I broke my chain and I am trying to find a new one to buy. I have tried looking though had no luck.
I have a 2014 giant boulder with 3 at the front and 8 at the back. So all I need is a size.
You need an 8 speed chain that is 3/32" wide. This is a "normal" bicycle chain. Often sold as a 6/7/8 speed. This chain will be somewhere between 112 and 118 links long.
You also need the old chain to figure out how many links it had. Lay them out side by side, and notice the old chain will appear to have longer links due to wear. Figure out which pin on the new chain lines up with the **same-number* of links on the old chain. Then use a chain breaker tool to mostly-remove a pin and take off the excess links.
You might prefer to use a quicklink to rejoin the chain, or refit the pin that is mostly-out.
Do not bother getting a 9, 10 or 11 speed chain. Its a waste of money for an 8 speed bike cassette.
However depending on the wear of your cassette, it may be worth replacing that too. Old chains and old cassette wear together, and just replacing the chain may give you chainskip. If that happens, time for a new 8 speed cassette.
The video says that you have to pass the chain for the big chainring and the big sprocket. Without going through the pulley wheels for rear derailleur. This gives the basic measure.
If your bike has double or triple front chainrings, you must add a full link (internal link and external link) to basic measure (minute 1:16 in the video). In case your bike has only one front chainring, you must add two full linlks (internal and external link) to basic measure (minute 1:28 in the video) for those bikes that only carry a front chainring.
As the saying goes, 'a picture is worth more than a thousand words'. In the video you can find the answer to how to measure the length of the string. As well as how to carry out the entire process.
I am not related in any way, neither with the store nor with the creators of the video.
Best regards Good luck!
The chain width is the most critical factor. Chains will almost assuredly be long enough out of the box and will need to be cut to length.
It is more than a matter of a 9, 10, or 11 speed being a waste of money - the wrong chain width may not work properly for your bike. It is important that you ask your retailer for an 8-speed chain.
Now, as far as the length goes, Criggie is correct, the easiest way (if you trust the previous installer) is to compare against your previous chain. However, if you loose your chain on a trail (as I have), all is not lost.
With a three ring in front, your chain should barely be long enough to fit on the largest ring in the front and the largest ring in the back. You should never use this combination when riding so the fit will cause the dérailleur to be fairly tight. On the other end (little ring to little ring), your dérailleur should still keep tension in your chain. If it doesn't you may not have a long enough dérailleur or its tensioning needs increased.
Putting the chain back on can be tricky if it doesn't contain a master link. One technique that I've learned over the years it to use a wire tie to hold the chain together while wrestling with my chain tool. If you do have to use a chain tool to push the pin in - the chain in that area will not articulate well. Bending it side to side (yes, side to side), will loosen it up.
Good luck to you.