Article — Bikini Size
Your bikini size is set by four numbers: bust, underbust, waist, and hip. A US bust of 89–91 cm puts you in size M (US 4, EU 36, UK 10); a cup is one letter per 2.54 cm of bust-minus-underbust difference. The American ASTM D5585 standard codifies these ranges, but every brand drifts a few centimeters either way — which is why sizing the top and bottom separately, and reading brand-specific charts, beats picking a single “S” off a tag.
This guide breaks down the US, EU, and UK systems, walks through accurate measurement, explains why most women buy bikini separates in different sizes, and gives the cup-depth math the calculator uses internally.
Bikini size systems compared
Three regional systems dominate global swimwear: US numeric (0, 2, 4, 6…), EU numeric (32, 34, 36, 38…), and UK numeric (6, 8, 10, 12…). The US-to-EU offset is +32, the US-to-UK offset is +6. So a US 4 equals EU 36 equals UK 10. The same body fits all three numbers; only the label changes.
Letter sizes (XS, S, M, L, XL) overlay on top, with one letter per two US numbers: XS=0, S=2, M=4, L=6, XL=8, XXL=10. Bra-derived sizes (34B, 36C) appear when a bikini top includes structured cups; these mix a band number with a cup letter and follow the same ASTM D5585 increments.
The bikini takes its name from Bikini Atoll in the Marshall Islands, where the United States conducted nuclear tests in 1946. French designer Louis Réard launched the two-piece that summer and named it after the atoll, betting his design would land with the same shock as the bomb tests dominating the headlines.
Bikini measurements step by step
Use a fabric tape. Measure in front of a mirror to keep the tape level. Wear thin, unpadded underwear — padding adds 2–4 cm to the bust reading. Stand naturally; do not suck in or push out.
Bust: across the fullest point of the chest, tape parallel to the floor at the back. Underbust: directly below the bust line, snug but not compressing. Waist: at the narrowest point, usually 2–3 cm above the navel. Hips: at the fullest point of the seat, roughly 18–23 cm below the waistline.
- Bust accuracy — off by 2 cm shifts one US size
- Cup-size step — 2.54 cm of bust-minus-underbust depth per letter
- EU band rounding — underbust rounded to nearest 5 cm
- Hip position — ~18–23 cm below the natural waist
- Time required — 5–10 minutes for all four measurements
- Best time of day — morning, before water retention shifts numbers
Modern push-up bras add roughly 2–4 cm to the apparent bust line. Measuring over one inflates the top size by one full US step and pushes the cup letter up by one. Always remeasure in an unpadded bra or none at all before ordering swimwear.
Bikini tops vs bottoms: size separately
Bikini tops are sized by bust and cup; bikini bottoms by hip and waist. A woman with a US 4 bust (89–91 cm) and a US 6 hip (99–102 cm) needs a size M top and a size L bottom — standard, not unusual. Many female adults have at least one US-size gap between their bust and hip measurements, a pattern documented in the CAESAR anthropometric survey of about 4,400 subjects in the United States, Canada, and Europe (1998–2002).
For this reason, almost every swimwear brand sells tops and bottoms separately. If a retailer only offers matched sets, check whether mixed sizing is allowed at checkout. If not, size to the larger of the two needs and accept that one half will be loose; tightening drawstrings or moving to underwire tops handles most of the slack at the bust end.
Bikini cup size explained
Cup size is set by the difference between bust and underbust circumference, not by the absolute bust. A woman at 90 cm bust and 70 cm underbust (20 cm difference) wears a larger cup than one at 90 cm bust and 80 cm underbust (10 cm difference) — even though both have the same bust number.
The ASTM D5585 standard uses a 2.54 cm (1 inch) step per cup letter: AA at 0–4 cm depth, A at 4–6.5, B at 6.5–9, C at 9–11.5, D at 11.5–14, DD/E at 14–16.5, F at 16.5–19. UK and EU systems use the same depth steps but spell larger cups differently: the UK doubles letters (DD, EE, FF), and EU often uses a single letter sequence through to F or G.
Cup sizes are relative, not absolute. A 32D and a 38D have the same 14 cm bust-minus-underbust depth, but the 38D bust is meaningfully larger because the band itself is wider. Cup letter alone tells you almost nothing without the band number alongside it.
Standard XS/S/M/L bikini tops are designed for A–C cups and offer little support above that. For D-cup and larger, look for tops sold in bra-style sizing (34D, 36DD, 38F) with underwire, molded cups, or wider straps. Plain triangle tops above C cup commonly result in spillage and shoulder pain.
Bikini styling by body shape
Bikini cut affects perceived proportions almost as much as size does. The four common shape categories — hourglass, pear, apple, athletic — each have cuts that flatter and cuts that fight the figure.
Hourglass (balanced bust and hips, defined waist) suits halter tops and mid-rise bottoms that follow the existing line. Pear (narrower top, fuller hips) is balanced by detailed tops — ruffles, patterns, lighter colors — with simpler high-cut bottoms. Apple (fuller torso, narrower hips) benefits from V-neck or plunge tops that elongate the torso, paired with high-waisted bottoms. Athletic (narrow hips, less waist definition) gains shape from ruched tops, side-tie bottoms, and contrasting colors top to bottom.
- Hourglass — halter top, mid-rise bottom, anything that follows the waist line
- Pear — detailed top, simple high-cut bottom, brighter top color
- Apple — V-neck top, high-waisted bottom, vertical patterns
- Athletic — ruched or padded top, side-tie or skirt bottom for hip illusion
- Tall torso — one-piece or long-line tankini avoids the gap problem
- Short torso — bandeau or low-band top keeps the silhouette balanced
Buying bikinis online
Always check the specific brand’s size chart, not just the letter or US number on the listing. A Brazilian-made bikini in size M may sit one US size smaller than an American or European M — Brazilian sizing tends to run small. Italian brands sometimes run small as well. German, Scandinavian, and most US brands track closer to ASTM D5585.
Read recent reviews filtered by users with similar measurements before ordering. Look for the words “true to size,” “runs small,” or “runs large” in the most recent 20 reviews; that signal beats any brand chart. Order from sellers with free returns whenever possible; a 25–40% online swimwear return rate is normal across the industry, so retailers have priced returns into the model.
Bikini fabric typically blends 80–88% nylon or polyester with 12–20% spandex. The spandex relaxes when soaked, which means a bikini that fits snugly dry will usually fit a little looser in the water. Do not size up “to be safe” if the dry fit is correct; the swim fit will be looser still.
Bikinis with under 12% spandex and unlined cups tend to lose shape after 15–20 wears, especially in chlorinated pools. A well-made bikini lasts 3–5 seasons. For frequent swimmers, the cost per wear of a $90 well-made set beats a $25 fast-fashion one within a single summer.