Bike Chain Length Calculator

Check inputs

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Setup preset:

Choose a starting setup, then replace the sample numbers with the bike's measured values.

Chainstay length:

Use the actual axle-to-bottom-bracket distance, not the chainstay tube length.

Largest chainring:

Enter teeth stamped on the largest chainring, for example 50T or 32T.

teeth

Largest rear cog:

Enter the largest rear sprocket, for example 34T road or 52T wide-range MTB.

teeth

Drivetrain setup:

Pick the physical sizing rule closest to this bike before cutting the chain.

Suspension chain growth:

Measure or estimate the added chainstay length at full compression.

Custom allowance:

Enter the added link count required by the frame, guide, or manufacturer note.

links

Calculation basis:

Compare simple and rigorous outputs in the results before making the final cut.

New chain supplied:

Count the replacement chain or use the package length so the tool can produce a cut plan.

links

Final link rounding:

Use even-link rounding unless a manufacturer or half-link setup gives you a different rule.

Smallest chainring:

For 1x, enter the same value as the largest chainring.

teeth

Smallest rear cog:

Typical smallest cogs are 10T, 11T, 12T, or 13T.

teeth

Rear derailleur capacity:

Use the derailleur maker's stated total capacity if known.

teeth

Connector note:

Use this to make exports clear for quick-link or connecting-pin chains.

Bike chain length cut plan
Item	Value	Shop use	Copy
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Bike chain length fit checks
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A bicycle chain has to be long enough for the hardest physical path it may need to cover, but not so long that the derailleur cannot take up slack in smaller gear combinations. The critical setup is usually the largest chainring and largest rear cog. That path protects the drivetrain from damage if the rider shifts into the big-big combination, even if that combination is not used often.

Chain length is measured in links, and one complete link pitch is one half inch. Cutting one link too short can make the drivetrain bind, overload the rear derailleur, or prevent installation. Leaving several extra links can make shifting loose, increase chain slap, and reduce the derailleur's ability to control the chain. Modern replacement chains often arrive with 114, 116, 118, 120, or 126 links, so the practical shop question is usually how many links to remove from the supplied chain.

Diagram of a bicycle chain path around the largest front ring and largest rear cog with chainstay and added-link allowance labels

Chainstay length is the frame measurement from bottom-bracket center to rear-axle center. Front and rear tooth counts add wrap around the sprockets, while an add-link allowance gives the derailleur room to articulate. A two-link allowance is a common starting point for many derailleur drivetrains. Wide-range 1x and rear-suspension bikes often need more margin because the biggest rear cog is larger and the effective chainstay may grow as the suspension compresses.

Single-speed and BMX setups are a different problem. There is no rear derailleur to absorb slack, so axle position, dropout length, half links, tensioners, and exact center distance matter more than a conservative big-big allowance. On derailleur bikes, chain length also has to make sense beside total wrap capacity: the difference between large and small chainrings plus the difference between large and small rear cogs.

A calculated link count is a cutting target, not the last safety check. Connector style, manufacturer instructions, suspension position, and chain end pairing can change what a real chain tool should remove. Before cutting, wrap the chain on the actual bike and confirm the largest-chainring and largest-cog path.

How to Use This Tool:

Use the calculator as a shop-bench planning pass before the physical wrap check.

Pick a Setup preset such as Road or gravel 2x, MTB 1x wide range, Full-suspension MTB, Single-speed or BMX, or Custom setup. The preset loads realistic starting values that remain editable.
Enter Chainstay length and choose mm or inches. Measure from bottom-bracket center to rear-axle center, or use the frame geometry chart when the bike is not on the stand.
Enter the Largest chainring and Largest rear cog. These are the sizes that drive the primary chain-length calculation.
Select Drivetrain setup. Standard derailleur uses a smaller allowance, wide-range 1x and rear suspension use larger allowances, and custom exposes the Custom allowance field.
Choose Calculation basis and Final link rounding. Auto selects the common simple equation unless the rigorous center-distance equation is a better fit for short or unusual setups.
Enter New chain supplied so the Cut Plan can say remove links, use the whole chain, or buy a longer chain. Use Advanced fields for small-ring, small-cog, derailleur capacity, and connector notes.
Read Fit Checks before cutting. If the stock chain is short, chainstay length is outside normal range, or derailleur capacity is over the entered rating, resolve that warning first.

The Link Sensitivity Map shows how nearby chainstay and rear-cog changes affect the rounded installed links, which is useful when comparing frames, cassettes, or borderline stock-chain lengths.

Interpreting Results:

Installed total is the main number. It is the rounded chain length after the selected formula, add-link allowance, and connector-count note. Cut from supplied chain translates that number into a work order for the stock chain length you entered.

