Why swimsuit sizing is broken, and what to use instead

A size M from one swimwear brand can be a size S from another. This is not a secret and it is not new. Vanity sizing, regional grading and stretchy fabric all push labels away from real measurements. The practical answer is to stop shopping by label and start shopping by numbers.

Why the label lies

Three forces are at work:

• Vanity sizing: brands move the M up over time so customers feel smaller. A 2026 M may match a 2018 L.
• Grading differences: a brand may grade up at the bust but not the hip, so the same letter fits two bodies differently.
• Fabric behaviour: a ribbed one-piece with 20% elastane will stretch to fit a size either side. A structured bikini top with 8% will not.

The label is a marketing decision. The measurements are geometry.

Use a three-number band, not a single size

Take bust, waist and hip. Find the row where at least two of the three land. If the third is two rows away, you are between sizes and the garment construction decides:

• One-piece: size up. You can take in a strap; you cannot add fabric.
• Bikini top: match the bust and underbust first. The band is the support.
• Bikini bottom: match the hip. A waist gap is easier to live with than a cut that digs in.
• Cover-up: size down if it is oversized by design.

Our swimsuit size chart lists the bands we use, with UK, US and EU conversions. It is a starting point, not a guarantee.

The 2cm rule

If your measurement is within 2cm of the top of a size band, you are effectively between sizes. Choose based on how the garment is constructed, not on hope. For anything with a fixed seam at the waist or a non-adjustable strap, take the larger size.

What a preview can and cannot tell you

A visual preview shows proportion and coverage: where the neckline lands, how the leg line reads, whether a print is too busy at full body scale. It cannot measure stretch, chlorine resistance or how a seam feels after two hours.

Use the AI swimsuit changer to eliminate the cuts that are obviously wrong for you. Use measurements to choose the size of the cut that survives.

A short buying routine

1. Measure bust, waist and hip in centimetres.
2. Find your band in the brand chart, not your usual letter.
3. Check fabric composition and stretch.
4. Preview the cut on your own photo to rule out proportion mismatches.
5. Buy one size, not three.

That routine will not make sizing perfect. It will make it predictable enough to stop guessing.