Here is a good tip for people changing strings on their electric guitar. Before you cut the strings in half and remove them, do make sure you have a full spare set! Especially if it is Saturday afternoon and all the shops are shut till Monday. I am feeling a little silly - I must have already used the B. 
Luckily I had a spare acoustic set and was able to pinch the B string from that - the gauge seems much the same and it sounds fine.
oops. Thatās a pain.
Lol.
To avoid that, i usually order 2 boxes of my favourite strings.
Pro tip: keep a set in your guitarcase, because you never know when a string will brake when your somewhere else.
Iām a little annoyed because I usually buy the DāAddario 3-packs. But they come sealed in these anti-corrosion plastic packs that have no indication at all on them of what is inside. And the acoustic and electric packs look identical.
Iāve decided I need to buy different brands for electric and acoustic.
A good tip is donāt cut your strings in half. Remove them one at a time and roll them up, put them back in the envelope that you take the replacement out of.
If you break just put the old string back on until you can get a new one.
There are places you can send your old strings to where they give them to musicians who canāt afford strings.
I bought 2 packs of Elixirs size 10s and opened one box. I ended up Changing to 12s for the next string change a few months later. So I never opened that second box. About a year after that. I got a different acoustic and wanted to put on the 10s so when I open that second box from the prior year there was no G string. There were only 5 sets of strings. Since I had just torn the top off, I could see the seal on the back opening was cleaverly put back on after being opened prior, but you could see it was a little off. I suspect it was a return or a employee. I sucked it up and just put the old G string back on temporarily.
Totally lame.
I used to do this, but for some reason I went over to cutting them. Probably because it was easier and faster. But I agree itās a good idea to keep them. And also to label everything!
Hereās my lesson from changing the strings on my Taylor this week. Double check which string envelope youāre picking up.
I had removed all the strings, cleaned the body, oiled the fretboard, polished the frets, tightened the nuts on the tuners, and started to put the strings on. I put the low E string envelope by the guitar, then realized it was time to feed the dog (because he was staring at me.)
So I went upstairs and fed him. When I came back down I grabbed the envelope on top of the pile, took the string out, secured the ball end with the bridge pin, them determined the proper length to give me the correct amount to wrap around the capstan, clipped it and started to wind it. Then I saw that the low E string was still in its envelope where I left it. I had clipped the A string to a length too short for its proper position.
Fortunately, I had another set of acoustic strings, so I took the A string out of that set and finished the job. But I now have a set of strings thatās one string short. 
Oops. Well, at least you can blame the dog!
I removed the strings from my guitar the evening before delivery of the new ones, so I could take the time to maintain everything in advance. Stripped it down and cleaned/polished everythingā¦ body, neck, frets, tuners, each individual saddle, even the screws holding the pick guard and pickups in place.
That was a week ago, and my shiny guitar is sitting there gathering dust while waiting for the elusive strings to arrive. 
how do you know what size strings to get, I have an acoustic that I got from someone I know and I donāt know the size
ooh thatās not fun me personally I would wait until whatever I ordered to arrive before doing work on anything so that it does not become a problem or anything, I hope they come soon
What brand of guitar? Do a search for the specs on your particular brand and model and you should be able to find what string gauges were originally on the guitar from the factory. 12s are pretty common on acoustics from the factory.
okay thanks for the info
Yeah good question.
But really @markr31 has it right you just basically look it up. I was looking up for a Luna. I went to the site and looked up what the string size was it came with and that became my base line. I also got some other good info like the nut material and also the specific woods types used and where. Pretty handy if I need any future patch work. But yeah, differemt strings for different things. Big question.
Most acoustics will handle the normal string gauges just fine. The only worry would be some boutique models or something very old in most cases.
Looking up what the manufacturer recommends or ships the guitar with, if that information is available, is a good place to start. Then you can decide on what you like the best. If you play mostly electric (or classical) where the strings you are accustomed to tend to be lower tension, you may want 11s on your acoustic and they do have brands that are lower tension (Straight up, Newtone Heritage, Martin Tommy Emmanuel, Curt Mangan and others). Too low tension and the guitar may sound a little thin, too high and you may find than hard to play.
Lots of choices and strings are relatively cheap fun. I have a very bright acoustic and like to find strings that are warmer that complement it. My Classical is the opposite.
i took off (most of the way) my old strings and put them back on (i was bored, donāt question it) and they sounded brighter, is there any reason for that, it was in tune before and i tuned it with the same app and everything.
Well, the expected delivery date as shown by Amazon was the next day, so I thought itād be fine. Lesson learnt.
According to the tracking info they ended up sitting in an intermediary warehouse for nine days for reasons unknown, before finally making their way here, arriving nearly two weeks after other items from the same order. Thing is that I do in fact have issues with that guitar now, and Iām starting to wonder if the neck being untensioned for so long is a contributuing factor. Ho-hum.
Guitars assembled in the factory might spend more than 2 weeks without strings on them, so I would not worry about any irreversible change. Give it a few days to settle and tune it again. Just bear in mind that the truss rod is for adjusting the neck relief (to make it straight or a little bowed, depending on your preference). If thereās still some string buzz, try changing the string height by adjusting the saddles/bridge. And finally, donāt forget about checking the intonation and adjusting the screws of the saddles if necessary.
If thereās still fret buzz after that, it could be due either to too much pressure applied to the strings or to fret wear which makes the surface of one or more frets uneven or ābumpyā.