Simple equation is the compact estimate often used for normal derailleur setups.
Rigorous equation uses center distance and tooth-count difference more explicitly, which can matter on short stays or unusual front and rear tooth spreads.
Big-big verification describes the physical check to make on the bike before cutting.
Derailleur wrap capacity compares the entered drivetrain spread with the capacity rating you supplied.

A green cut plan does not prove every gear will shift well. It means the entered measurements produce a plausible link count. Confirm the actual chain path, suspension compression if relevant, connector instructions, and derailleur capacity before the final cut.

Technical Details:

Both formulas work in inches because bicycle chain pitch is one half inch. Chainstay length is converted from millimeters when needed, tooth counts are rounded to whole teeth, and the selected add-link allowance contributes one half inch per added link before the result is rounded to links.

Formula Core

The simple length estimate is:

L = 2 C + \frac{F}{4} + \frac{R}{4} + 0.5 A

L is chain length in inches, C is effective chainstay in inches, F is largest chainring teeth, R is largest rear cog teeth, and A is added links. Raw links are then L / 0.5.

The rigorous estimate uses the sprocket tooth-count difference in the center-distance term:

L = 2 \sqrt{C^{2} + {0.0796 \times (F - R)}^{2}} + \frac{F + R}{4} + 0.5 A

With a 410 mm chainstay, 50T ring, 34T cog, and +2 links, the simple estimate is about 54.8 inches, or 109.6 raw links. Even-link rounding returns 110 installed links.

Bike chain length rule and check fields
Field or rule	How it affects the result	Boundary to verify
Suspension growth	Adds to measured chainstay only for rear-suspension mode.	Use the longest effective axle path.
Add-link allowance	Adds practical chain length after the shortest big-big path.	Standard modes use +2 or +4 links; custom uses the entered value.
Even-link rounding	Rounds up to the next even installed link count.	Use for most closed-loop derailleur chains.
Wrap capacity need	Large ring minus small ring plus large cog minus small cog.	Compare against the rear derailleur rating.
Stock chain length	Does not change the target; it changes the cut action.	If stock links are less than target links, buy a longer chain.

Auto method selection favors the rigorous formula for single-speed setups, short chainstays, or large front-rear tooth differences. The JSON output records both the selected formula and comparison formula so repeat calculations can be audited later.

Accuracy Notes:

Chain sizing affects drivetrain safety, so measurement and installation checks matter as much as arithmetic.

Use the actual largest chainring and largest rear cog installed on the bike.
For full-suspension bikes, check the longest chainstay path through travel or follow the frame maker's service instructions.
Connector notes are planning aids; follow the chain manufacturer's quick-link or connecting-pin instructions before cutting.
Do not cut at a previously joined reinforced pin or damaged link.

Worked Examples:

Road 2x replacement: A 410 mm chainstay, 50T largest ring, 34T largest cog, and 116-link supplied chain produces an Installed total of 110 links with the simple formula and +2 allowance. Cut from supplied chain says to remove 6 links.

Wide-range 1x MTB: A 435 mm chainstay with 32T front and 52T rear in 1x large cog mode uses a +4 allowance. The target often lands close to a 116-link installed total, and the Large-cog 1x allowance check confirms that the larger allowance is active.

Short stock chain warning: If a full-suspension setup needs 120 installed links but New chain supplied is 116, the summary reports that the chain is short. Buy a longer chain or correct the stock-chain entry before cutting.

FAQ:

Why use the largest chainring and largest rear cog?

That combination creates the longest drivetrain path. Sizing for it helps prevent binding if the bike is shifted into big-big.

Why does the result round up to an even number?

Most bicycle chains alternate inner and outer link plates, so an even installed link count is the usual closed-loop cutting target.

When should I use the rigorous formula?

Use it for single-speed, BMX, short chainstay, or unusual tooth-spread setups where the compact simple equation may hide center-distance effects.

What if the supplied chain is shorter than the target?

Do not join a chain that is too short. The Cut Plan will say you need more links; buy a longer chain or verify that the entered stock length is wrong.

Glossary:

Chainstay: Distance from bottom-bracket center to rear-axle center.
Big-big: The largest chainring and largest rear cog path used for chain sizing checks.
Add-link allowance: Extra links added after the theoretical shortest path to give the drivetrain practical movement.
Wrap capacity: The amount of chain slack the rear derailleur must manage across gear combinations.
Raw links: The unrounded link count before the selected cutting rule is applied.

References:

How to Check Your Chain Length, Shimano.
Rear Drive System Technical Service Instructions SI-0026A, Shimano, July 2005.
Chain Length Sizing, Park Tool.