I usually have a bunch of chain sitting around, and at the very least if I’m ordering chain for a build I go from the chain specs of the original bike. But say I want to be legit engineering-like and spec a chain length while I’m designing the bike? I go here, to the awesome Sprocket and Chain Length calculator on the RB Racing site. Right? Who doesn’t love a calculator that tells you to round up if you get an odd number of links? It’s also going to tell you your ratio, and stuff like that.
You’re gonna need this too: Chain Breaker
I’ve broken more than my fair share of chain tools, so I’d suggest something pretty robust. You really don’t need to get one that’s a “rivet tool” either, as long as you use a masterlink. As far as that goes, I’ve been using the RK Racing Chain for the past few years, and have been fine with it. Chain, honestly, is chain, and you just want something that’s decent and will last. Gentlemen do not run chain that comes in colors.
OK, as long as we’re on the subject of chains, I typically will match my chain with my sprockets, and when one has to be replaced, I replace all three – front, rear and chain. It may be obsessive, but it’s served me well for the 45 years I’ve owned bikes. Also, don’t neglect the lube.
This stuff is what I’ve had in the shop for a while, and I use it for all of my bikes: Maxima Chain Wax. A good chain lube = a nasty mess on your bike, and running degreaser on the chain defies all logic and common sense on keeping your chain lubed properly. The light, not messy goey chain lubes? Not even worth the effort. An old timer, long ago told me to lube the chain after a ride – when it was all nice and warm. I have no idea if that’s the way to do it, but I’ve done it since then, and it seems to work fine… but when in doubt, lube the chain, warm or cold.