First Bra Sizing by Measurement, Not by Age
Two measurements and one subtraction give the answer: underbust snug, overbust relaxed, and the difference between them decides both the garment type and the size. Age charts circulating online almost never state a sample and the age ranges they use overlap so heavily that they carry close to no information. The stage table below is indexed to centimetres, with age shown only so you can see how wide the overlap actually is.
By the Think Bra Editorial Team
Measure the underbust snugly and the overbust relaxed, subtract one from the other, and read the difference off the table below. That number decides two things at once: what kind of garment is appropriate and what size to buy in it. Nothing else needs deciding.
Age is on the table because people look for it, and it is there mainly so the overlap is visible. The measurement bands below correspond to age ranges that run five or six years wide and sit on top of each other, which means age can move a recommendation by at most one step and gets it wrong often enough not to be worth using.
The term "training bra" is worth retiring while we are here. It describes a soft unstructured garment with no wire and usually no adjustable band. It doesn't train anything, and the name has caused a fair amount of confusion about whether the garment is doing a job — it is: it provides a smooth layer, a bit of coverage and a small amount of movement control, and that is a legitimate specification.
Take the two measurements
A soft dressmaking tape, a mirror, and a thin non-padded top. Two minutes.
- Underbust: tape directly beneath the breast tissue, level all the way round, pulled snug enough that it stays put when you let go but not tight enough to compress. Stand upright, arms down, breathe out normally.
- Overbust: tape across the fullest part, level, resting rather than pulling. Arms down. Tape tension here is the single biggest source of error — too tight and the reading drops a full centimetre.
- Subtract underbust from overbust. Roughly 2.5 cm of difference is an A cup, 5 cm a B, 7.5 cm a C, 10 cm a D.
- Take both readings twice. If they disagree by more than a centimetre, the tape moved; do it again rather than averaging.
- Write down the centimetre figures, not just the resulting size. Tracking the raw numbers over a few months is what turns "is it time for the next size" from a judgement call into a lookup.
The stage table
Stages here are defined by the measured difference and by nothing else. They are a lookup index, not a description of anything about the person being measured.
Stage Overbust − underbust Underbust range Typical age range (overlapping heavily) Garment type Size to buy What triggers the next size 0 Under 1 cm 56–66 cm 7–12 None needed; a vest or camisole if wanted By chest measurement, S/M/L Difference reaches 1 cm, or a preference for a layer 1 1–2.5 cm 56–68 cm 8–13 Crop top or fitted vest, no cups Chest measurement, XS–M Difference reaches 2.5 cm 2 2.5–5 cm 58–70 cm 9–14 Non-wired bralette with a fixed band XS–M, or 28AA–30A where a band-and-cup size is offered Difference reaches 5 cm, or the top edge starts marking 3 5–7.5 cm 60–74 cm 10–15 Soft-cup bralette with an adjustable back and a proper elastic band 28A–32B Difference reaches 7.5 cm, or tissue sits outside the side edge 4 7.5–10 cm 62–78 cm 11–16 Seamed or lightly moulded soft cup, two-hook adjustable band 28B–34C Difference reaches 10 cm, or the cup edge cuts across the front 5 Over 10 cm 64–84 cm 12 and up Adult size run applies; structured wire-free or wired 28C upward, measured properly Normal adult re-measurement intervals from here Find the difference in centimetres, buy the garment type in that row, and use the last column as the only trigger to move up. Note how far the age column overlaps — a 12-year-old appears in four of the six rows, which is why age is the wrong index. Underbust ranges overlap between stages for the same reason. Ribcage size and cup difference are independent variables, and a narrow ribcage with a 7 cm difference is Stage 3 exactly as much as a wide ribcage with a 7 cm difference is.
Where a garment offers a band-and-cup size rather than S/M/L, take it. A 30A is a specification; a medium is a hope. The step from S to M in youth ranges commonly spans 8–10 cm of chest measurement, which is three band sizes' worth of band tension in one jump.
