The same bust reads as a 36D in one shop and a 32G in another. Here are the four systems behind that, which brands use each, and how to read your own size in any of them from two measurements.
Most of us can recite two or three of our own bra sizes, one from the department store, one from a newer brand, maybe a plain large from somewhere online, and every one of them is a different number. It is easy to read that as a failing of your body, as though you are somehow awkward to fit. You are not. American bra sizing has never had a single standard, and once you can name the systems running side by side, the mess turns into something you can simply read.
Four systems are in use at the same time, and a brand picks one without ever telling you which. Below are the four, the brands that use each, a table that converts your body into all of them, and the plain reason your old size was never wrong in the first place.
What actually differs between one shop and the next?
Two things vary independently, which is why the confusion multiplies instead of settling.
The first is the band. Some brands take the measurement under your bust and add four inches to it. Others take that measurement exactly as it comes off the tape. On the same body that produces a band of 34 in one shop and 30 in another, with nothing about you having changed between the two fitting rooms.
The second is the cup letters above D. One tradition counts A, B, C, D, DD, DDD, G, H. Another counts A, B, C, D, DD, E, F, FF, G. Both call themselves standard, and they diverge from the sixth letter onward, which is exactly where a fuller bust lives.
Where the extra four inches came from
The plus-four band is a leftover from an older way of making bras. Decades ago, before elastic was woven through everything, a band was cut from stiff fabric with little give, so it had to be made several inches longer than the body just to stretch around and fasten. Adding about four inches was the practical fix.
Elastic solved that problem long ago, but the convention stayed, which is why a department-store fitting can still hand you a band two sizes larger than your ribcage. The arithmetic outlived its reason, and half the market simply never updated it.
How does each of the four systems work?
Traditional +4
Band equals your underbust measurement plus four inches, or plus five if the number is odd. The cup letter is your bust minus your band, at one inch per letter, using the American ladder of DD, DDD, G. This is the oldest of the four and the one most American women were first measured by.
Modern US direct
Band equals your underbust measurement, rounded to the nearest even number, with nothing added. The cup is calculated the same way and uses the same American ladder. The band change alone moves you two sizes down and several cup letters up compared with Traditional +4, though nothing about your body has moved.
Modern UK
The band works exactly as it does in Modern US direct. The cups are where it separates: after DD the letters continue E, F, FF, G, GG, so the same volume that reads as DDD in America reads as E in Britain. This system reached the American market through fuller-bust fit specialists and is now used by a small, visible group of them.
Hybrids
Roughly a fifth of brands mix the two axes. Some pair a plus-four band with British cup letters. Some skip DDD and jump from DD to F. Some run their cup letters a step or two larger than the American ladder. A hybrid cannot be worked out from the label alone, so a size that fits at one hybrid brand tells you nothing about the next until you read that brand’s own chart.
Which brands use which system?
Checked against the published size charts of American brands in mid-2026. Treat it as a map rather than a promise, since a brand can quietly change its method between seasons.
| System | Share of brands | Where you meet it |
|---|---|---|
| Traditional +4 | 43 percent | Bali, Aerie, Warner’s, Playtex, Maidenform, and most of what sits on the shelf in Target and Walmart |
| Modern US direct | 21 percent | Victoria’s Secret, Knix, current Wacoal |
| Modern UK | 14 percent | Curvy Kate, Panache, Bravissimo, Freya, Elomi |
| Hybrid | 22 percent | Cuup, ThirdLove, SKIMS, and most retailer-own labels |
The American cup ladder of DD, DDD and G is the language of about two thirds of the market, because both Traditional +4 and Modern US direct use it. The British ladder is the minority spelling, not a different measurement.
What is my size in each system?
Take your underbust and your bust, subtract one from the other, and read your line across.
Reading the second row: a woman measuring 30 inches under the bust and 38 inches around it is a 34D at Bali, a 30H at Victoria’s Secret and a 30FF at Curvy Kate. One body, three labels, no disagreement at all about the actual volume.
How do the cup letters line up?
The two ladders agree for the first five letters and separate after that.
An American DDD and a British E are the same cup. So are an American H and a British FF. The letters differ, the volume does not.
