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Bra size calculator

Two measurements. No four inches added to anything. You get the band, the cup, the arithmetic behind both, your sister sizes, and the same size in seven countries' systems.

The six-measurement method of the A Bra That Fits community. The band comes from the snug underbust; the cup from the leaning bust and the loose underbust. The tight underbust does not change your size - it adds a note on how firm a band will feel - so you can leave it out. Implemented independently and not affiliated, so a result can differ from theirs. How to take all six.

Not sure where the tape goes? See how to take both measurements.

If your breasts sit low or are soft, add one more measurement

Sample · 32" under, 36" bust

This is a sample, not your size. Enter your two measurements to see yours.

32DUS / Canada sizing

UK 32D · EU 70D

band 32"4" difference → Dbust 36"

Sister sizes - same cup volume, different band

30DDband one step down, cup one letter up
34Cband one step up, cup one letter down

If the band digs in, go up a band and down a cup. If it rides up your back, go down a band and up a cup. The cup holds the same volume either way.

How that was worked out

Band
32" underbust, rounded to the nearest even inch → band 32
Difference (in inches - the cup ladder is defined in inches)
36" bust - 32" band = 4"
Cup
4 inches of difference → D cup

No four inches were added anywhere. The band is your ribcage measurement, and the cup is the gap between your bust and that band - one letter per inch. Why that matters.

The same size, in every system

Lead with:
SystemSize
US / Canada32D
UK32D
EU70D
Australia / NZ10D
JapanD70
France85D
Italy2D
Not the size you wear now? Here is whyTried it and it does not fit? Check the fit

Taking the two measurements

A soft tape and, ideally, a mirror. Everything here is about keeping the tape level, because a tape that rides up at the back is the most common reason a result comes out wrong.

Where to place the tape for each measurementA torso seen from the side, facing right. The upper dashed line runs level through the fullest point of the bust; the lower solid line runs level through the crease directly beneath it, around the ribcage.21BACKFRONT
Underbust - tape snug
Against the ribs, directly beneath the bust. Breathe out.
Bust - tape loose
Around the fullest part, resting rather than pulling.
Both measurements are taken with the tape level all the way round - check the back in a mirror, because a tape that rides up behind you is the single most common cause of a wrong result. Measure in a thin unpadded bra or nothing; a padded cup adds up to two inches to the bust reading and will push the answer two cup letters too high.

Where it goes wrong

Four ways to get a plausible-looking answer that is simply incorrect

You measured over a padded bra

Padding adds up to two inches to the bust reading, and one inch is one cup letter. The same 32-inch ribcage reads as a 32D measured bare and a 32DDD measured over a push-up. Measure without one, or in something unlined.

Your ribcage lands on an odd inch

A 33-inch underbust is genuinely between two bands, and the two answers are not one cup apart - they are two. At 33 under and 38 bust you are either a 34D or a 32DDD. The calculator gives you both. Keep whichever band you can still breathe in.

The tape rides up at the back

It adds length to both measurements, and unevenly, so the error does not cancel out. Check it in a mirror rather than by feel - it is almost always higher behind you than you think, because your arms are in front of you while you hold it.

You measured on the wrong day

Bust measurement can move by a full cup letter across a menstrual cycle, and by two or more during pregnancy and while feeding. A size taken at the peak will be loose for the rest of the month. If you are between sizes, measure twice a week apart.

Try it

Move the measurements and watch the size change

Both drawings are the same body: from the side, and sliced across and seen from above. Drag either measurement and both redraw.

Measurements

One body, two views

from the side
from above
Size
34D
Bust
38"
Across the chest
6.64"
Weight, each
423-879 g

The same body, measured twice

A 30" ribcage and a 36" bust is not a 34B. It is a 30DDD.

