Here is something the swimwear industry does not talk about.

The sizing system you are measuring yourself against was built in the 1940s using body data from men in the military. Not women. Not swimmers. Not anyone who was going to wear a one-piece into the ocean. Men. In uniform.

Eighty years of adjustments have not fixed the core problem. The system was never designed for the range of actual women's bodies. It was designed for standardised production.

On top of that, women's bodies shift naturally throughout the month. Hormones, fluid retention, cycle timing — all completely normal, all completely ignored by every sizing chart you have ever looked at.

So when you find yourself between a medium and a large across three different brands, that is not a "you" problem.

The sizing system is what is broken. Not your body.

Performance swimwear follows different rules than the dress hanging in your wardrobe. Here is what actually matters:

If you are between two sizes, go up.

Here is something that makes this decision easier than it sounds: the difference between consecutive sizes is generally only 1–2 cm. That is it. You are not jumping into a dramatically bigger suit. You are giving yourself a centimetre of breathing room that disappears the moment the fabric hits water anyway.

A suit that sits slightly loose will compress against your body when it gets wet. Water tightens the fit naturally. A suit that is too tight does the opposite of what you need — it restricts your breathing, limits your shoulder range, digs at the leg line, and turns every stroke into a fight against your own gear.

You can swim in slightly loose. You cannot swim in too tight.

Check your torso length, not just bust and hips.

For one-piece swimsuits and springsuits, torso length is usually the measurement that determines whether the suit actually fits. It is not the one most people check first. If a suit pulls at the shoulders or rides up at the bottom, it is almost always a torso length issue — not a size issue.

Ignore the mirror. Test with movement.

Raise both arms overhead. Simulate a stroke. Twist at the waist. Squat. The suit should move with you without shifting, riding, or digging. If it passes the movement test, it fits. Forget what the tag says.

Your size is not your identity. It is a starting point on a chart that was never built for you in the first place. The number that actually matters is zero — as in, the number of times you should have to adjust your suit once you are in the water.

That is the standard. Everything else is just a label.