How do I translate the size I already wear?
You need two numbers and about two minutes.
- Measure under your bust with the tape level and snug, breathing out, and round to the nearest whole inch.
- Measure around the fullest part of your bust with the tape resting rather than pulling.
- Subtract the first from the second. That difference is your cup in every system, before it gets a letter.
- For a Traditional +4 brand, add four to your underbust for the band. For a Modern brand, use the underbust as it is.
- Read the letter off the ladder that brand uses.
If you land between two sizes, take the larger one. Research comparing women’s self-chosen sizes with professionally fitted ones found most women in the wrong size, and the most common error by far was a band too loose paired with a cup too tight [1]. Sizing up on the cup and down on the band corrects both at once.
Does this mean my old size was wrong?
No. A label describes a garment, not a body. A brand that calls your cup a DDD and a brand that calls it an E have measured you identically and spelled the answer differently.
What is worth checking is whether the fit itself was right, because that is a separate question from the number. Across 309 women assessed on their own bras, incorrect fit showed up most often in the cups, the front of the band and the straps, and those three behaved differently depending on breast size [2]. The number can be right on the tag and the fit still off, which is why the mirror matters more than the label.
Which system does ILARIA use?
The fourth one. Our band follows the Traditional +4 rule, so an underbust of 31 to 32 inches gives you a band of 36. Our cup letters run about two steps larger than the American ladder, which places us in the hybrid group alongside Cuup, ThirdLove and SKIMS.
Being a hybrid only causes trouble when a brand keeps its chart to itself, so here is ours in plain numbers. A difference of 7 inches between your bust and your underbust is a D with us, 8 inches is a DD, 9 inches is a DDD and 10 inches is a G. Our 36D sits close to a 32G in Modern US direct and a 32F in British sizing.
Our size guide is built on your underbust and bust rather than on letters, so you can find your size straight from the tape without converting anything first. Take your two numbers to the ILARIA size guide, and if you would rather have the charts and fit help in one place, the Fitting Room gathers them.
What to do with all of this
You do not need to memorise four systems. You need your two measurements, written down once, and the habit of reading a brand’s own chart from them instead of trusting the letter on your last bra. That single move ends the guessing at every shop, ours included, and it is the reason a bra that arrives finally fits.
If it helps to have other women working the same thing out, our community is an open door and never a hard sell. And whenever you want the fast version, your underbust and bust plus our chart give you your size in under a minute, with a free first exchange if you land close and want to adjust. You can also see our bras grouped by cup once you know your number.
Frequently asked questions
Why is my bra size different at every shop?
Because four different sizing systems are in use in the American market at once, and brands do not label which one they follow. The two variables are whether four inches are added to your underbust to get the band, and whether the cups above DD are counted as DDD, G or as E, F, FF.
Which of my sizes is the real one?
All of them, within their own system. The only figures that stay constant are your underbust and bust in inches. Every size label is a translation of those two numbers, and different translations can be equally correct.
Is a DD the same as an E?
Not quite. An American DD and a British DD are the same cup, and the American DDD and the British E are the same cup. The confusion happens one step later than most people expect, at the sixth letter rather than the fifth.
Why does British sizing have no DDD?
The British ladder continues alphabetically after DD, so the next letter is E. The American ladder repeats the D instead, giving DDD before it moves on to G. Both add one inch of volume per step, so the two ladders describe the same cups under different names.
Do I need to measure again when I switch brands?
No. Measure once, write both numbers down, and convert from them. Measure again when your body changes, and at least once a year, since band and cup both shift with weight, pregnancy, feeding and the years around menopause.
Sources
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McGhee DE, Steele JR. Optimising breast support in female patients through correct bra fit. A cross-sectional study. Journal of Science and Medicine in Sport, 2010.
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Coltman CE, Steele JR, McGhee DE. Which bra components contribute to incorrect bra fit in women across a range of breast sizes? Clothing and Textiles Research Journal, 2018.
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Wood K, Cameron M, Fitzgerald K. Breast size, bra fit and thoracic pain in young women: a correlational study. Chiropractic and Manual Therapies, 2008.
