Size 40O Bikinis
Also labeled UK 40K and EU 90O
If you wear a 40O bra, start your bikini top search in the same size. Then check that the band stays put and the cups hold you without spilling at the sides. Our Cup-Sized Bikini Size Chart shows how 40O is labeled in UK and EU sizing.
When a 40O Bikini Band Loosens in Water
A 40 band spans a wide back, and a bikini band that is even slightly loose will move as soon as you swim. A hook band with more than one setting lets you adjust as the fabric relaxes over time. Give the top a gentle tug at the back; a good band springs back to the same place.
Check the straps last on a 40O top. They should sit flat on your shoulders and leave no deep marks after a few minutes.
Checking the Cups on 40O Bikini Tops
At this depth, the cups need full coverage at the sides and enough room at the base so the band can sit flat. If the cups feel full but the band rides up, firm up the band before looking at a larger cup. Both cups should sit level with each other when the band is straight.
On UK labels, this size is 40K. UK charts stop at K for this cup, since they double letters lower down.
When a 40O Bikini Top Shifts
A bikini top has no torso fabric to hold it in place, so movement tests it quickly. Reach overhead and bend down in the 40O top, then check the band at the back. A band that stays level means the cups can do their job.
For everyday styles, browse our 40O bras.
Frequently Asked Questions About Size 40O Bikinis
What is a 40O bikini top in UK sizing?
In UK sizing 40O becomes 40K, the top of most UK charts, and in EU sizing 90O. Read the size system on every tag.
What are the signs a 40O bikini band is too tight?
Watch for pinching at the sides, marks that last after you take it off, or a band you cannot slide two fingers under. With an O cup, a band that is a touch too tight often feels fine standing and uncomfortable when you sit.
What are the signs a 40O bikini band is too loose?
The cups tip forward, the back climbs toward your shoulder blades, or you can pull the band well away from your ribs. With an O cup, those signs show quickly, so firm up the band before you touch the straps.
What is the first fit check for a 40O bikini top?
Start with the band, then the cups, then the straps. A level band lets you judge whether the cups hold an O cup without spilling or gapping.
Where does the support in a 40O bikini top come from?
Mostly from the band and the cups working together. On a large band, the straps should only fine tune the fit, not lift the weight.
Can I tighten the straps to make a 40O top feel secure?
No, not as a fix. Tight straps on an O cup dig into your shoulders and pull the band up, so firm up the band first.