#1
Well, it seems my acoustc's intonation is out, but only on one string, the high E.

I was wondering what could've caused this, and what can be done to solve it? I swear I never used to have this problem, and when the string is in tune open, the 12th fret is almost a semitone above.

Any ideas guys? I haven't changed the strings in a long time, could that be it?
#2
Your obvious starting point is going to be to change strings and see how it is from there. No sense in trying to nail down intonation prbs with old strings on. Stick with the same gauge of string too otherwise you might compound the problem.
#4
Sure. Measure them. But in order to do that, you'll need a micrometer or caliper. Something that will accurately measure down to 1000th of an inch (that's three places to the right of the decimal point by the way). Actually, you should only need to measure the high E string's thickness. If it's a .012, then you've got a set of .012's on it. .010 = .010's and so on. They could be a custom set, but for the most part just stick with the basic set.
If you can't measure the strings, then just buy a set of .012's. They are the most common for acoustics. They're also called "Lights", meaning light tension vs. medium or extra light tension. And here are a couple of pics of the tools used to measure things that small, in case you were wondering, and are available in english or metric:

This is a micrometer


This is a dial caliper


Both are available with digital readouts, although they can get quite expensive. Either borrow one, or just put .012's on and see what happens.
#5
Quote by LeftyDave
Sure. Measure them. But in order to do that, you'll need a micrometer or caliper. Something that will accurately measure down to 1000th of an inch...


If you can't do that, just put 11s on and see how it sounds. You wont hurt anything.
#6
Quote by GC Shred Off
If you can't do that, just put 11s on and see how it sounds. You wont hurt anything.


Chances are 12's are on them. Most guitars are set up for 12's. Probably best sticking with 12's.
#7
Intonation is all based on the length of the string. If you somehow raise the nut or saddle it will change the length of the string. this in turn changes the point at which the string is in tune. The frets on your neck area all set up for a certain length. Change the length and the string will be in tune on one end and not the other. Take it to your local guitar tech and have him set it up properly. You won't be disappointed. It will play easier too.