Chain Length Calculator
A new chain that is two links too long slaps the chainstay and shifts sloppily, and one that is two links too short can tear a derailleur off in the big-big gear. This chain length calculator sizes a bicycle chain from your chainstay length and your largest chainring and cog, using the Park Tool formula, and returns the link count, the length in inches, and metric conversions. A second mode covers any plain chain by links times pitch.
Measure the chainstay from the center of the bottom bracket to the center of the rear axle. Common values: road 410 mm (16.1 in), gravel 430 mm (16.9 in), MTB 450 mm (17.7 in).
How it was calculated
Bike sizing uses the Park Tool formula L = 2C + F/4 + R/4 + 1 (inches), cross-checked against the precise geometric variant. Weight estimates in links mode are rough approximations. This tool sizes bicycle drivetrain chains and plain chains; for a boat, the separate anchor chain and rope length calculator works out rode length from depth and scope instead.
Quick Answer
Key takeaways
- Chain length is set by three numbers only: chainstay length, largest chainring teeth, and largest cog teeth. Cassette size changes the answer, marketing names do not.
- The link count always rounds up to an even number, because inner and outer plates must pair up for the chain to join.
- The Park Tool formula and the precise geometric formula rarely differ by more than one link pair. When they do, cut to the longer count and confirm on the bike.
- A new chain almost always needs links removed. Chains ship at 114 to 126 links; most bikes need 104 to 122.
- Quick Answer
- How to use this bicycle chain length calculator
- The chain length formula, explained term by term
- Worked example: sizing a chain for a 420 mm road frame
- Too long or too short: reading the symptoms
- Sizing steps, in the order a mechanic works
- What competing calculators miss
- Frequently Asked Questions
- Add This Calculator to Your Website
- References
How to use this bicycle chain length calculator
Start in the Bike chain sizing tab. Enter your chainstay length and pick its unit, then the tooth counts of your largest front chainring and largest rear cog. Press Calculate and the headline result is the link count to cut to, with the physical length in inches, millimeters, and centimeters underneath it.
Measure the chainstay from the center of the bottom bracket to the center of the rear axle, not to the edge of the dropout. If you do not have the bike in front of you, the frame maker's geometry chart lists the figure, usually in millimeters. Tooth counts are printed on most chainrings and cassettes; if the print has worn off, count the teeth on the biggest ring and biggest cog directly.
The Chain by links tab runs the calculation in reverse. Give it a link count and a per-link length and it returns the total length in three units with a rough weight estimate, which is handy for plain roller chain, a replacement conveyor chain, or checking a chain already hanging in the garage. Riders tracking the energy a ride costs can pair a correctly sized chain with the bicycle calorie calculator when planning longer days.
The chain length formula, explained term by term
The bicycle industry standard is the formula published by Park Tool chain sizing guidance: L = 2C + F/4 + R/4 + 1, with everything in inches.
The 2C term is the two straight runs of chain, top and bottom, between the cranks and the rear wheel. The F/4 and R/4 terms are the wrap around roughly half of each gear: a chainring's circumference in link terms is its tooth count, and about a quarter of the combined wrap enters this simplified form. The final +1 is the derailleur inch, the slack the rear derailleur cage needs to tension the chain through its pulleys.
The precise variant, 2 times the square root of (C squared plus (0.0796 times (F minus R)) squared), plus (F + R)/4, models the chain path as geometry instead of averages. The 0.0796 factor converts the tooth difference into the offset between the two gear centers. For most bikes the two formulas agree within one link pair, and this calculator is the only common one that prints both side by side so you can see the agreement yourself instead of taking it on trust.
Worked example: sizing a chain for a 420 mm road frame
The calculator is pre-filled with this bike, and every number below is what it returns.
| Step | Working | Result |
|---|---|---|
| Chainstay in inches | 420 mm / 25.4 | 16.54 in |
| Park Tool formula | 2 x 16.54 + 50/4 + 34/4 + 1 | 55.07 in |
| Raw link count | 55.07 / 0.5 | 110.1 links |
| Rounded to even | Round up to the next even number | 112 links |
| Cut length | 112 x 0.5 in | 56.0 in (1,422 mm) |
| Precise formula check | Geometric variant | 54.17 in |
The precise formula lands about one link pair below the Park Tool result, which is the normal pattern: the simple formula builds in a small safety margin through its derailleur inch. Cut to 112 links, then confirm on the bike. In the big-big combination the derailleur cage should sit angled forward with two slight bends in the chain at the pulleys, not folded flat and not stretched straight.
