This is a hard one, and it really comes down to personal preference. As a rule, we’d advise you to go for your usual bra size when you’re buying a bikini or swimsuit, at least as a starting point. But we’re also aware that people don’t want their usual lingerie to fit when it comes to their swimwear in case it leaves them feeling too exposed when they’re relaxing on the beach. Some people like to size up to ensure they get good rear coverage in their bikini briefs, or to make sure their swimsuits are comfortable.
If this sounds like you, try this swimwear sizing strategy. Go up one back size bigger than your normal bra, while selecting one cup size smaller, to ensure you keep the same volume in the cup. For example, if you usually wear a 36E bra size but want more room in the body of your swimming costume, try a 38DD. Just remember though, this might affect the level of support you get.
You also need to remember that all swimwear stretches slightly over time, and baggy bikini bottoms and swimsuits will no longer be flattering and may become uncomfortable. Excess material will eventually become a nuisance, leaving you feeling uncomfortable and self-conscious. The last thing you want is to have to adjust your swimwear constantly while you're trying to relax in the sun. Buying your exact size (or even, shock horror, a size smaller) can be slimming, so try it.