Swimsuit size by bra size: what to buy in a bikini top or swimsuit
If the swim top is sold in a band and a cup, buy your bra size. If it is sold in XS to XXL, the brand's own chart decides, and one brand prints it. Either way there is one trap: swimwear brands write the cup in US letters, so a UK F is a G.
Your UK bra size in swimwear letters
| Your UK size | Swimwear label (US) | Convert it |
|---|---|---|
| 34D | 34D | Convert 34D |
| 34DD | 34DD | Convert 34DD |
| 34E | 34DDD | Convert 34E |
| 34F | 34G | Convert 34F |
| 34FF | 34H | Convert 34FF |
| 34G | 34I | Convert 34G |
| 34GG | 34J | Convert 34GG |
| 34H | 34K | Convert 34H |
"bathing suit with cup size" gets about 6,600 Google searches a month in the US, 2,400 in the UK and 140 in Australia; "swimsuits in bra sizes" is the same cluster (Google Ads figures, September 2026).
Two kinds of swim top
What the tag says decides which page of this guide you need.
Cup-sized: a band and a cup
Fantasie describes its swimwear as cup-sized, "which means you can enjoy the same signature Fantasie support" as in its bras, and answers "Should I go up a back size in swimwear?" with "you can take your usual bra size in any of our bikini tops, tankini tops and swimsuits" (Fantasie, "Swimwear Fitting Guide", opened 29 September 2026). Its cup-sized range goes up to an M cup. So the size to buy is your bra size, in the letters that brand prints.
Letter-sized: XS to XXL
Non-wired tops and swimsuits are usually sold in letters. Fantasie prints the conversion from bra size to letter for its Merissa non-wired bikini top and swimsuit, in the table below. Other brands publish theirs on the product page: it is the chart that counts, not a general rule.
The cup letters are the US ones
Fantasie, Freya and Elomi each state that their lingerie and swimwear use US underband and cup sizing (Fantasie, "International Size Guide"; Freya, swimwear (US store); Elomi, "International Size Guide"). US ladders run DD, DDD, G, H, I, so a UK 34F is a 34G on the tag. The table above converts eight cups on a 34 band.
Bra size to XS-XXL: Fantasie's chart
The one brand chart we found that converts a bra size to swimsuit letters. Reproduced as printed, opened 29 September 2026.
| Band | Cup D | Cup DD | Cup E | Cup F | Cup FF | Cup G |
|---|---|---|---|---|---|---|
| 30 | XS | XS | S | S | M | M |
| 32 | S | S | M | M | L | L |
| 34 | M | M | M | L | L | XL |
| 36 | L | L | L | XL | XL | XXL |
| 38 | XL | XL | XL | XXL | XXL | XXL |
Read down a column and the letter grows with the band; read across and it grows with the cup. The chart's headings run D, DD, E, F, FF, G. E, F and FF are UK letters (the US ones would be DDD, G, H), yet Fantasie says its swimwear uses US sizing and does not say which convention this chart follows. If you sit between two letters, or wear a UK F or FF, check the product's own chart or ask the brand. The chart starts at D: for a smaller cup it has no answer.
How far cup-sized swimwear goes
The largest cup each brand says its swimwear reaches, and the UK letter behind it.
| Brand | What it says | Largest US cup | UK letter |
|---|---|---|---|
| Fantasie | bra-sized bikini tops, tankini tops and swimsuits "in up to an M cup" | M | J |
| Freya | cup sizes "from a B to an O cup across Lingerie, Swimwear and Active" | O | K |
| Elomi | swimsuits, bikinis and tankinis "in up to an N cup" | N | JJ |
Three checks in the fitting room
Fantasie's swimwear fitting guide, opened 29 September 2026.
- The centre. The piece of fabric between the cups should sit flat against your chest and should not move when you raise your arms.
- The wire. The end of the underwire sits behind the breast tissue, on the ribcage, with no digging in.
- The band. Snug and parallel to the ground, without riding upwards.
These are the same three things you check in a bra. If one fails, the fit check turns it into the size to try next, and the sister-size chart shows the neighbouring sizes.
When the swimsuit has no cups
Tankinis, suits and briefs sold in numbers follow dress size, not your bra band and cup.
| Country | Size 1 | Size 2 | Size 3 | Size 4 | Size 5 | Size 6 | Size 7 |
|---|---|---|---|---|---|---|---|
| UK/AUS/NZ | 14 | 16 | 18 | 20 | 22 | 24 | 26 |
| EUROPE | 40 | 42 | 44 | 46 | 48 | 50 | 52 |
| FRANCE | 42 | 44 | 46 | 48 | 50 | 52 | 54 |
| USA | 10 | 12 | 14 | 16 | 18 | 20 | 22 |
Quick answers
Should I go up a band size in swimwear?
Not in cup-sized swimwear. Fantasie says its swimwear is cup-sized, so you can take your usual bra size in any of its bikini tops, tankini tops and swimsuits. Check the fit the way you would a bra: the centre stays flat, the wire sits behind the breast tissue and the band stays level.
My bra size is UK. What do I buy in a US swimwear brand?
Keep the band and convert the cup. Fantasie, Freya and Elomi all say they use US underband and cup sizing, so a UK 34F is a 34G, a UK 34E is a 34DDD and a UK 34FF is a 34H. The table above shows the row for each cup on a 34 band.
What if my cup is smaller than D?
Fantasie prints its bra-size-to-letter chart for cups D to G and bands 30 to 38 only. Below D, use the size chart on the product page you are looking at; we have not found a brand chart that covers it.
Does my bra size decide a swimsuit that is sold by dress size?
No. A tankini, suit or swim brief sold in numbers (Elomi's run UK 14 to 26) follows dress size, not the bra band and cup. The last table converts those numbers between countries.
Where to go next
- Bra size calculatorTwo methods side by side, and why they disagree
- How to measureBand, bust and the leaning check, step by step
- Fit checkTick what feels wrong; get the size to try next
- Sister sizesThe whole chart, and what to try when the band rides up
- UK to US converterAny size in any of six systems
- International size chartUK, US, EU, FR, AU and Japan on one grid
- Cup sizes guideWhat a cup letter means, and why it changes by band
- Plus size brasBands 40 and up, cups DD and up: sizing, fit and styles, with the numbers
- Brand size chartsVictoria's Secret, ThirdLove, Wacoal, SKIMS, M&S and Freya, read side by side
Sources
Each source was opened and checked on the date shown, or on 25 September 2026. Where sources disagree, the page says so instead of picking one silently.
- Fantasie, "Swimwear Fitting Guide": cup-sized swimwear up to an M cup; "take your usual bra size"; three fit signs (opened 29 September 2026)
- Fantasie, "International Size Guide": US underband and cup sizing; the Merissa non-wired bikini top and swimsuit chart from bra size to XS to XXL (opened 29 September 2026)
- Freya, swimwear (US store): cup sizes from B to O across lingerie, swimwear and active; US underband and cup sizing (opened 29 September 2026)
- Elomi, plus size swimwear (US store): swimwear in up to an N cup; US underband and cup sizing (opened 29 September 2026)
- Elomi, "International Size Guide": US underband and cup sizing, B to O cups and 32 to 48 bands; UK, EU, FR, AU/NZ, US and JP rows (UK cups to KK, US to P) (opened 29 September 2026)
- Wacoal America, "Size Charts": US and UK cup ladders side by side (opened 26 September 2026)
- Wikipedia, "Bra size": UK and US cup ladders, the +4 method, EN 13402 and JIS cup steps