What the +4 method gives

34B

Band
30" ribcage + 4" = 34
Cup
36" - 34" = 2" difference

What the tape actually says

30DDD

Band
30" ribcage = 30
Cup
36" - 30" = 6" difference

2 band sizes and 4 cup letters apart - from one tape, on one body, on the same afternoon. The four inches were added because pre-war band fabric had no stretch. Elastic solved that decades ago. The four inches stayed. They are the reason a band rides up your back while the cup cuts across the front.

Why the four inches are still there

For most of the twentieth century, bra bands were made from fabric with almost no stretch. A band cut to the exact measurement of a ribcage could not be got into, so fitters added four inches and the industry standardised on it. Elastic solved that problem decades ago. The four inches stayed, because the size charts, the stock and the training all assumed them.

The band does around 80% of the supporting work. So the version with the loose band and the small cup fails twice over: it rides up your back because nothing is anchoring it, and it spills at the front because the cup was calculated from a number four inches too big. The straps then take over the support the band abandoned, which is where the shoulder grooves come from.

This calculator uses your ribcage measurement as the band, rounded to the nearest even inch, and takes one cup letter per inch of difference between bust and band. It shows you that arithmetic on every result, so you can check it rather than trust it.

One inch is one cup letter

The whole system runs on a single number: how much larger your bust measurement is than your band. Four inches is a D, five is a DD, six is the one every country names differently. Drawn at a matched band, consecutive cups look like this.

32A

1" diff

0.9" deep

4.35" across

32B

2" diff

1.35" deep

5.18" across

32C

3" diff

1.58" deep

5.93" across

32D

4" diff

1.87" deep

6.32" across

Looking down from above — 32A and 32D

32A

32D

Sliced across the body at the fullest point and seen from above. The dashed line is the ribcage on its own — the 32 band. A bigger cup does not only stand further out from the chest, it covers more of it, and the gap at the breastbone narrows. That is the part a side view cannot show you.

A bigger cup does not mean a lower one. The shapes barely change with size because size is not what decides how far a breast sits down — weight and having breastfed matter far more.

Four consecutive cups on a matched 32 band, starting from an A. Each step adds exactly one inch of difference, so the steps are equal. Both the depth and the width across the chest are the ones the model solves. Notice the shape changes, not just the size - the wider a breast sits on the chest the less room there is to spread further, so past a point the extra volume goes forward instead. Across these four the width grows from 4.35" to 6.32" while the depth grows from 0.9" to 1.87".

Sister sizes: when the answer is close but not right

A cup letter does not describe a fixed volume. It describes a volume relative to its band, so a 34C and a 32D hold the same amount - the 32D just holds it on a tighter band. Those are sister sizes, and they are what you reach for when the cup is right and the band is not.

Sister sizes of 34C: every row holds the same cup volume.
BandCupSize
30DD30DD
32D32D
34C34Cthe 34C in this example
36B36B
38A38A
Every row holds the same cup volume as 34C. Each step down the table loosens the band by two inches and drops the cup a letter; each step up tightens the band and adds a letter. What changes is where the support comes from, not how much room the cup has.

The sister size calculator works the whole ladder out from any starting size, in any of four sizing systems.

Converting between countries

Every result gives you all seven systems. These are the full tables. They matter most above a D cup, where the systems stop agreeing: a UK E is a US DDD, and a UK F is a US G.

Band sizes

US and UK bands are the same number of inches. Everything else is metric: the EU band is the ribcage measurement in centimetres rounded to the nearest 5, France adds 15 to the EU number, Italy counts those same 5cm steps as 1, 2, 3, and Australia subtracts 22 from the inch band.

USUKEUAU/NZJPFRIT
282860-6075-
303065865801
3232701070852
3434751275903
3636801480954
38388516851005
40409018901056
42429520951107
4444100221001158
4646105241051209

Cup letters

Every system agrees up to D. Above it they split three ways, and this is where almost all conversion errors happen: the UK inserts single letters between its doubles, the US stacks Ds and then jumps to G, and the continental systems just carry on through the alphabet. A UK E is a US DDD, not a US E. Past a 15-inch difference American labelling simply stops, which is why the US column runs out before the others do.

