A period swimsuit that shifts on a dive, digs into the shoulders during butterfly or leaves gaps around the legs is not going to feel reliable when you are trying to focus on a set. Knowing how to choose period swimwear size starts with treating it like any other piece of training kit: the fit needs to be secure, comfortable and suited to the sessions you actually swim.
Period swimwear is designed to look and feel like regular swimwear while offering built-in protection for use in and around the water. The right size helps that protective layer sit where it should, while also giving you the freedom to streamline, tumble turn and train with confidence.
Start with the brand size chart
Do not rely on the size you usually buy in leggings, school uniform or everyday underwear. Swimwear sizing varies between brands, and performance-focused cuts can feel different from fashion swimwear. A size chart for the specific product is always the best starting point.
Take fresh measurements before ordering, particularly if you are buying for a growing junior swimmer. Measure the fullest part of the bust, the natural waist and the fullest part of the hips. For a one-piece, torso length can matter too, especially for taller swimmers or anyone who often finds shoulder straps tight.
Use the measurement that places you in the larger size when you sit between two sizes. A swimsuit should feel supportive, but period swimwear that is overly tight can be uncomfortable for a full training session and may pull away from the body at the leg openings. The aim is close contact, not restriction.
How should period swimwear fit?
A correctly fitted period swimsuit should feel snug when dry. Most swim fabrics relax slightly once wet, so a suit that feels loose in the changing room is unlikely to become more secure in the pool.
The protective gusset needs to lie flat against the body. If there is visible wrinkling, bagging or a gap at the crotch, the size or style may not be right. Equally, the suit should not pull sharply across the hips or create deep marks after only a few minutes of wear.
Check the fit in the same way you would assess a training costume. Lift your arms overhead, squat, bend forward and mimic a streamline position. For swimmers who train regularly, it is worth checking that the straps stay in place through shoulder movement and that the leg line remains comfortable when kicking.
A good fit usually means:
- the fabric sits flat across the bust, hips and bottom
- straps feel secure without cutting into the shoulders
- leg openings sit close to the body without pinching
- there is no excess fabric around the waist or crotch
- you can move freely through a full range of swimming strokes
Choose the right style for your sessions
Size is only part of the decision. The best style depends on how and where you swim. A one-piece is often the most practical option for lessons, squad training and club sessions because it provides consistent coverage and stays in place through repeated lengths, dives and turns.
Two-piece styles can work well for recreational swimming or lower-intensity sessions, provided the bottoms fit securely around the waist and legs. If you are buying period swimwear for a young swimmer, consider how confident they feel changing at the pool and whether a familiar one-piece shape will make the transition easier.
For competitive swimmers, it is worth separating training kit from race kit. Period swimwear is a useful option for everyday pool time, warm-ups, technique work and training blocks. However, swimmers should follow the rules of their competition and speak to their coach where racewear requirements are specific.
Do not size up for extra protection
Absorbency comes from the built-in protective layers, not from buying a larger costume. Going up a size to gain more coverage can create gaps that reduce the close fit needed for the swimwear to perform as intended.
Instead, check the product description for its stated absorbency level and recommended use. Some period swimwear is designed for spotting or lighter days, while other options offer more protection. Your flow, session length and confidence level all matter.
If you have a heavier flow or are returning to the pool after a long break, consider using period swimwear as part of a wider plan rather than expecting one garment to suit every situation. A shorter technical session, easy access to changing facilities and a spare swimsuit in your mesh bag can make pool time feel far more manageable.
Fit advice for junior swimmers
Parents buying for swimmers in lessons or squads often face the same question: should you buy for now or allow room to grow? With period swimwear, a close fit is more useful than extra growing room. A costume that is too large can move during jumping in, kicking practice and turns, which can affect both comfort and confidence.
That said, do not force a junior swimmer into a smaller size because they are close to the next bracket on the chart. If the torso feels short, straps are digging in or the fabric is stretched thin across the hips, move up. Comfort is especially important for swimmers who are new to periods and may already feel self-conscious about training.
Give them a chance to try the suit on at home before the first pool session, following the retailer's hygiene and returns guidance. Encourage a few movement checks rather than a quick glance in the mirror. If they can stretch into a streamline, squat comfortably and move without adjusting the suit, it is a positive sign.
Check your fit before the first full session
A new suit can feel different in the water, particularly when a swimmer is used to a standard training costume. Wear it for a shorter swim first if possible. Notice whether the straps remain comfortable, the legs stay in place and the lining feels secure during push-offs and turns.
For regular squad swimmers, having a second period swimsuit can be practical. It means there is always a dry option for back-to-back training, galas, school swimming or a weekend away with the club. Rinse the suit in cool fresh water after use and allow it to air dry fully, away from direct heat, to help maintain the fabric and fit.
When the size looks right but does not feel right
Sometimes the measurements point to the correct size, but the cut simply does not suit your body shape. This is common with swimwear. A swimmer with a longer torso may prefer a style with more length through the body, while another may need a different strap design for better shoulder comfort.
Do not keep a suit that distracts you every length. Try another style or brand, compare the size charts rather than the label alone, and prioritise how it performs in movement. The best period swimwear should feel like dependable training kit - one less thing to think about when you are on the blocks, in the lane or building towards your next session.