How to Size Swimwear That Actually Fits

The Problem With Swimwear Sizing

Your Alaïa bodysuit fits like a second skin. Your Toteme trousers required no alterations. So why does ordering swimwear online feel like a gamble with worse odds than finding a taxi in August? The answer lies in construction. Unlike ready-to-wear, which relies on darts, seams, and strategic draping to accommodate your shape, swimwear uses stretch fabrications that must grip, support, and flatter without gaping or digging in. A single size differential in the bust can mean the difference between chic and catastrophic. This swimwear sizing guide will help you navigate the measurements that matter.

What to Measure (and How)

Before consulting any brand's size chart, you need three numbers: bust, waist, and hip. Use a soft tape measure and take readings over bare skin or the thinnest possible underwear.

Bust: Measure around the fullest part of your chest, keeping the tape parallel to the floor. Don't pull tight. If you're between sizes here, consider whether you prefer more or less coverage. Brands like Eres and Melissa Odabash cut their tops generously through the band, while others such as Hunza G (which knits everything in one size) rely entirely on the fabric's give.

Waist: Find the narrowest point of your torso, typically an inch or two above your navel. This measurement is critical for one-pieces and high-waisted bottoms. Many Italian brands including Oséree size their swimsuits closer to contemporary sportswear, meaning their 'small' corresponds to a UK 8 or US 4, not the UK 10 / US 6 you might expect.

Hips: Measure around the fullest part of your hips and backside, again keeping the tape level. This dictates bottom fit. Brazilian-cut styles require less ease than full-coverage options.

How Luxury Swimwear Sizing Differs

Ready-to-wear uses woven fabrics with minimal stretch, so a size 40 Jacquemus dress will fit much like a size 40 from The Row. Swimwear, however, is knit from nylon-elastane blends (or increasingly, recycled polyamide) that can stretch anywhere from 20% to 200%. This variability means sizing becomes brand-specific, sometimes wildly so.

Take Lisa Marie Fernandez: her crepe fabric has almost no stretch, so the brand recommends sizing up if you're between measurements. Conversely, Norma Kamali uses her signature double-knit jersey with substantial give, allowing a single size to accommodate a broader range of measurements. This is why a proper swimwear sizing guide must account for fabrication, not just numbers on a chart.

European brands (Zimmermann, Eres, Matteau) often run smaller through the bust and more generous through the hip than American contemporary lines. If you typically wear a 34D bra, you may need a French size 40 top but a 38 bottom. Separates exist for this reason.

Brand-Specific Considerations

Some practical notes from the fitting room:

• Solid & Striped runs true to US sizing but cut their tops for a B-C cup. Larger busts should look for their styles with underwire or adjustable ties.
• Hunza G knits everything in one size that theoretically fits UK 8-14, but the fabric's tension works best on UK 10-12 frames. Smaller or larger bodies may find the ruching awkward.
• Eres uses French sizing and assumes a B cup across all tops. Their bottoms, however, are generous and forgiving.
• Matteau cuts Australian sizes that run large. Their size 1 fits like a US 2-4.

If a brand offers tops in bra sizes (30D, 32B, etc.) rather than S/M/L, use that system. It's infinitely more reliable.

When to Size Up, When to Size Down

As a general rule for this swimwear sizing guide: if you're between sizes and prioritizing support or coverage, size up. If you want minimal tan lines and a sportier fit, size down—but only if the fabric has at least 50% stretch.

For one-pieces, let your longest torso measurement guide you. Many brands list a 'torso length' on their size charts, measured from high point of shoulder, through the crotch, to the same point on the back. If yours is 60cm and the garment is drafted for 58cm, you'll be tugging at the straps all day.

Bottoms are more forgiving. Most feature at least some adjustability through side ties or a slightly elasticated waistband. The exception: structured high-waisted styles, which gap unattractively if too large.

The Final Word

Swimwear sizing remains an imperfect science, but measuring yourself properly and consulting brand-specific charts (not just selecting your usual dress size) will dramatically improve your odds. When in doubt, reach out to customer service. Most luxury brands have fit specialists who can recommend sizing based on your measurements and the specific style you're eyeing. And remember: even the best swimwear sizing guide can't replace the option to order two sizes and return one.