DifferenceUSUKEUAU/NZJPFRIT
1"AAAAAAA
2"BBBBBBB
3"CCCCCCC
4"DDDDDDD
5"DDDDEDDEEE
6"DDDEFEFFF
7"GFGFGGG
8"HFFHFFHHH
9"IGIGIII
10"JGGJGGJJJ
11"KHKHKKK
12"LHHLHHLLL
13"MJMJMMM
14"NJJNJJNNN
15"OKOKOOO
16"-KKPKKPPP
17"-LQLQQQ
18"-LLRLLRRR

Next

What people ask once they have a size

About this calculator

This is a free tool and nothing else. It sells no bras, takes no affiliate commission and links to no shop, which is the reason it can tell you that the +4 method is wrong without that being awkward. More on who runs it and why is on the about page. Your measurements never leave your browser - the privacy policy sets out exactly what is and is not collected. There is also a contact page if a conversion looks wrong to you, and the terms of use.

Questions people actually ask

Why is my calculated size so different from the size I have been wearing?
Almost always because the size you were given used the +4 method and this one does not. Adding four inches to a 30-inch ribcage produces a 34 band, and because the cup is measured from that inflated band, the cup letter comes out several sizes too small. A 34B under the old arithmetic is a 30DDD under this one, on identical tape measurements. Both the number and the letter change, which is why the new size looks alarming and fits better.
My breasts sit low. Does that change my size?
Not the band - that is your ribcage, and it does not move. It can change the cup. Standing, low or soft tissue settles against the body and the tape reads small. Lean forward from the hips so it hangs away and measure again. On the calculator, open "If your breasts sit low or are soft" and enter that second number: you get a size range instead of one size, and it tells you to start at the larger end.
Should I measure wearing a bra or not?
For the underbust it makes no difference - the tape sits on your ribs either way. For the bust, measure either without a bra or in a thin unlined one. A padded or push-up cup adds up to two inches to the bust reading, and since one inch is one cup letter, that is the difference between a 32D and a 32DDD.
My ribcage measures an odd number. Which band do I use?
Both are defensible, and the calculator shows you the other one when this happens. A 33-inch ribcage sits exactly between a 32 and a 34 band. Take the smaller band if you want firmer support and it is comfortable, the larger one if the tighter band restricts your breathing. Note that the cup letter moves two steps when the band moves, not one: at 33 under and 38 bust, the answer is either a 34D or a 32DDD.
One of my breasts is larger than the other. What do I do?
Asymmetry of a cup size or so is ordinary and not a fault in the measurement. Take the bust measurement as it falls, which will land between the two, then fit to the larger side and take up the slack on the smaller one - a slightly padded or moulded cup does this without adjustment, and a stretch-lace cup will simply accommodate it.
Does the calculator store or send my measurements?
No. The arithmetic runs in your browser and nothing is transmitted or saved. If you use the copy-link button, your two numbers are placed in the URL you copy - that link will reproduce the result for anyone who opens it, so treat it the way you would any other link with personal information in it.
How often should I re-measure?
Whenever your weight moves by roughly 4-5kg, during and after pregnancy, after any change to hormonal contraception, and after breast surgery of any kind. Ribcage size in particular moves with weight in a way most people do not expect, and it is the band, not the cup, that does the supporting.
Is A the smallest cup size?
No. Below A there is AA, for less than half an inch of difference between bust and band, and this calculator gives it when that is what you measure. Some petite and small-cup brands also make AAA, a step smaller again. There is no agreed measurement for AAA - size charts contradict each other by a whole cup - so it is a size to look for on a label rather than one a tape can give you. If AA cups gape on you, that is when to look.
Is a bra size calculator accurate enough to buy from?
It gives you a correct starting point, not a guarantee. Cut varies enough between brands and between styles within a brand that a size which fits perfectly in one balconette will gape in another. Use the calculated size as the one to try first, and use the sister sizes on either side of it when it is close but not right.