Bikini Sizing Guide: What Size Bikini Am I? (Complete Reference)
By the Bikini Uni team. Updated June 2026.
One page that tells you how swim sizing works, where the numbers come from, and which guide to read next for your specific question.
How bikini sizing works (the 28-40 system)
Most athletic and competition swimwear sells in numerical sizes from 28 to 40 rather than S, M, L, or XL. The number is a body measurement in inches:
- Women's bikini tops and one-pieces. The size number tracks your bust measurement at the fullest point. A 32-inch bust generally fits a size 32 top.
- Women's bikini bottoms. The number tracks your hip measurement at the fullest point. A 38-inch hip fits a size 38 bottom.
- Men's jammers, diver-cuts, and trunks. The number is your waist at the natural waistline (about an inch above the navel — not where pants sit). A 32-inch waist is a size 32 jammer.
The system has two advantages over S/M/L. First, it removes the guesswork — you know what "32" refers to. Second, it lets a brand offer top and bottom in different sizes on the same body, which most women need.
The full reference chart
Use this as a quick lookup. For a measurement walkthrough see our two-piece sizing guide.
| Size | Women's bust (top / one-piece) | Women's hip (bottom) | Men's waist (jammer / trunks) |
|---|---|---|---|
| 28 | 27–29 in | 31–33 in | 27–29 in |
| 30 | 29–31 in | 33–35 in | 29–31 in |
| 32 | 31–33 in | 35–37 in | 31–33 in |
| 34 | 33–35 in | 37–39 in | 33–35 in |
| 36 | 35–37 in | 39–41 in | 35–37 in |
| 38 | 37–39 in | 41–43 in | 37–39 in |
| 40 | 39–41 in | 43–45 in | 39–41 in |
The chart shows the inch range each size is built to fit comfortably. If your number lands between two sizes, see "Between sizes?" below.
Why letter sizing (S/M/L) doesn't map cleanly
You've probably owned a "Medium" bikini from one brand that fit and a "Medium" from another that didn't. That's not your body — it's the format.
S/M/L is a brand-by-brand assignment. There is no agreed inch range for "Medium." Brand A may build Medium for a 33–35 bust; Brand B for a 36–38. Three inches at the bust is the difference between a top that fits and one that gaps.
Numerical sizing solves this because the number IS the measurement. A size 34 from any maker that uses 28-40 sizing is built for roughly a 34-inch bust. Variance still exists — fabric stretch, cut shape, edition — but the anchor is the body, not the brand.
Rough equivalence if you're translating from S/M/L (use as a starting point, then check the maker's actual chart):
- XS ≈ 28
- S ≈ 30–32
- M ≈ 32–34
- L ≈ 34–36
- XL ≈ 36–38
- XXL ≈ 38–40
Most women are different sizes on top and bottom
Bust and hip rarely match. A 34 top with a 38 hip is normal. A 36 top with a 32 hip is also normal. Forcing both halves of a matched bikini set to share a number guarantees one won't fit.
The fix is independent top and bottom sizing — order each half in the right size separately. Most Bikini Uni two-pieces support this. For the full breakdown see our mix-and-match guide.
A one-piece can't use independent sizing — it's one continuous garment. For a body with more than a 3-inch bust-to-hip gap, a two-piece with separate sizing will almost always fit better than any one-piece in any size.
Where to go next (decision tree)
This page is the reference. For your specific question, the deep-dive lives elsewhere:
- "I wear a 34B bra — what bikini top size do I order?" See bikini top sizing 101. Bra band size measures underbust; bikini size measures bust. They're three inches apart on average.
- "I don't know my measurements — how do I find them?" See what size bikini am I. Step-by-step measurement walkthrough with the fit tests.
- "My top fits, but my bottom is loose — what now?" Independent sizing on the next set, or the four-option fix recipe in the two-piece guide.
- "I'm a man, what jammer/trunks size?" See our men's jammer, diver, trunks buying guide. Most men size 1–2 inches down from their pants because pants sit lower than the natural waist.
- "One-piece or two-piece for lap swimming?" Sizing differs slightly between the two. See one-piece vs two-piece for lap swim.
- "Cinched-waist or one-shoulder — does the chart still apply?" Yes with adjustments. See cinched-waist bikini guide or one-shoulder swimsuit guide.
"Numerical sizing isn't fussier than S/M/L — it's actually less. The number is just your body in inches. The guesswork is in the letters."
