There are several relevant measurements for a chain.
Pitch: distance between the rollers - always the same (1/2") on modern bikes.
- Internal width: distance between insides of the plates - almost always the same (3/32" for the common derailleur-equipped bike, 1/8" otherwise).
Bikes with one rear cog, track, singlespeed and internally geared bikes traditionally use 1/8" chains. A 1/8" chain will work on a 3/32" cog but will not work on a derailleur bike. A 3/32" chain will not work on a 1/8" chainwheel and sprocket.
- External width: greatest outside width of the chain and is generally the width of the pins that go through each roller and connect the plates. This width varies for 3/32" chains, and the width required depends on the number of speeds that the chain needs to work with on the rear cog cluster. eg 8,9,10 speed.
The width of the chain basically depends on the number of cogs in back (speeds) since clusters with more speeds have their cogs spaced closer together.
In general 8 speed chains (7.1mm wide) can be used on 5,6,7 and 8 speed bikes, whilst 9 speed chains (6.8mm) and 10 speed chains ( 6.2mm) are best used on 9 & 10 speed drivetrains.
- Length - the total end-to-end length of the chain, which can be varied by adding/removing links.
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