Why the age column is nearly useless
Age-indexed bra charts are everywhere and they share one property: no stated sample. There is no published dataset mapping bra size to age with a described sampling method that would survive being cited, and the charts that circulate are back-filled from retail size runs rather than from measurement.
That does not mean nothing is published. It means the things that are published measure something adjacent, and it is worth knowing which is which before trusting a number.
Source What it actually publishes Sample stated? Usable for sizing? Brand youth size chart Chest circumference in cm mapped to XS–L for that brand's own garments No Within that brand only, and only for band tension ISO 8559-1 and 8559-2 Where to place the tape for each body measurement, and how a size label should be constructed from body dimensions Not applicable — a definitions standard Gives you the measuring method, not a size EN 13402 Size designation of clothes by body dimensions, including the pictogram label format Not applicable — a labelling standard Tells you the label should carry the body measurement itself, which is the right principle ASTM body measurement standards for girls' sizes Body measurement tables underlying US girls' garment sizing Derived from anthropometric survey data; the basis varies by revision Chest circumference only — no bust-minus-underbust figure National child anthropometric surveys Chest circumference percentiles by age Yes, stated in each survey Sets a plausible underbust, says nothing about cup Age-based bra charts on retail blogs A band and cup letter per year of age No No Only two rows here have a stated sample, and neither publishes the measurement that determines cup size. Take the method from the standards, take the size from your own tape. The useful conclusion from that table is the EN 13402 principle: the most honest size label is the body measurement itself. A garment labelled "fits 66–71 cm underbust" is telling you something checkable. A garment labelled "age 12–14" is telling you what the buyer at that retailer assumed.
Checks that work on a soft-cup garment
- Band level. Run a finger round the band at the back — it should sit at the same height as the front, not ride up between the shoulder blades. This is the same check as on an adult bra and it fails for the same reason: the band is too big.
- Two fingers under the band, not four. On a fixed-band bralette you can't adjust it, so this is a buy-or-don't test rather than a tightening one.
- No red line after twenty minutes of ordinary wear. A faint temporary mark from the band is normal; a line that is still visible half an hour after taking it off means the size is wrong.
- Strap check. Straps should carry almost nothing on a soft-cup garment. If loosening them noticeably drops the front, the band is not doing its job.
- Side edge. Tissue sitting outside the side edge of the cup is the clearest signal in the whole list that the garment has been outgrown.
- Movement, not shape. The garment is doing its job if walking downstairs feels different with it on. That is the only performance question worth asking at these stages.
What actually signals a size change
Three signals, and a list of things people mistake for signals.
- The measured difference has crossed into the next row of the table. This is the real one and it is the only one that needs acting on immediately.
- The band no longer sits level, or leaves a mark that persists. Band before cup, always.
- Tissue is visible outside the side or top edge of the cup when standing normally.
- Not a signal: a birthday. Not a signal: what someone the same age is wearing. Not a signal: a garment feeling different after a hot wash, which is shrinkage in the elastic rather than a change in the body.
- Not a signal: the garment feeling tight on one particular day. Underbust measurement varies by a centimetre or so across a day and across a cycle; re-measure at the same time of day before concluding anything.
Buy this, skip that
Two garments in rotation beat five bought at once. Elastic recovers between wears, and something that gets worn every day for six months is finished long before something worn every other day for a year.
Opinions, stated plainly.
- Buy adjustable-back garments from Stage 3 onward. The adjustment is what lets one purchase survive a few centimetres of change instead of none.
- Buy cotton or a cotton-rich knit for the early stages. It is more comfortable under a school shirt than a polyester moulded cup and it costs less.
- Buy two. Rotation is the cheapest life-extension available for any elastic garment.
- Skip push-up padding at every stage on this page. It adds volume to a cup that has none spare and it changes nothing about how the garment performs.
- Skip adhesive cups entirely. They have the lowest Hold Index of any construction in the category and their failure mode is public.
- Skip wire until Stage 4 at the earliest, and only then if the difference is genuinely at or above 7.5 cm and a structured wire-free option has already been tried.
- Skip anything sized only by age. If the label has no body measurement on it, the manufacturer has told you how much thought went into it.