Training-bra sizing is confusing because there's no single system everyone uses. One brand uses kids' clothing sizes, another uses XS–XL, another uses band-and-cup sizing. We treat the label as a starting point and the actual fit as the real test.
Our sizing rule, in three steps
Measure → check that brand's current size chart → run the 5-point fit check. Don't use age instead of a real measurement, and don't assume a Medium at one brand means the same thing at another.
Measure
Where to put the tape, how snug is snug, and why measuring twice actually matters.
How to measure →Read the size chart
How kids' sizes, XS–XL, and band/cup sizing are actually different from each other.
Training bra size guide →Check the real fit
Band, straps, front, movement, and comfort — checked after actually trying it on.
Fit guide →Not sure where to start? Try the Size Assistant
It won't invent a universal bra size for you — nobody can do that from just a couple of answers. It just asks what you already know and points you to the right next step.
The three sizing systems you'll run into
| System | Looks like | What it tells you | The catch |
|---|---|---|---|
| Kids' clothing sizes | 10/12, 14/16 | Roughly matches clothing sizes | Not based on actual bust/underbust measurements |
| Letter sizes | XS, S, M, L | Groups a range of body sizes together | The ranges differ between brands |
| Band/cup | 28AA, 30A | A band number plus a cup letter | Brands calculate and fit these differently |
Something feel off? Figure out what, not just "wrong size"
Band rides up
Usually the band isn't anchoring well — though the style and how stretchy it is can matter too.
Fix band riding up →Straps fall down
Could be the strap adjustment, shoulder shape, how stretchy the fabric is, or the size.
Fix falling straps →Front gaps open
Can come down to size, shape, style, or just a cut that doesn't match this particular body.
Fix front gaping →Growing changes everything
A bra that fit a few months ago might not fit now. Growth, stretch, and normal washing can all change how something fits. Our growing-out guide looks at what to actually watch for, instead of guessing by age.
Why there's no single universal size chart here
Because one wouldn't actually be accurate — it would just look precise. Our size chart guide explains how to read a manufacturer's own chart and compare charts without pretending every brand's labels mean the same thing.
The order that actually works
Think of sizing as a few steps, not a single calculation. Measure first. Then find out what sizing system this specific product uses. Then use that brand's chart to pick a starting size. Then check how it actually feels once it's on and you're moving around. Skip a step, and a size that looked right on paper can still feel wrong on the body.
Why this matters more for a first bra
First-bra shopping usually happens while your kid is still figuring out what feels comfortable at all. A confident-sounding size answer can feel reassuring, but it can also create pressure to make one size "work" when it just doesn't. It's better to know it's okay to try something, compare it, and say no if it's not right.
Learn to describe what's actually wrong
"This size is wrong" doesn't tell you much. "The band rides up," "the straps fall," "the front gaps," or "it feels tighter than it used to" each point to a different fix. That kind of specific language makes it a lot easier for parents, kids, and stores to actually solve the problem — instead of just judging a body.
Keep it practical, keep it private
Measuring and checking fit don't need to be a big production. Your kid can measure themselves, ask for help from someone they trust, or use a store's fitting service if they want one. The goal is just good information and a bra that actually feels good.