Between sizes? How to break the tie
Your measurement lands at 33.5 inches and the chart shows 32 and 34. Which do you order? Depends on the garment and your use.
Athletic / training cut: size down. Fabric loosens 0.5–1 size over the first dozen wears. A snug-when-new athletic top hits its right wear-in fit by month two.
Leisure / beach cut: size up. Comfort over the long sit on hot sand beats fit precision; leisure suits aren't expected to grip through movement.
Bottom with a cinch or tie: size up at the hip. You can cinch tighter — you can't add fabric. The cinched-waist guide covers this.
One-piece: size to your larger measurement (usually hip if you're pear-shaped, bust if you're broader on top). A one-piece tight in one zone and loose in another rides up in the loose one.
Brand variance: why "size 34" isn't universal
The 28-40 system is the closest the industry gets to a standard, but cuts and fabrics differ. Two factors create real variance even within numerical sizing:
Stretch grade. Higher elastane (18%+) sits closer to the body; the same size 34 in a high-elastane suit fits snugger than in a 10% elastane suit. Most athletic swimwear runs 18–22% elastane.
Cut shape. An "athletic" size 34 top and a "halter" size 34 top hit the body differently. The number is the measurement target; the cut decides how the fabric distributes around it.
The practical move: check the maker's own size chart, not just the size number. For Bikini Uni, the chart is on every product page beneath the variant selector.
Six common ordering mistakes
If you've been frustrated with bikini fit before, one of these is usually why.
- Ordering your bra band size as your bikini top size. Three inches small in most athletic cuts. Conversion guide.
- Ordering your jeans waist as a swim bottom or jammer. Jeans sit on the hip; bikini bottoms use hip; jammers use natural waist. Jeans waist ≠ swim sizing on either end.
- Buying the bottom size when your top and bottom should differ. A 32/38 body in a "size 34" matched set won't fit either half right. Separates exist for a reason.
- Trusting an old suit as your size reference. Old swimwear is stretched out. Re-measure each season.
- Sizing a one-piece to your smaller measurement. It'll ride up at the bigger measurement. Default to the bigger one.
- Skipping the wet test. Swimwear loosens about half a size when wet. A dry-perfect fit can be too loose at the pool.
Frequently asked questions
What's the difference between bikini sizes 28-40 and S/M/L?
The numeric system labels each size with the inch measurement it fits — size 32 is built for a 32-inch bust (or hip, or men's waist, depending on the garment). S/M/L is a brand-assigned label with no agreed inch range, which is why two "mediums" from different makers can fit three inches apart.
Why does the same size fit differently in different brands?
Two factors: stretch grade (higher elastane sits closer to the body) and cut shape (athletic vs leisure vs halter distribute fabric differently around the same number). Always check the maker's own size chart, not just the number.
Are men's and women's swim sizes the same?
Numbers overlap (28-40 for both) but they reference different body parts. Women's tops and one-pieces use bust; women's bottoms use hip; men's swim uses natural waist. A size 34 men's jammer and a size 34 women's top reference completely different measurements.
Do bikini bottoms and one-piece swimsuits use the same size?
No. Bottoms size by hip; one-pieces size by bust. If you wear a size 36 bottom (36-inch hip) and have a 32-inch bust, you'd order a size 32 one-piece — not a 36.
What size bikini should I order if my measurements don't match any single row?
Size top to bust and bottom to hip — they don't need to match. If a single measurement lands between two rows, size down for athletic suits and size up for leisure. For one-pieces, size to the larger measurement so the suit doesn't ride up.
What's the most common bikini sizing mistake?
Ordering your bra band size as your bikini top size. The band measures underbust; bikini size measures bust. They're roughly three inches apart in most cases. See the top sizing 101 conversion table.
Where do I find the Bikini Uni size chart?
This page is the cross-product reference. The exact chart for any specific style is on its product page, under the size selector. Browse bikinis, one-piece swimsuits, or men's swim.
Can I message Bikini Uni for a size recommendation?
Yes. Send your bust, waist, and hip measurements and we'll tell you the size we'd ship to a friend with the same body. Free, almost always faster than guessing and returning.
Find your fit
Bikini Uni runs 28-40 across the women's mainline two-pieces — Bella Cintura, La Sirenita, Ameri-Cali, and Dos Tierras — sewn by hand in Cali, Colombia, with independent top and bottom sizing.
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