A first bra does not need to be complicated. Teen and girls’ bras are usually sold in simple letter sizes from XS to XL, based on one measurement around the ribcage. When the ribcage grows beyond that range, women’s band-and-cup sizes take over. This calculator reads your underbust and bust measurements and shows both: the girls’ size from Nike’s chart and the band and cup size.
How to use the teen bra size calculator
- Measure the underbust: wrap a soft tape around the ribcage right under the chest, level all the way round and snug.
- Measure the bust: around the fullest part, tape level and not tight.
- Enter both in inches or centimetres.
- Read the results: the girls’ size and the numeric clothing size it corresponds to, and the band and cup size.
Teen bra size formula
band = underbust rounded to the nearest even inch
cup = bust − band, in inches (1 = A, 2 = B, 3 = C)
The girls’ ranges are Nike’s teen bra guide (retrieved 2026-10-15). The band and cup method is the one described on Wikipedia’s bra measurement page. A girl whose underbust is between two ranges (for example 25.75 in) should take the larger.
| Girls’ size | Numeric clothing size | Underbust (in) | Underbust (cm) |
|---|---|---|---|
| XS | 6–7 | 24.5 – 25.5 | 62.2 – 64.8 |
| S | 8–9 | 26 – 27 | 66.0 – 68.6 |
| M | 10–12 | 27.5 – 28.5 | 69.8 – 72.4 |
| L | 14–16 | 29 – 30.5 | 73.7 – 77.5 |
| XL | 18–20 | 31 – 32.5 | 78.7 – 82.5 |
Worked example
Underbust 27.5 in, bust 31 in.
- 27.5 in falls in the 27.5 to 28.5 range → girls’ size M (numeric 10–12).
- Band: 27.5 rounds up to 28. Cup: 31 − 28 = 3 inches, a C: 28C.
Choosing a first bra
- Comfort first: soft, seamless, unwired styles are common choices for a first bra.
- Re-measure often: bodies change quickly during puberty, so check the fit every few months.
- Fit check: the band should sit level and snug, the straps should not dig, and the fabric should lie flat.
- Sports bras are often a good everyday option; see the Sports Bra Size Calculator.
The results are general information. Parents and teens can also ask a fitter in a shop, and a doctor if there are health concerns about development. For international sizes see the Bra Size Converter (US/UK/EU/AU/FR/JP).
Sources
Every figure above comes from these pages, read directly on the dates shown. Brand names are used only to identify published charts.
- Nike — teen bra guide (girls’ sizes by underbust, numeric sizes, fit advice) — checked 2026-10-15
- Wikipedia — Brassiere measurement (band and cup method) — checked 2026-10-15
Frequently asked questions
What bra size should a teenager wear?
It depends on her measurements, not her age. Nike’s teen bra guide uses the underbust measurement: 24.5–25.5 inches is an XS, 26–27 an S, and so on to 31–32.5 for an XL. Above that, women’s band and cup sizes are used.
How do I measure for a first bra?
Use a soft tape around the ribcage right under the chest, level and snug, but not tight. Nike’s advice is to keep the tape level, and to choose the larger size if between sizes.
What if she is between two sizes?
Nike says that if you are between sizes, go for the bigger one, because a little extra room is better than too tight.
When should a girl start wearing a bra?
There is no set age. Breast development begins at different ages, and the right time is when she is comfortable and wants support. This calculator only helps with the size.
What is the difference between girls’ and women’s sizes?
Girls’ bras are sized by letter and by an underbust range, and are often soft and unpadded. Women’s bras use a band and cup. The calculator shows both so you can see where the underbust moves from one to the other. See Bra Size Converter (US/UK/EU/AU/FR/JP).