In short
- 🔑 The number is the underbust measurement and the letter is the difference from the bust. Every 2 cm of difference is one more letter.
- In Spanish sizes the number is about 15 above the measurement: a 90 corresponds to an underbust of 73 to 77 cm.
- The same letter is not the same amount of breast: a C on an 85 holds less than a C on a 100. That is why sister sizes exist.
- Most women wear a size that is not their own: in two studies, 80% and 85% of the women studied.
- When it comes to surgery, the letter is a guide, but the plan is not made with it: in an augmentation, the implant is chosen from the measurements of the chest; in a reduction, the plan is made in grams.
A bra size has two parts. The number is the measurement around the ribcage just below the breasts, and the letter — the cup size — comes from a subtraction: the bust measurement, taken at nipple level, minus that underbust measurement. On Spanish size charts, each letter is about 2 cm more of difference: 14 to 16 cm is a B, 16 to 18 a C and 18 to 20 a D.
Knowing this helps you buy better and, above all, understand what a letter means when breast surgery is being discussed: the cup is a ratio between two measurements, not an amount of breast. This page explains how to measure yourself, why two bras in the same size do not fit you the same, and what changes — and what does not — after an augmentation or a reduction.
If you are thinking about surgery, the operation itself is covered in the breast augmentation guide and in the breast reduction guide; this page is about size, which is the language almost everyone speaks when they arrive at the consultation.
How to read a bra size: the number and the letter
The number is the underbust. It is measured with the tape horizontal just below the breasts, in the fold.
In the sizes used in Spain, that number is not the measurement in centimetres but about 15 more. The size chart of the manufacturer of the garments we use at the clinic shows it:
| Size | Underbust |
|---|---|
| 80 | 63 to 67 cm |
| 85 | 68 to 72 cm |
| 90 | 73 to 77 cm |
| 95 | 78 to 82 cm |
| 100 | 83 to 87 cm |
| 105 | 88 to 92 cm |
The letter is a subtraction. The bust is measured at its fullest point, at nipple level, and the underbust is subtracted from it. The difference is the cup:
| Bust − underbust | Cup |
|---|---|
| 14 to 16 cm | B |
| 16 to 18 cm | C |
| 18 to 20 cm | D |
Below the B comes the A, and above the D come the E and the rest, on the same 2 cm scale. An example, the same one our size guide gives: a 79 cm underbust and a 96 cm bust are 17 cm of difference, which makes a 95C.
That is the scheme, but it is not a standard everyone follows. Each country numbers sizes its own way — in the United Kingdom and the United States sizes are given in inches — and each brand adjusts its own chart. A plastic surgeon wrote as much in 1998 in Plastic and Reconstructive Surgery: between the lack of standardisation and the different styles, finding a bra that fits well is more trial and error than measurement. And a 2015 review on standardising cup measurements makes the same point: the cup is a continuum, and until patients and surgeons speak the same sizing language, the letter lends itself to misunderstandings.
How to measure yourself at home
It takes two measurements and five minutes. What makes them come out right is how they are taken:
- A cloth tape measure, not a rigid one.
- Standing, with your back straight and your arms relaxed.
- In your underwear, or with very little on.
- The tape parallel to the floor, snug but not tight. It works better if someone helps you.
First, the underbust: just below the breasts, in the natural fold, with the tape horizontal. Then, the bust: at the fullest part of the breasts, at nipple level. With both, look up the number in the first table and the letter in the second.
If you land right on the border between two sizes, or the result looks nothing like the label you wear, that is not unusual: the measurement gives a starting point, and the fit is checked by putting the bra on.
Why two bras in the same size do not fit the same
Because sizing is designed to approximate, and on top of that most women do not wear their own size.
A study of 104 women compared the size each one chose — or the one that came from measuring — with the size given by a professional’s fitting criteria. 85% were wearing a bra that did not fit them properly, and neither trying on several styles nor using measuring charts put it right: the sizes chosen that way were still different from the correct one.
Another study, of 30 young women with upper back pain, found the same by a different route: 80% were wearing the wrong size, nearly always smaller than the one that fitted them (70%), and the poor fit was more common the larger the breasts.
The practical conclusion is modest and useful: the label you wear is not a good measure of your breasts. If you are going to talk about sizes — to buy clothes or at a surgical consultation — measuring is more use than remembering.
Sister sizes: why the cup is not a volume
Here is the key to everything that follows.
The letter is worked out relative to the underbust. That is why the same C is smaller on an 85 than on a 100: the 16 to 18 cm difference is spread around a narrower or a wider chest. And for the same reason, going up one band size and down one letter gives a cup of similar capacity: a 90B and an 85C are more alike than an 85C and a 100C. In the trade these are called sister sizes.
