1/8 VS 3/32

The fraction is the inside width of the chain — the gap the sprocket tooth sits in. 1/8″ is 3.18mm, 3/32″ is 2.38mm. Track drivetrains are usually 1/8″; anything that came off a geared bike is usually 3/32″. On a fixed gear both end up on the same shelf, so the question comes up constantly.

Which direction mixes

A 3/32″ chain will run on 1/8″ teeth. A narrow gap over a narrow tooth is fine — the tooth simply does not fill the chain. The reverse does not work: a 1/8″ chain has plates too far apart to engage a 3/32″ tooth properly, and on a derailleur-spaced cassette it does not fit between the sprockets at all.

So the only mix worth discussing is wide teeth, narrow chain.

What changes when you mix

Lateral play. The chain sits on the tooth with room to move sideways, and on a geared bike that slack is absorbed by the derailleur and never noticed.

Fixed gear removes the thing that was hiding it. Stop pedalling and the drive load reverses; skid and the load goes sideways as well. Play that was invisible on a road bike becomes the margin between the chain staying on and the chain coming off. That is why the default answer for fixed is 1/8″ on all three parts — chainring, chain, cog.

Chainline matters more than either

Matching widths does not save a drivetrain whose front and rear centres are offset. The track figure is around 42mm, and a few millimetres out shows up as noise first and a thrown chain later.

Chainline calculator — it takes measured values only. We do not estimate chainline from spindle length, because the estimate is wrong often enough to be worse than no number.

Half-link chains, briefly

Half-link (“magic link”) chains exist in both widths and let you adjust length in half-inch steps, which is useful on a frame with short track ends. They are not a third standard — a half-link 1/8″ chain is still a 1/8″ chain, and the same mixing rules apply.

Checking a specific combination

The fit check returns compatible with a caveat for the wide-teeth/narrow-chain case rather than a plain yes, with the reason attached — because “it fits” and “it is the right choice” are different answers. The widths themselves are at 1/8″ and 3/32″.