My fixed gear bike is going to want a new chain soon, how do I figure out which width I should use?
There two sizes of chains that can be used on a fixed wheel bicycle 1/8" and 3/32". It all depends on the width of the teeth on the cog and the chain ring. The most common one is 1/8" as that is what is used on the track and BMX. On bikes that have been converted from gears to fix are more likely to be 3/32". My Son's bike has a 3/32" cog, chain ring and chain.
Get a pair of calipers and measure the width of both the chainring and the cog. If the width is much larger than 3/32" then use 1/8".
If you don't have calipers, but have a piece of 1/8" chain, then put the chain on the cog and the chainring and see if there is a large visible gap between the plates of the chain and the teeth, if so then can probably use a 3/32" chain, if it's pretty snug you probably need a 1/8" chain. If you have a 3/32" chain instead, then put it on both cog and chainring, if the teeth fit through the holes in the chain and the chain rests evenly on the cogs, you're probably fine with a 3/32" chain, if it sits unevenly at all, you probably need 1/8".
If you don't have a chain, then you could check for width markings on your chainrings and fixed cog, if they are single speed chainrings they tend to have a width marking on them (to distinguish between 1/8" and 3/32" sizes).
Keep in mind, If either the rear cog or front chainring is larger than the other in width, you need a larger 1/8" chain (which really won't run poorly on a smaller chainring, but a smaller 3/32" chain just won't fit on a 1/8" cog or chainring).
Also, If you're buying a 3/32" chain, you'll probably find that as a 6/7/8 speed chain at most bike shops. You probably don't want to try to use a 9 or 10 speed chain on a single speed bike. 1/8" chains are used exclusively for single speed setups.
In addition to the other answers, I found it helpful to read that the nominal width of a chain (1/8 or 3/32) actually refers to the width of the sprocket. Quote:
There is some confusion in these numbers because the actual width of a 1/8" sprocket is typically 1/8"(3.175 mm) and the gap between the inner side plates of the chain must be slightly wider to fit over the teeth. The width of the teeth on derailer-equipped bicycles with 5 or 6 rear sprockets was traditionally 2 mm, and the 3/32" (2.30 mm) chain would fit over those teeth -- but the smaller widths with larger number of sprockets are not as well standardized.