This is not a fitting-room curiosity. A study of 209 patients (395 breasts) compared which measure best predicted the real weight of the breast, weighed after a mastectomy. The letter alone fell short: its correlation with weight was 0.67, against 0.76 for the sister-size group, which combines band and cup. In the analysis that took several factors into account at once, only the sister-size group and body mass index still predicted weight; cup and band on their own did not.
In plain terms: a letter does not tell you how much breast there is. It tells you what the breast is like relative to that chest.
What your size means if you are having surgery
In a breast augmentation
The size you would like to be is very useful information for whoever is going to operate on you: it says what result you are after. What it does not do is choose the implant.
The implant is chosen from the measurements of your own chest — the width of the base of the breast, the elasticity of the skin and the thickness of the tissue that will cover it — which is what determines what will fit without showing or stretching too far. Two surgeons published a decision process based on five measurements in 2005: in 1,664 augmentations with up to seven years of follow-up, only 0.2% needed a second operation to change size. How that choice is made is on the page on implant size.
Given what sister sizes show, there is no figure in cubic centimetres that equals “one cup”: the same volume goes up more letters on a narrow chest than on a wide one, and the starting point counts too. What can be given is a guide to the usual range, and that is what that page does.
Expectations about the letter matter more than they seem to. In the 2015 review on standardising cup measurements, the authors point out that not reaching the expected cup size is still the main cause of dissatisfaction after an augmentation. That is why size is best discussed at the consultation with measurements in front of you, and not just with the label of the bra you wear. A 1998 paper proposed precisely measuring the breast itself to work out the cup, and found that the size after an augmentation or a reduction could be predicted accurately that way.
If you have already had surgery and are worried that you went too small or too big, that is covered in regret and revisions after an augmentation.
In a breast reduction
The aim is not a letter: it is to remove the weight that causes the symptoms and leave a proportionate breast. That is why a reduction is planned in grams and measurements, not in sizes.
Even so, the cup size you would like is discussed at the consultation, and that decision is yours. It is not a commitment: it is the guide that tells the surgeon how much volume to keep, which is why it cannot be guaranteed 100%. What is removed, what predicts it and why removing more is not better are covered in what cup size you will be after a reduction.
After a substantial reduction, you can expect to go down several letters. If large breasts are giving you back or skin problems, the line between a size and a health problem is drawn in breast hypertrophy.
When to measure your new size
After either operation, the final size is not measured straight away: the breast is swollen and, in an augmentation, the implant takes time to settle. After an augmentation, the result is clearly visible at around 6-8 weeks and the final appearance takes a few more months, as explained in recovery after augmentation.
To buy the bras you will wear from then on, at ICPM we recommend measuring yourself six months after surgery, after both an augmentation and a reduction: by then the result can be considered very close to final. Anything measured earlier describes a breast that is still changing.
The post-surgical bra: what it is and how its size is chosen
After a breast augmentation at ICPM, you leave theatre in a dressing, and the post-surgical bra goes on at 2-3 days. Some surgeons put it on straight after the operation and skip the dressing: there is no evidence favouring one option over the other, and both are valid.
It is worn for a month, day and night, taken off only to shower, and the regime is adapted to how you progress: sometimes it has to be worn for longer. After a reduction the timetable is different, and it is set out in bras after a breast reduction. In both cases, the surgeon tailors the regime to how each case is progressing.
What all the styles have in common:
- No underwire, because the wire sits right on the inframammary fold, which is usually where the scar is.
- They support the weight of the breast so that it does not pull on the stitches, and they contain the swelling.
- After an augmentation, with a chest band if the surgeon prescribes one: a strap above the breasts that holds the implant in place while the tissue adapts. It can be separate or built into the bra itself.
- Front-fastening is better: after an augmentation, you should not raise your arms above shoulder height for the first two weeks.
There is no need to buy one before your operation. At ICPM, the size of the post-surgical bra is measured after surgery, at the first check-up, and the clinic keeps bras in every size so that the one that best fits each patient can be chosen there. It is measured like any other bra — the underbust and the difference from the bust — but on the breast that has already been operated on, which is the one it has to support. And here getting it right matters more than with lingerie: a garment that is too big does not support and one that is too small hurts. The two styles we use, with their size charts, are also in the post-surgical bras in our shop.
Book an appointment
You can book your own appointment. Pick the day and time from the free slots in the diary, in a panel that opens right here, without leaving this page.
Frequently asked questions
How do I know my bra size?
Measure your underbust just below the breasts, with the tape horizontal, and your bust at nipple level. The underbust gives the number (in Spanish sizes, about 15 above the measurement) and the difference between the two measurements gives the cup: 14-16 cm is a B, 16-18 a C and 18-20 a D.
What is the difference between a B, a C and a D cup?
About 2 cm of difference between bust and underbust for each letter. But the letter is relative to the band: a C on a size 85 is not as much breast as a C on a 100.
What are sister sizes?
Sizes with a cup of similar capacity: going up one band size means going down one letter, so a 90B and an 85C are alike. In a study of 209 patients, the sister-size group predicted breast weight better than the letter alone.
