Bra Size to Swim Size Conversion: A Brand-by-Brand Chart

This page converts a UK or US bra size into the swim size each reviewed brand actually sells. It is a reference table, built from each brand’s own published size guide in September 2026. One rule sits above the rest: buy from the brand’s own chart, never from your dress size. A 34F body maps to a different label at a bra-sized brand than at a dress-sized one, and neither matches your jeans.

How to take the two measurements that matter

Two numbers decide your bra-derived swim size: your underbust and your bust. Everything else is a brand’s reading of those two.

Measure your underbust snug. Wrap the tape around your ribcage directly below your bust, parallel to the floor, and pull it firm enough that it does not slip when you drop your arms. That number, rounded, is your band. Then measure your bust at the fullest point, standing relaxed, tape level. Do not pull tight here.

Your cup is the difference between the two. Subtract the underbust from the bust. Roughly one inch of difference is an A, two is a B, three a C, four a D, and each further inch adds a letter. PrimaDonna publishes the same method with an inch-by-inch cup table on its own measurement guide, and Monday Swimwear’s fit guide uses bust and underbust the same way. Take both measurements twice. A half-inch error moves you a full cup.

For the full four-measurement protocol, including torso length and hip, see our guide to measuring for swimwear.

UK to US cup size conversion

UK and US cup letters match through DD, then split. This is where most cross-brand errors happen. A UK DD and a US DD are the same cup, but a UK E is a US DDD, not a US E.

The table below follows the mapping the UK bra-sized swim brands publish in their own size guides.

UK cup	US cup
D	D
DD	DD
E	DDD
F	G
FF	H
G	I
GG	J
H	K
HH	L
J	M
JJ	N
K	O

The DD, DDD, and E band is the danger zone. A US shopper who reads “E” on a UK-sized brand can end up a cup bigger than she thinks. Note the top of the table too, because it is where the size guides in this category most often go wrong: UK J is US M, UK JJ is US N, and UK K is US O. A roundup that prints “up to a K cup (UK)” for a brand whose own guide stops at J has quietly promised a size that does not exist.

Bra size to swim size, brand by brand

Here is the conversion nobody else publishes. Rows are UK bra sizes. Columns are the reviewed brands, each read from its own published size guide. Bra-sized brands map directly. Dress-sized brands need a translation and carry a note. Where a brand’s range does not reach a size, the cell says “not published” rather than guessing.

UK bra size	Fantasie swim	Aerie bikini tops	Monday Swimwear (dress-sized)
30D	30D	30D	S, or the Small Band Large Cup S/L if the cup runs fuller
32DD	32DD	32DD	S/L
34D	34D	34D	S
34F	34F (US G)	not published (Aerie bikini tops stop at DD)	S/L covers a 28 to 34 inch underbust to a G cup
36G	36G (US I)	not published	no direct cup equivalent
38FF	38FF (US H)	not published	no direct cup equivalent
40H	40H (US K)	not published	no direct cup equivalent
42J	not offered (Fantasie’s J-cup swim stops at a 40 band; the largest cup it sells in a 42 band is GG)	not published	no direct cup equivalent

Three brands are deliberately absent from the grid. PrimaDonna publishes its swim cup range, B to I on bikini tops and C to I on swimsuits, but does not publish which bands its swim comes in, and its own pages do not state which lettering system those cups use. Panache publishes a swim ceiling on its category pages, up to a J cup on its UK site and up to an N cup, which is UK JJ, on its US site, but its size-guide pages render their charts by script and returned no band range. CUUP does not sell swim tops in straight band-and-cup sizes at all: it uses an adaptive system of 23 sizes that each map to several bra sizes, from Size 1 (30A to 32A) to Size 23 (38H and 40G). None of the three can honestly be reduced to a single cell. Check each on the brand’s own site before ordering.

Read your own UK bra size down the left and across. Two patterns stand out. The bra-sized specialists carry you well past a G cup, though not always on the smallest bands: Fantasie’s swim range starts at a 30, so a true 28 band is out of range there even though the brand’s lingerie goes lower. The fashion labels stop early, and they stop in the cup before they stop in the band.

