The best training bra size isn't based on a birthday, a grade in school, or a size borrowed from a different brand. Start with real measurements, use that specific brand's current chart, then check the actual fit.
Quick answer
Training bras usually use kids' or teen clothing sizes, letter sizes like XS–XL, or band-and-cup sizes. The same letter or number can mean a different measurement range depending on the brand.
The three main sizing systems
Kids' or teen clothing sizing
Some first bras are sized like regular kids' or teen clothes. That's easy when the fabric stretches a lot, but it only gives you a rough starting point. Check the brand's body-measurement chart if they have one.
Letter sizing (XS–XL)
XS, S, M, L and XL group a range of body measurements together. It's easy to shop, but the exact cutoffs are different for every brand — never assume a Small at one brand matches a Small at another.
Band-and-cup sizing
Traditional bra sizing separates the band measurement from the cup size. It can be more precise, but the actual formulas and fit still vary a lot by brand.
A better way to pick a starting size
- Measure the underbust and bust right now — not from memory.
- Open that exact brand's current chart for the style you're looking at.
- Match the actual measurement range, not the age.
- Stuck between two sizes? Check the brand's own advice on that, and think about how stretchy the fabric is.
- Once it's on, run the 5-point fit check.
Why age doesn't work as a sizing tool
Kids the same age can have very different heights, builds, and stages of development. Age-based sizing might be convenient for a store shelf, but real measurements tell you a lot more about how something will fit. That's why we don't publish "age equals bra size" charts.
What if the measurements fall between two sizes?
There's no single rule that says always round up or always round down. A stretchy pullover style behaves differently than a firmer sports bra. Check the brand's own guidance first, then think about what feels more comfortable and what the bra is actually for.
The chart gets you close — the fit check confirms it
Even a bra that matches the chart perfectly can still fit badly. Check that the band stays level, the straps stay put, the front sits comfortably, it stays stable when moving, and your kid isn't thinking about it.
Common sizing mistakes
- Buying based on age alone.
- Assuming the same letter size works the same at every brand.
- Pulling the measuring tape way too tight.
- Sticking with a bra that constantly needs adjusting just because "the chart said it fits."
- Ignoring style: a racerback, pullover, and sports bra can all feel different in the same size.
Stretch changes how a size feels
Two bras labeled the exact same size can feel completely different because the fabric behaves differently. A stretchy cotton blend, a smooth seamless style, and a firmer sports bra could all say Medium and still fit very differently. That's why brand and style matter just as much as the letter on the tag.
Don't stock up before you know it fits
If it's a new style, try just one first if you can. Wear it long enough to really check the band, straps, seams, and movement. Once you know the style actually works, then it makes sense to buy more colors or extras.
Keep a simple note
For a kid who's still growing, a quick note can save time later: brand, product, size bought, date, and whether it ran loose, snug, or just right. That's more useful than assuming the same size will still work months from now — or at a different brand.