How many cubic centimetres does it take to go up one cup size?
There is no fixed figure. The same volume goes up more letters on a narrow chest than on a wide one, and the starting point counts too. The implant is chosen from the measurements of the chest; the usual range is on the page on implant size.
What size will I be after a breast augmentation?
It can be estimated, not promised. The size you are after is the most useful information you bring to the consultation, but the implant is chosen from the measurements of the chest, and the final letter also depends on the style and brand of the bra.
And after a reduction?
You can expect to go down several letters. The cup size you would like is discussed at the consultation, and that guide tells the surgeon how much volume to keep, but a reduction is planned in grams and a letter cannot be guaranteed.
When can I measure myself to buy new bras after surgery?
Six months after surgery, after both an augmentation and a reduction: that is when the result can be considered very close to final. Before then, the breast is still changing and the size you measure is not the one you will end up with.
Do I have to buy the post-surgical bra before my operation?
No. At ICPM its size is measured after surgery, at the first check-up, and the clinic keeps bras in every size so that the one that best fits each patient can be chosen.
Why has my size changed without surgery?
Mainly because of weight. In a study that measured the breasts of 356 women in three dimensions, volume depended on body mass index — it was two to three times greater with overweight and obesity — and not on age.
Why does my bra label not match what I measure?
Because each brand adjusts its own chart and because most women wear a size that is not their own: in two studies, 80% and 85% of the women studied. Measuring is a better starting point than the label.
Sources
- Pechter EA. A new method for determining bra size and predicting postaugmentation breast size. Plast Reconstr Surg. 1998;102(4):1259-1265. The usual system for calculating bra size fails so often that it is useless, and there is a lack of standardisation between brands and styles; it proposes working out the cup by measuring the breast directly, and the method accurately predicted size after augmentation and reduction surgery. https://pubmed.ncbi.nlm.nih.gov/9734454/
- Bengtson BP, Glicksman CA. The Standardization of Bra Cup Measurements: Redefining Bra Sizing Language. Clin Plast Surg. 2015;42(4):405-411. The cup is a continuum; not reaching the expected cup size is still the main cause of dissatisfaction; common guidelines and standards between patients and surgeons are needed. https://pubmed.ncbi.nlm.nih.gov/26408432/
- Jolkovsky E, Miller MN, Barkhordarzadeh AD, Piva S, Alnaseri T, Slack GC. Improving Documentation in Plastic Surgery: Sister Bra Size Group and BMI Are More Indicative of Breast Weight Than Cup Size. Aesthet Surg J. 2025;45(10):1017-1025. 209 patients (395 breasts) who had a mastectomy; correlation with specimen weight: sister-size group ρ = 0.76, cup 0.67, body mass index 0.61, band 0.48; in the multivariate analysis only the sister-size group and body mass index remained significant. Level of evidence 4. https://pubmed.ncbi.nlm.nih.gov/40581068/
- McGhee DE, Steele JR. Optimising breast support in female patients through correct bra fit. A cross-sectional study. J Sci Med Sport. 2010;13(6):568-572. 104 women; 85% were wearing ill-fitting bras, and the sizes obtained by their own choice or with measuring systems were significantly different from the correct size according to professional fitting criteria; trying on several bras did not improve the choice. https://pubmed.ncbi.nlm.nih.gov/20451452/
- Wood K, Cameron M, Fitzgerald K. Breast size, bra fit and thoracic pain in young women: a correlational study. Chiropr Osteopat. 2008;16:1. 30 nulliparous women aged 18 to 26; 80% were wearing the wrong size (70% too small, 10% too large), with poorer fit the larger the breasts; no relationship was found between bra fit and thoracic pain in that population. https://pubmed.ncbi.nlm.nih.gov/18339205/
- Coltman CE, Steele JR, McGhee DE. Breast volume is affected by body mass index but not age. Ergonomics. 2017;60(11):1576-1585. Breast volume measured by three-dimensional scanning in 356 women aged 18 to 84: from 48 to 3,100 ml; it did not depend significantly on age but did on body mass index, with volumes two to three times greater with overweight and obesity. https://pubmed.ncbi.nlm.nih.gov/28532249/
- Tebbetts JB, Adams WP. Five critical decisions in breast augmentation using five measurements in 5 minutes: the high five decision support process. Plast Reconstr Surg. 2005;116(7):2005-2016. A decision process based on five tissue measurements, drawn from more than 2,300 primary augmentations; in 1,664 cases with up to 7 years of follow-up, overall reoperation of 3% and reoperation to change implant size of 0.2%. https://pubmed.ncbi.nlm.nih.gov/16327616/
Book your consultation
If you come to the consultation with a size in mind, bring it: it is the best clue to what you are after. Bring the two measurements from this page too; with them in front of you, the conversation about the result — how much can be added or how much it makes sense to reduce in your case — starts where it should.
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