Too long or too short: reading the symptoms
Length errors announce themselves in the two extreme gear combinations, which is why mechanics check those first.
| Symptom | Likely verdict |
|---|---|
| Chain sags or slaps the chainstay in the small-small gears | Too long |
| Rear derailleur cage folds back until the chain nearly touches itself | Too long |
| Shifter refuses the biggest cog, or the derailleur looks stretched straight | Too short |
| Chain feels guitar-string tight in big-big | Too short |
| Skipping under power with a brand new chain fitted | Not length Worn cassette or chainring teeth are the usual cause |
One caution before blaming length for skipping: a worn cassette chews a new chain no matter how perfectly it is sized. If the old chain measured well past 0.75 percent elongation, budget for a cassette too. And if you are deciding whether the whole bike is worth the new drivetrain parts, the bicycle value calculator gives a realistic resale figure to weigh against the repair bill.
Sizing steps, in the order a mechanic works
Bottom bracket center to rear axle center, to the nearest millimeter. On a full suspension bike, note the frame maker's chain growth figure as well.
Largest chainring teeth and largest cog teeth. These two combinations, big-big and small-small, are where length errors show up.
Enter the three numbers in the calculator and note both the Park Tool result and the precise check. Cut to the even link count it returns.
Thread the chain big-big without the derailleur, pull snug, and confirm the cut point leaves the derailleur inch the formula promised. Then route it through the derailleur and join it.
What competing calculators miss
The best-known rival shows both formulas, and credit where due: its math is sound. The gaps are around the edges, and they are the edges where home mechanics actually get stuck.
Miss 1: chainstay in one unit only. Several ranking tools want the chainstay in inches and nothing else, so a metric geometry chart means a hand conversion before you can start. This calculator takes millimeters, centimeters, or inches and converts for you.
Miss 2: inches with no cut decision. A result of 55.07 inches is not something you can cut to. The useful answer is 112 links, so that is the headline here, with the physical length kept as supporting detail.
Miss 3: no second direction. Rival bike tools cannot answer the plain question of how long a 116-link chain actually is. The links times pitch mode answers it for any chain, with conversions and a weight estimate, which no bicycle-only rival offers.
One honest limit: no formula sees your derailleur's cage length or a frame's chain growth curve. Treat the result as the cut target, then make the final big-big check on the bike before joining the chain for good.
Frequently Asked Questions
Measure in a straight line from the center of the bottom bracket, where the crank axle sits, to the center of the rear axle. A tape measure read to the nearest millimeter or eighth of an inch is accurate enough, and most frame makers also publish the figure in the geometry chart for your model.
Most derailleur bikes land between 104 and 122 links: roughly 106 to 112 for road bikes, 110 to 114 for gravel, and 112 to 122 for mountain bikes. Your exact count comes from your chainstay length and your largest chainring and cog, which is what the calculator uses, so measure instead of copying a range.
A bicycle chain alternates inner and outer link plates, and the two ends can only join if the chain contains complete inner and outer pairs. An odd count leaves two ends of the same type facing each other, so the calculator always rounds up to the next even number.
Almost always. New chains ship with 114 to 126 links so one product fits many bikes, and most bikes need fewer than that. Size the chain with the calculator first, then remove the extra links with a chain tool and confirm the length on the bike in the big-big combination before the final join.
Wrap the chain around the largest chainring and the largest rear cog without threading it through the derailleur, pull the ends together until the chain is snug, then add two links, which is one inch, before cutting. It is the hands-on check mechanics use to confirm a formula result on the bike itself.
Yes. Rear suspension movement lengthens the distance between the bottom bracket and the rear axle as the wheel travels, which is called chain growth. Size the chain at the point of maximum growth, with the suspension compressed, or follow the frame maker's stated link count for your model.
It works the other way around: enter a link count and the length of one link, called the pitch, and it returns the total chain length in millimeters, inches, and feet, plus a rough weight estimate by chain type. It suits plain roller, jewelry, and other non-bicycle chains as well as checking a chain you already own.
Add This Calculator to Your Website
References
- Park Tool, the bicycle tool maker whose sizing formula this calculator uses, explains chain length sizing, the big-big method, and full suspension chain growth. Park Tool: Chain Length Sizing. Accessed October 2026.
- Icebike's chain length guide walks through matching an old chain, rivet-by-rivet counting, and why a worn chain reads longer than its true link count. Icebike: Bike Chain Length and How to Size a Chain. Accessed October 2026.
- Sheldon Brown's chain pages cover chain construction, the half-inch pitch standard, and wear measurement, the background for why link counts and pitch drive this calculator. Sheldon Brown: Bicycle Chains. Accessed October 2026.