Bikini Top Sizing 101: Why Your Bra Band Size Isn't Your Swim Size
Bikini Top Sizing 101: Why Your Bra Band Size Isn't Your Swim Size
By the Bikini Uni team. Updated May 2026.
Your bra size is built around a band-and-cup system. Bikini sizing isn't. Here's how to actually find the right top.
Why bra size doesn't equal bikini size
You wear a 34B bra. You order a "Medium" bikini top, or a "size 34," and it doesn't fit right. You assume you measured wrong, or that the brand runs small. Neither is true.
The reason is structural. A bra is built around two independent measurements — the band (underbust) and the cup (bust volume). A 34B and a 36A are technically different but contain similar volumes; this is "sister sizing."
A bikini top, especially athletic or competition cut, is sized around one measurement: your bust. There's no separate band. The cup is built into the top in a fixed shape, often with elastic at the bust line and side seams that grip without underwire. When you order a "size 34" swim top, you're ordering for a 34-inch bust measurement, full stop.
Your 34B bra and a "size 34" bikini top reference different numbers entirely. The bra's "34" is your underbust; the bikini's "34" is your bust. Order based on the bra's number and you'll get a top 3-4 inches too small.
The two measurements that matter
You need bust. You may also want underbust for context.
Bust: measure around the fullest part of your bust, tape parallel to the floor. Wear a non-padded, non-push-up bra (or no bra) so you're measuring your actual chest. Stand straight, arms relaxed.
Underbust (optional): measure directly under the bust, where a bra band sits. Same as your bra band size.
Write the numbers down.
Bra size → swim size conversion
Not exact (cut and fabric vary), but a starting point:
| Your bra size | Approximate swim top size |
|---|---|
| 30A / 32A / 32B | 28 to 30 |
| 32B / 34A / 34B | 30 to 32 |
| 34B / 34C / 36A / 36B | 32 to 34 |
| 34C / 34D / 36B / 36C | 34 to 36 |
| 36C / 36D / 38B / 38C | 36 to 38 |
| 38C / 38D / 40B / 40C | 38 to 40 |
This is a guide, not a guarantee. A 34D and a 36B contain similar volumes but sit differently in a fixed-shape bikini top. The reliable approach: measure your bust directly. See our full two-piece sizing guide.
Four common fit problems and what they mean
Cups gap at the top. Top is too big in the bust. Size down by one.
You spill out the top or sides. Too small in the bust. Size up by one.
Band rides up your back. Too small overall, or strap configuration wrong for your torso length. Size up and check for longer-torso version.
Straps dig red lines into shoulders within an hour. Either too small (size up) or strap placement wrong for shoulder width. If sizing up doesn't fix it, try a different cut — halter or wider-band.
"The single fastest way to get the wrong bikini top is to order your bra band size. They measure different things."
How to test a bikini top fit at home
Try on dry first, then wet (a quick rinse works). Run through these:
- Pinch test. Pinch fabric at the side of the cup. Pull more than half an inch from body = too big. Can't pull any = too small.
- Arm-raise test. Raise arms over your head. Cups shouldn't lift off your chest. If they do, band too loose.
- Bend test. Bend forward at the waist. Mirror check. If you see daylight between chest and cups, too big.
- Wet test. Wet the suit, repeat tests. Swimwear stretches when wet — perfect dry fit can be loose wet, so a touch-snug dry is correct.
Why athletic vs leisure tops fit differently
Athletic bikini top — designed for lap swimming or active beach use — has elastic at the bust line, thicker band, minimal padding. Built to grip through movement.
Leisure bikini top — designed for sand-and-cocktail use — has more padding, less elastic, softer band. Built to look good standing still.
The same body fits these differently. Size by bust = right number. Same number in athletic vs leisure = different garments. If you swim, choose athletic. If not, leisure is honestly more comfortable for sitting on sand.
Frequently asked questions
What size bikini top should I order if I wear a 34B bra?
Roughly a size 32-34. A 34B has roughly a 35-inch bust, which lands at 34 in most athletic-cut sizing. Measure your bust directly and order to that number.
Why is my bikini top loose even though I ordered my "bra size"?
Bra band size measures underbust; bikini size measures bust. A 34-inch underbust corresponds to roughly a 36-37-inch bust, so a "size 34" bikini ordered against a 34-band bra runs ~3 inches small.
Can I wear a sports bra under a bikini top?
Yes, especially for athletic use. Consider whether a higher-support cut (sports bikini with internal elastic) would work better instead. Layering adds drag and dries slower.
Do bikini tops stretch out over time?
Yes. Elastane degrades with chlorine, UV, heat. Handmade bikini top cared for properly lasts 18 months-3 years; fast-fashion often loses shape after a season. See care guide.
How do I know if a bikini top will hold up for a bigger bust?
Check three things: wider band at bust line (1+ inch), real elastic sewn into bust band, adjustable straps. Top that depends entirely on tied-string straps will fail on a bigger bust regardless of size.
Should I size up or size down between two sizes?
Size down on athletic tops — fabric loosens with wear. Size up on leisure tops if comfort matters more than fit precision.
Why does my bikini top fit dry but feel loose wet?
Swimwear fabric stretches when wet (varies by elastane grade). A dry fit should feel a touch snug to land in the right wet fit.
My bust is a different size from my hips by 4 inches. What do I do?
Buy from a brand that offers independent top and bottom sizing. Order your top by bust measurement, bottom by hip measurement. Most matched-size sets are designed for women with a 2-inch gap; bigger gaps need separates.
Find your fit
Most Bikini Uni bikinis are sold with independent top and bottom sizing. Sizes 28-40, sewn by hand in Cali. Browse the Bella Cintura, La Sirenita, Ameri-Cali, or Dos Tierras.
Between two sizes? Message us with your numbers — we'll tell you which size we'd ship to a friend with the same measurements.
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