If you're holding a bikini up in the changing room wondering whether it's the right one, the honest answer is you won't know until you've had it on and walked around a bit, not just stood still in front of the mirror. Sit down in it. Raise your arms like you're actually swimming, not posing. If the top shifts or the bottoms dig in when you do that, they'll do it worse once you're wet and moving in actual water.
Before you buy, check the lining is proper swim fabric and not just a thin layer that'll go see through the second it's submerged, and have a feel of the seams where the cups meet the band, because that's where cheap ones pull apart first. On underwired styles, check the wire sits flat against your ribcage rather than poking at the sides, that's a fit issue, not something that'll settle. On bandeau styles, look for some kind of grip or boning inside the band, otherwise you'll spend the whole holiday hitching it back up. Ties on triangle bikinis are worth tugging gently in the shop, you want to know they're not going to work loose with movement, not find that out mid swim. Over months, the thing that ages a bikini fastest isn't the sun, it's chlorine and hot washes, so rinse it in cool water after every wear, chlorinated pool or not, and let it dry flat rather than wringing it out or draping it over a radiator, which kills the stretch and fades the colour unevenly.
These aren't the right choice if you want something to do serious swimming laps in, a triangle or bandeau top will not hold still through front crawl, and black ones, lovely as they look, show chlorine fade and salt marks faster than you'd expect because the loss of colour is so obvious against dark fabric. They're also not kind to anyone who wants a lot of coverage or support, there's only so much an underwire can do before you're better off in a swimsuit with proper structure.
On sizing, go by your actual measurements rather than your usual clothes size, bikini brands run small and inconsistently across styles. Buy top and bottom separately where you can, since most of us aren't the same size top and bottom anyway. Watch for gaping at the sides of triangle cups, that means the size is right but the shape isn't yours, and watch for bottoms that cut in at the hip on sitting down, which usually means sizing up rather than a different cut entirely. A bikini that's slightly loose in the shop will often feel right once it's wet, a bikini that's tight in the shop never loosens at all.
Picked by Leanne Doyle. More in Swimwear.