Shimano and SRAM add 2 on road bikes; on a 1x mountain bike hardtail you add 4.
What happens if a bike chain is too short or too long?
If it is too short, in the big ring and big cog it can pull the derailleur tight enough to break it or rip off the hanger. If it is too long, in the small gears it sags, the derailleur cannot take up the slack, and it skips or rubs.
When should I replace my bike chain?
According to Park Tool, once it has stretched 0.5% on 11-speed or more and 0.75% on up to 10-speed. With a ruler: 24 links measure 12 inches (304.8 mm) when new; if they go past 12 1/16 in (about 306.3 mm), replace it.
How do I size a chain for SRAM Transmission?
You do not use the big-big method: SRAM publishes a chart by chainstay length and chainring size (from 112 to 136 links), and the SRAM AXS app calculates it too. Measure the chainstay with the suspension fully extended.
Sources
- Shimano: chain dealer's manual DM-CN0001
- Shimano: 12-speed MTB rear derailleur dealer's manual DM-MARD001
- Shimano: how to determine chain length
- SRAM: how do I size a chain for a SRAM Eagle AXS drivetrain?
- SRAM: how do I size a chain for a SRAM eTap AXS 2x drivetrain?
- SRAM: Eagle Transmission frame fit specifications, chain length chart
- Park Tool: chain length sizing
- Park Tool: when to replace a chain on a bicycle
- Park Tool: CC-3.2 chain wear indicator