Swimwear sizing is genuinely confusing, and it is not your fault if you have never found a bikini top that fits properly. The problem is structural: most swimwear brands size cups from a single block and scale up or down, which means the cup shape changes with the size. Here is how to measure correctly and what to look for in a brand that sizes properly.

Why Standard Swimwear Sizing Fails

Most swimwear is sized by a single measurement. Usually the underbust or the overall dress size. And the cup is scaled proportionally from a base block. This works reasonably well for a narrow range of body types and fails for almost everyone else. A woman who wears a 32D bra and a woman who wears a 38D bra have very different cup volumes, but in most swimwear brands they would both be offered the same cup shape in different band sizes. The result is a top that either gaps or overflows.

How to Take Your Measurements

You need two measurements: your underbust circumference (measured directly under the bust, snug but not tight) and your bust circumference (measured at the fullest point, with a slight ease). The difference between the two gives you your cup size in the standard UK/EU system. Write both numbers down. Not just the letter. When you contact us for a virtual fitting, these are the first two numbers we ask for, along with your height and your preferred level of support.

What Separate Cup Grading Actually Means

At Stream Lab Core Nex, the cup in every swimwear style is graded separately from the band. This means the cup volume, the cup depth, and the underwire channel (where applicable) are recalculated for each size rather than scaled from a single block. The pattern-maker we work with in Osaka has been doing this for 18 years and it shows in the fit. It also means you can order a top and a bottom in different sizes. Which is how most women's bodies actually work.

When a Virtual Fitting Makes Sense

If you have ever bought a bikini top that fit in the band but not the cup, or vice versa, a virtual fitting is worth 30 minutes of your time. We look at your measurements, ask about your preferred silhouette, and tell you exactly which size and style to order. We have done this with clients in Tokyo, Paris, Singapore, and Sydney. The fit rate is significantly better than ordering from a size chart alone. It is free and there is no obligation to purchase.

If you are unsure about sizing, book a virtual fitting before you order. It takes 30 minutes and it is free. Details on the contact page.