How do I figure out a man's bet size?

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I'm not big on buying clothing for men, but I sure know how to pick by style and fashion when it comes to selection. I found a really rad looking belt at some store, but I wasn't sure what size my guy would be. He's a 31x31 in pants... So before that my guestimate is that his belt size in the US system would maybe be 32 - 34? Because I heard a 32 waist would take a 34 belt because of the allowance between the skin and the clothing... o.O I'm not 100% sure on this, mind helping me out? Thanks. :D SSSHHH!!! I'm trying to squeeze him into slim jeans too because of his figure. I don't want him to change at all, but I'm just curious of how nicely he'd probably fit in them in our next trip shopping. :P Slim jeans look hot on thinner guys! Baggier clothing makes him look kinda shrunk down? I still love him no matter what he looks like and all, I mainly love him as a person his wardrobe needs a sight revamping. Don't get me wrong he's gorgeous. n_n