Rod guide size replacement.

[FishinFang]

I am going to order rod tips and a couple of guides to replace a few I've mangled.  How do I know what size to buy.  I see on tips there are sizes like #4, #4.5, #5, #5.5.  Guides are listed as #7, #8, #10, etc.  These are going on baitcasting rods, so I know I need baitcasting guides.  However, how do you know what is the correct size for my particular rods?  I'm replacing 2 tips and the first guide from the tip.  Thanks for any help.