Let’s be honest: clicking “add to cart” on a new bikini can feel like a gamble. When you can’t try it on, how do you really know if it will fit? This uncertainty is amplified when you’re buying a sport bikini, where the right fit isn’t just about how you look—it’s about how you perform. An ill-fitting suit can lead to chafing, slipping, or a wardrobe malfunction right when you’re focused on your activity.

But it doesn’t have to be a guessing game. Understanding how to properly size a sport bikini is the single most important step in ensuring it performs as intended. This guide will walk you through everything you need to know to choose your size with confidence, so you can get back to what you love without giving your swimwear a second thought.

The first thing to understand is that sport bikini sizing is fundamentally different from regular fashion sizing. Your dress size or the size you wear in a casual bikini is likely not your correct size for performance swimwear. As we covered in our guide on sport vs. regular bikinis, a sport bikini is engineered for a snug, secure fit that stays put during intense activity. It’s designed to feel like a supportive second skin, not a loose covering.

This means you should expect a new sport bikini to feel tighter than you’re used to. It’s a compressive fit that’s essential for providing support and preventing the suit from filling with water or shifting when you move. Embracing this “snug fit” philosophy is the key to unlocking the performance benefits of your gear.

Your most powerful tool in this process is a simple measuring tape. Taking accurate measurements of your body is the only way to reliably match yourself to a brand’s size chart. Forget your usual size and start fresh with these key measurements:

1. Bust: Measure around the fullest part of your bust. Keep the tape level and parallel to the floor. Don’t pull it too tight; it should be snug but not constricting.
2. Under-Bust: This is a critical measurement for bikini tops. Measure directly underneath your bust, around your ribcage. This measurement is the foundation of your top’s support system.
3. Waist: Measure around the narrowest part of your torso, which is usually just above your belly button.
4. Hips: Measure around the fullest part of your hips and bottom. Again, keep the tape level and snug.

Once you have these numbers, you can confidently compare them to the size chart on the product page. This is your source of truth, far more reliable than guessing based on other brands or your t-shirt size.

Many of us have made sizing mistakes in the past, but they are easy to avoid once you know what to look for. The most common errors often stem from applying fashion logic to performance gear.

• Buying Your Usual “Size”: Ignoring the size chart and just picking a “medium” because you usually wear a medium is the fastest way to get the wrong fit. Every brand is different, so always trust the chart.
• Fearing the Snug Fit: Many women try on a properly sized sport bikini and think it’s too small because it feels tight. Remember, that snugness is a feature, not a flaw! It will relax slightly in the water.
• Only Measuring Your Bust: For tops, the under-bust measurement is arguably more important than the bust size. It determines the fit of the band, which provides most of the support.
• Not Considering Your Body Type: As we explored in our guide to fitting different body types, your unique proportions might mean you need to prioritize certain fit features, like an adjustable top for a larger bust.

Once you have your measurements and you’re ready to choose, use these pro tips to ensure you get the best possible fit for your active lifestyle.

• Buy Tops and Bottoms Separately: Very few people are the same size on top and bottom. Buying separates is essential for getting a perfect fit for both halves of your body.
• Prioritize the Under-Bust Band: For tops like our Parla Bikini Top, the fit of the band is everything. It should be snug and firm. If it’s loose, the top will not provide adequate support.
• Do an “Activity Test” at Home: When your new bikini arrives, don’t just stand in front of the mirror. Jump, twist, bend over, and swing your arms. Does everything stay in place? If so, you’ve found a great fit.
• Check for Gaps and Digging: The fabric should lie flat against your skin. If there are gaps, the suit is too big. If the edges are digging in uncomfortably, it’s likely too small.

Getting the right size in a sport bikini boils down to a simple strategy: measure yourself accurately, trust the size chart, and embrace the snug, supportive fit that performance requires. A well-fitting bikini is the foundation for all the top 10 features you should look for, from support to durability. When your gear fits perfectly, you build a sense of trust that allows you to pour all your energy into your sport. For a deeper dive into the technical aspects of fit, our complete guide on what to look for in performance bikinis is a great next step.

For performance swimwear, we almost always recommend sizing down if you’re between sizes. A suit that’s slightly too snug will relax to a perfect fit in the water, while a suit that’s slightly too big will only get looser and fill with water, causing drag and slipping.

It should feel noticeably tighter than a regular bikini or your everyday clothes. Think of it like a firm, supportive hug. It should not be painfully tight or restrict your range of motion, but you should feel securely held in place.

Your size should remain consistent, but the style you choose might change. This is where versatile swimwear comes in. A high-impact activity like surfing might require a top with more coverage and support, while a lower-impact activity like paddleboarding might allow for a different style, but the underlying size based on your measurements should be the same.

While it might be a close starting point, sports bra sizing can vary, and more importantly, swimwear fabric behaves differently in water. A sports bra isn’t designed to handle the forces of water or resist stretching out when wet. Always rely on your direct measurements and the swimwear-specific size chart.

They should be snug around your hips and waist without digging in, and the fabric across your bottom should lie flat. If you can easily fit more than a finger or two under the waistband, they are likely too big. Do a few squats and leg lifts—if they stay put, you’re in good shape.

A high-quality sport bikini made from fabric like ECONYL® will have excellent shape retention and will not stretch out permanently like cheaper swimwear. It will relax slightly when wet but should recover its shape after being rinsed and dried. This is why starting with a snug fit is so important.

Choosing the right size is an act of self-care that sets you up for success in the water. Take the time to do it right, and you’ll be rewarded with a bikini that feels like it was made just for you.

Because you deserve swimwear that works as hard as you do.