Measure your bust, natural waist, and fullest hip, then match each number to the brand’s own chart — and size the top and bottom separately when the brand allows it.
A bikini that gaps at the cup or rides up with every wave is almost never the wrong “size” in some universal sense. It’s the wrong size for that brand’s chart, measured the wrong way. Here’s how to measure once, read a chart correctly, and check the fit before you commit.
How Do You Measure For A Two-Piece Swimsuit?
Use a flexible tape, keep it level to the floor, and don’t pull it tight. Measure over minimal clothing and record numbers in whatever unit the brand’s chart uses — many US charts list inches.
- Bust: wrap the tape around the fullest part of the bust, horizontal all the way around.
- Waist: measure the narrowest part of your torso — the spot where you bend side to side.
- Hips: stand with feet together and measure around the fullest part of the hips and seat.
- Cup-sized tops: measure overbust in a well-fitting bra, plus underbust around the rib cage directly below the bust.
Torso length — shoulder, over the bust, through the legs, and back up — matters most for one-pieces, though a few brands include it in general swimwear charts.
Reading A Size Chart (And Where People Go Wrong)
Size charts list body measurements, not garment measurements, and they belong to one brand only. A medium in one label is a large in another.
Nike’s US women’s chart is a good working example of how the numbers map:
| US Size | Bust / Waist / Hip | Extended Sizing |
|---|---|---|
| XS, 0–2 | 31.5–33.5 / 24–26 / 34.5–36.5 in | — |
| S, 4–6 | 33.5–35.5 / 26–28 / 36.5–38.5 in | — |
| M, 8–10 | 35.5–37.5 / 28–30 / 38.5–40.5 in | — |
| L, 12–14 | 37.5–40.5 / 30–33 / 40.5–43.5 in | — |
| XL, 16–18 | 40.5–43.5 / 33–36 / 43.5–46.5 in | 1X, 16W–18W: 44–48 / 38–42 / 47–51 in |
| 2XL, 20–22 | 43.5–46.5 / 36–39 / 46.5–49.5 in | 2X, 20W–22W: 48–52 / 42–46 / 51–55 in |
| — | — | 3X, 24W–26W: 52–56 / 46–50 / 55–59 in |
Two rules from Nike’s own guidance solve most boundary cases. Measurements that land on a line: take the smaller size for a tighter fit, the larger for a looser one. Waist and hip pointing at different sizes: follow the hip. REI’s swimsuit guidance adds the fix that matters for bikinis — buy the top by bust or cup and the bottom by hip and waist, using mix-and-match sizing when the brand offers it.
Common measuring errors: putting the tape at the navel instead of the natural waist, measuring hips with feet apart, and forcing a bikini top into generic dress sizing when it actually needs cup and underbust data.
How Should A Two-Piece Actually Fit?
A correctly fitted suit stays put when you move — no tugging, no readjusting. Fit each piece against its own checklist.
For the top: the band sits snug and level, cups fully contain the bust, and straps support without cutting into the shoulders. Too small shows up as spillage, a restrictive band, or straps digging in. Too large shows up as a band that rides up, slipping straps, sagging fabric, or wrinkles.
For the bottom: it should stay in place without digging into the waist or leg openings and without riding up. Cut changes the math — high-waisted styles add abdominal coverage, cheeky cuts remove seat coverage, and underwire tops give fuller busts more structure than wire-free designs. Tested 2-piece swimsuit picks for every body type can show how these cuts look on different frames before you order.
Then move. Walk, bend, and stretch in the fitting room or mirror at home. Bravissimo’s swimwear guide is direct about the test: a chart predicts nothing about how the suit behaves in water, so trust movement over numbers. Check the retailer’s return and final-sale rules before you remove tags or hygiene liners — once those come off, the suit is usually yours.
Fabric stretch, lining, tie closures, rise, and cup construction all shift the fit inside the same labeled size. The chart is the starting point; the movement check is the verdict.
What Do The Different Numbers And Letters Actually Mean?
The letter is a summary; the measurements are the real answer. US chart sizing runs XS through 3X, with “W” sizes (16W–26W) cut for a fuller figure at the same height — which is why a 1X and an XL are not interchangeable.
When bust and hip numbers put you in different letters, buy two different sizes. That’s the whole advantage of a two-piece: the top fits your bust and the bottom fits your hips, and neither has to compromise for the other.
Common Questions
Should I size up or down if I’m between sizes?
Go down for a snugger fit and up for a looser one, per Nike’s sizing guidance. For bikinis specifically, lean on the hip measurement for the bottom and the bust or cup measurement for the top rather than forcing one letter to cover both.
Do I measure my waist at the navel?
No. The natural waist is the narrowest part of your torso, roughly where you bend side to side — typically above the navel. Measuring at the navel adds inches and pushes you into a size that will gap at the back and dig in at the leg openings.
Why do two brands give me different sizes?
Each chart is written for that brand’s own patterns and fit philosophy, so the numbers only apply to that label’s suits. Always re-measure against the specific product’s chart instead of assuming your usual size carries over.
Sources
- Nike. “Women’s Swimsuit Size Chart” Source for US size measurements, extended sizing, and boundary-fit guidance.
- REI. “How to Choose Swimsuits” Supports separate top-and-bottom sizing by bust and hip.
- Bravissimo. “Bikini and Swimwear Size Guide” Informs the movement-based fit check over chart numbers alone.