It’s time to change the strings on your guitar, but you have no idea what gauge they are. What do you do? There are a few ways to approach the problem:
First, if you just bought your guitar and this is the first time you are changing the strings, then you should know that most manufacturers will put a set of 9’s (Amazon) on an electric guitar and a set of 12’s (Amazon) on an acoustic guitar. There is no harm in purchasing a set of strings and seeing if they fit.
Second, you may be asking this question because you are not sure if your electric guitar is strung with a set of 9’s or a set of 10’s. If this is the case, then try buying a single low E string of each set and seeing if one fits. A set of 9’s has a .042 gauge string (Amazon) and a set of 10’s has a .046 gauge string (Amazon).
Finally, if you still are not sure, then purchase a caliper (Amazon) and measure the thickness of your strings with it.