Bra sizing starts with two numbers: the underbust and the bust. Neither is difficult to measure, but both are easy to measure wrongly — and being two centimetres out usually means a whole row of hooks was bought for nothing.
§ 1What to have ready
- A soft measuring tape. Without one, a length of non-stretchy string plus a ruler works; it simply adds a step.
- A mirror you can see your whole torso in, to check the tape is not riding low at the front.
- No bra at all, or a thin unpadded, unwired one.
- Paper and pen, or a note on your phone — you are recording more than one number.
You do not need anyone to help. Measuring yourself is often more accurate, because you can feel directly whether the tape is pressing in. For more on size and fit, see the Sizing section.
§ 2Measuring the underbust
- Stand upright and run the tape horizontally around the ribcage, at the crease under the bust — the line where a bra band sits. The tape should lie against the body and stay level all the way round.
- Read the number at the end of a normal exhale. Do not inhale and do not deliberately tighten. The ribcage expands by one to two centimetres on an inhale, and that reading will keep you on the loosest hooks for the life of the bra.
- Snug, not tight: you should just be able to slide one finger between tape and skin. If the tape leaves a mark on the skin, it is too tight.
- Round to the nearest centimetre, then repeat the measurement twice and take the larger of the two. If the two differ by more than two centimetres, the tape was not level; start again.
Common error
Many people measure where the band of the bra they are wearing currently sits, rather than where the ribcage is. The band rides up, so the two are often slightly different. Measure the body, not the bra.
§ 3Measuring the bust
The bust is the horizontal circumference around the fullest part of the chest. Read it at the end of an exhale as well, and do not flatten the tissue with the tape. It has exactly one use: subtracting the underbust from it gives the cup step. On its own it means nothing.
Leaning forward makes the reading smaller; deliberately lifting the chest makes it larger. A relaxed standing posture with the arms down gives a number you can actually use.
§ 4How the two numbers become a size
| Item | How it is worked out | Notes |
|---|---|---|
| Band size | Round the underbust to the nearest 5 cm step | 78 cm, for example, corresponds to the 80 step. Between two steps both usually work; it comes down to whether you prefer the band firmer or looser |
| Cup | Bust minus underbust, in steps of roughly 2.5 cm | About 10 cm is an A, 12.5 cm a B, 15 cm a C, 17.5 cm a D |
| Equivalent sizes | One step down in band with one step up in cup often holds the same volume | 80B and 75C frequently hold a similar volume; the band tension is what differs |
| Different systems | China and Europe step in centimetres; the UK and the US step in inches | The steps do not line up exactly after conversion, so do not simply copy numbers across |
The limits of this conversion
It gives a starting range, not a definite answer. The same 80B can differ by half a cup between brands, because cut, band elasticity and cup shape all vary. No measuring method replaces trying a garment on. The value of measuring is that your starting point is close enough, not that it removes the fitting.
§ 5When to measure again
A measurement is not valid for life. It is worth repeating when any of the following applies.
- Your weight has changed by more than two or three kilograms.
- The band rides up even on the tightest hooks, which means the band has loosened and you should go one step down.
- There is a gap along the top edge of the cup, or the wire is pressing into breast tissue.
- During the days of the cycle when breast volume changes noticeably. Measurements taken then are not a good long-term standard.
- A bra has been in use for around six months or more; band elastic loses tension and the fit is no longer what it was when new.
A note touching on health
Persistent marks, pressure points or ribcage pain usually mean the size or cup shape is wrong rather than that it should be endured. This journal offers general information only and does not give medical advice; if pain persists, please consult a doctor. For why a band loosens with wear, on the mechanical action and drying marks, see Care Label Symbols.
Sources and uncertainties
- Bands in 5 cm steps and cups in steps of roughly 2.5 cm of bust minus underbust are the sizing convention generally used in China and Europe.
- The UK and the US work in inches and subdivide differently, so the steps do not align exactly on conversion. This page therefore gives no cross-system conversion table, because the way brands implement it is itself not uniform.
- Taking the larger of two readings and reading at the end of an exhale are practices for reducing random error, not standard requirements. They do not change the conclusion that a size is only a starting point.