If your band is 28 or 30 and your cup is above DD, our small-band, large-cup sourcing guide narrows the field to brands that publish those combinations.

Methodology and limits. Every cell above was read from the named brand’s own published size guide or swim category page in September 2026, never from a roundup or a size-chart aggregator, because secondary sources in this category are frequently wrong by a cup or a band. For Fantasie, that meant checking which band and cup combinations the brand actually sells, not just the headline range: its swim runs bands 30 to 44 and cups D to J, but not every combination inside those limits exists. Three brands are not in the table because we could not meet that standard for them, and we say so rather than filling the gap. Brands revise their ranges, so reconfirm the specific style before ordering rather than trusting any chart, including this one. Where a brand publishes nothing for a combination, the cell reads “not published” rather than an estimate. Monday Swimwear is dress-sized, so its cells give the closest recommended size, not a cup match.

Sister sizes, and when to use one

Two sizes are sisters when the cups hold the same volume on different frames. Go up one band and down one cup, or down one band and up one cup, and the space for your bust stays the same. A 34F and a 36E are sisters. A 34F and a 32FF are sisters the other way.

Bravissimo, a UK bust-specialist retailer, publishes a sister-size chart and states the rule plainly: if you go up a cup size, go down a back size. That is the mechanism, and it holds across bra-sized brands.

Use a sister size when your exact size is sold out, or when a band runs tight or loose in one brand. The honest caveat: a sister size is a compromise, not an equivalence. The cup volume matches, but the weight sits differently. A larger band spreads the load lower and wider and carries less of it, so a 36E does not support the way a 34F does. If you have to move, move down in the band and up in the cup rather than the reverse.

Dress-sized brands: how to read their charts

Dress-sized brands sell swim in letters like PP, P, S, M, L, V, VV and VVV, and those letters describe your frame, not your cup. Monday Swimwear runs nine sizes, PP, P, S, S/L, M, L, V, VV and VVV, with each tied to a bust and underbust range on its fit guide, which rates each style individually, from A to D on the narrowest-fitting tops up to A to G on the widest. The S/L is the exception in that run: it is the Small Band Large Cup size, and the brand’s collection page states its suits fit cup sizes AA to G.

The gap in this system is cup support. One dress size covers a spread of cups, so a D and an F wearing the same S get different coverage from the same top. This is why fuller-busted shoppers struggle with fashion swim: the frame fits, the cup does not.

Some dress-sized brands address this with a hybrid. Monday Swimwear’s Small Band Large Cup sizing is built for a small frame with a fuller bust, and the brand states it fits a 28 to 34 inch underbust and a DD to G cup. It exists in three underwire tops rather than across the range, which is a real limitation worth knowing before you plan around it. If you are a 30F or a 32E who never fits a straight S, that is the cell to read first.

Where the brands disagree

Building this table surfaced three real inconsistencies, and they are exactly where a shopper gets the size wrong.

First, brand range does not equal style range, and a brand’s headline figure often does not equal its own category pages either. One bra-sized swim brand states a ceiling of a UK J cup on its UK category page and a US N cup, which is UK JJ, on its US one, while secondary roundups routinely print a UK K for the same brand. Any individual style then stocks a narrower band run than whichever figure you trust. The size you want, in the print you want, may never have been cut. Always check the band run on the product page, not the brand’s size guide.

Second, the same letter means different volumes in different systems, and brands are inconsistent about saying which system they are using. An “E” on a UK-sized brand is a US DDD. An “E” on a US-sized brand is a different cup entirely. Where a page does not state the system, treat the letter as unusable until you find one that does.

Third, US-facing pages of UK brands sometimes convert and sometimes do not. The same brand can show UK letters on one regional site and US letters on another, for the same garment. Read the guide attached to the store you are actually buying from.

Who this is not for

If you already shop one bra-sized brand and never cross-shop, you do not need a conversion. You need that brand’s own chart, and nothing here beats it. This page is for the shopper moving between a lingerie-derived label and a fashion label, where the same body carries two different size names.

This page is a reference, not a recommendation. It will not tell you which suit to buy. It tells you which size to order once you have chosen, and it is refreshed in place when a brand changes its chart.