Bicycle Chain Length Calculator
Calculate the correct chain length for your bike
Why Correct Chain Length Matters
Getting your bicycle chain length right is critical for smooth shifting, drivetrain longevity, and rider safety. A chain that's too short creates excessive tension on the derailleur when shifted into the largest chainring and largest cog combination (the "big-big" gear). This can snap the chain, bend the derailleur cage, or even damage your frame's derailleur hanger.
Conversely, a chain that's too long will sag excessively in small-small gear combinations, leading to dropped chains, poor shift quality, and accelerated wear on chainrings and cogs due to inefficient power transfer.
How Chain Length is Calculated
The most widely accepted method for determining chain length uses a mathematical formula that accounts for three key variables: chainstay length (the distance between your bottom bracket and rear axle), the size of your largest front chainring, and the size of your largest rear cog.
This formula ensures the derailleur has enough capacity to tension the chain properly across all gear combinations while providing a small safety margin to prevent over-tensioning in extreme gears.
The Standard Formula
The industry-standard formula for chain length is:
L = 2(C) + (F/4) + (R/4) + 1
- L = Chain length in links
- C = Chainstay length (converted to chain links)
- F = Largest front chainring (teeth)
- R = Largest rear cog (teeth)
- +1 = Safety margin for derailleur wrap
Example Calculation
Chainstay: 410mm, Largest chainring: 50T, Largest cog: 32T
First, convert chainstay to inches: 410mm ÷ 25.4 = 16.14 inches
Then convert to links: 16.14 ÷ 0.5 = 32.28 links
L = 2(32.28) + (50/4) + (32/4) + 1
L = 64.56 + 12.5 + 8 + 1 = 86.06 links
Round to nearest even number and add 2 links: 88 links
Chain Link Basics
Bicycle chains are measured in links, with each full link consisting of an inner plate pair and an outer plate pair. A standard bicycle chain has a pitch of 0.5 inches (12.7mm) between pins.
Because chains are constructed in pairs, you must always use an even number of links. This is why the formula result is rounded to the nearest even number. If your calculation yields 87 links, you'd round to 88.
The additional 2 links added to the formula result provide adequate derailleur cage wrap and prevent the chain from being stretched to its absolute limit in big-big combinations, which cyclists should avoid anyway for efficiency reasons.
Different Methods
The Big-Big Method
An alternative physical measurement method involves routing the chain around the largest chainring and largest cog without threading it through the derailleur. Find the point where the two ends meet with minimal tension, then add two full links (four half-links). This empirical method can be useful for unusual frame geometries or when you don't have exact measurements.
However, the big-big method requires you to have the bike fully assembled and doesn't account for derailleur capacity differences. The formula-based approach is generally more accurate and allows you to order the correct chain length before installation.
Limitations
While this calculator provides an accurate starting point, always verify the result physically before cutting a new chain. Factors like unusual frame geometry, high-capacity derailleurs, or aftermarket chainring sizes can affect optimal chain length.
After installation, shift through all gear combinations to ensure the derailleur cage doesn't hyperextend in small-small gears or become over-tensioned in big-big combinations. Fine-tune by adding or removing two links (one full link) if needed.
Frequently Asked Questions
How do I measure chainstay length?
Measure from the center of the crankset spindle (bottom bracket) to the center of the rear axle. Most bike manufacturers list this spec in millimeters.
Why is chain length important?
A chain that is too short can snap or damage the derailleur when shifting into big-big gears. A chain that is too long will sag and cause poor shifting or chain drops.
Should I always add 2 links?
The formula includes a 1-link safety margin, which gets rounded to 2 links (even number). This provides adequate derailleur wrap and prevents over-tensioning.
Does this work for single-speed bikes?
No, this calculator is designed for derailleur systems. Single-speed bikes require a different chain tensioning approach based on horizontal dropout adjustment.