"What's better, a compact or a standard crank?" is one of the questions that pops up every once in a while and often lights a lively debate about the details that confuses the novice cyclist rather than help with his decision, so I figured it would be nice to have the basics, that are the same for everybody, covered, so that everybody can then discuss on the same page.

So, what's the difference?

That's pretty easy, actually. The only difference is the bolt circle diameter (bcd), sometimes called pitch circle diameter.

A standard crank has a bcd of 130mm, a compact crank has a bcd of 110mm. That means two different things. First is, that you cannot mount a 130mm chainring to a compact crank, and vice versa. The second is that you can not mount a chainring smaller than 38t to a standard crank, as the bolts would have to be higher than the teeth.

On the other hand you can mount a 53t chainring to a compact crank, if the chainring has a 110mm bcd. So you can say that a compact crank is more versatile.

The possible downside is that the distance from the bolt to the teeth that the chain has to climb to while shifting to the big ring, is greater, so that the lever is longer and more area to flex over, which, in theory, would make shifting of a 53/39t chainring combination on a compact crank "worse" than on a standard crank. If that turns out to be noticeable in the real world depends on what you compare to, what your shifting technique is like and how you perceive your shifting.

Any major problems would most likely be due to a factor other than a difference in bcd, like comparing a very stiff Dura Ace 7900 chainring to a dremeled down super light chainring, or a mal-adjusted derailleur.

In practice the only true "problem" is rather that the choice of chainrings with 53 teeth and a 110mm bcd is limited and you might not find one that you like the looks of or feel like paying the price that's being asked for it, as it's usually a specialty product and not massed produced.

Now that you know the difference, which one is better for you?

This answer is definetly more complicated, as it's not just a matter of what will fit with what, but it actually depends on you, your power output and your self selected cadence and is something that the nice people on this forum are able to help you with, once you know the basics.

Right now, this is pretty much just a draft and I invite people to point out mistakes, help make things even clearer and whatnot and I'll edit this post.