If you have ever been measured in a shop, bought the size they gave you, and quietly never worn it — this page explains what went wrong. It is almost certainly not you, and it is almost certainly not your measurements. It is the formula.
Why two calculators give you two different sizes
There are two methods in circulation, and they disagree dramatically.
- The +4 method. Measure under your bust, then add four inches (five if the number is odd) to get your band size. This is what most department stores used for decades, and what a great many online calculators still use.
- The modern method. Your band size is your underbust measurement, rounded to the nearest even number. Nothing added.
The +4 rule is not arbitrary — it made sense when it was invented. Bras in the 1940s had no elastic in the band. The fabric did not stretch at all, so you needed about four inches of slack simply to fasten the thing. Every bra sold today has a stretch band engineered to fit snugly against your actual ribcage. The four inches of slack is now four inches of nothing.
We ran both formulas across every plausible pair of measurements. The +4 method comes out two band sizes too big and four to five cup letters too small, every single time. A woman who is a 34D by the modern method is handed a 38AA by the old one.
And for roughly a quarter of real measurement pairs, the +4 method produces no valid size at all — it recommends a band larger than the wearer’s own bust measurement, which leaves no cup to speak of. Our calculator tells you when that happens instead of quietly rounding to something plausible.
A band that is two sizes too big cannot hold anything up. It rides up your back, the cups pull away and gape, and all the weight transfers to the straps — which is why they dig in, and why so many women assume they need wider straps when what they actually need is a smaller band.
How to measure yourself
Two measurements, one soft tape, two minutes. Wear a non-padded bra or none at all — padding adds to the bust reading and hands you a cup size that does not exist on your body.
Directly under your bust, level all the way round. This one wants to be firm — snugger than feels natural. The band carries the weight, so a loose tape here is the single most common reason a calculated size comes out wrong.
Around the fullest part, level, tape resting rather than pulling. Breathe out normally. Check in a mirror that it has not slipped down at the back — that is what makes two people get different numbers from the same body.
Your band comes from the first measurement. Your cup comes from the difference between the two: one inch of difference is one cup size.
Is DD bigger than D? The letters, explained
Yes — DD is one cup larger than D. The confusion is understandable, because the three major sizing systems stop agreeing with each other the moment you pass D.
- The US doubles the D — D, DD, DDD — then jumps to G. Confusingly, plenty of American brands print E and F instead of DD and DDD for the identical cup.
- The UK doubles alternate letters: D, DD, E, F, FF, G, GG, H. It never uses I, to avoid it being read as a 1.
- Europe simply runs through the alphabet with no doubling at all: D, E, F, G, H.
So one body wearing one bra can be a UK 34F, a US 34G and a European 75G. None of those is wrong.
| Difference | US | UK | EU / FR / JP |
|---|---|---|---|
| 1 inch | A | A | A |
| 2 inches | B | B | B |
| 3 inches | C | C | C |
| 4 inches | D | D | D |
| 5 inches | DD (or E) | DD | E |
| 6 inches | DDD (or F) | E | F |
| 7 inches | G | F | G |
| 8 inches | H | FF | H |
| 9 inches | I | G | I |
| 10 inches | J | GG | J |
Sister sizes: the trick worth knowing
Cup letters are not absolute. A D cup on a 32 band is a different volume from a D cup on a 38 band — the letter describes the difference between band and bust, not a fixed quantity. Which leads to the most useful piece of bra knowledge there is:
Go down a band, go up a cup. Go up a band, go down a cup. The volume stays identical. A 34C, a 32D and a 36B all hold exactly the same amount — they just sit on different ribcages.
This is why the calculator gives you sister sizes alongside your result. If your size is out of stock, if the cup is right but the band feels loose, or if a particular brand runs small, a sister size is where you go next — not a random guess.
Six signs your bra does not fit
These tell you more than any number. If you only remember one thing from this page, make it the first one.
- The band rides up at the back. It should sit level all the way round, like a shelf. A band that creeps upward is too big — go down a band and up a cup.
- The straps dig in or leave grooves. Straps are for shaping. If they are carrying the load, the band is not doing its job.
- The cups gape or wrinkle. Space you are not filling means the cup is too big — or, if it gapes at the top while fitting elsewhere, the wrong cup shape.
- Tissue spills over the top or under the arm. The cup has run out of room. Go up a cup, keeping the band the same.
- The centre panel floats away from your breastbone. It should lie flat against you. If it does not, the cup is usually too small.
- The underwire sits on breast tissue. It should sit on your ribs, behind the breast entirely. Wire on tissue is uncomfortable and means the cup is too small or the wrong shape.
One more, easy to miss: a new bra should fit on the loosest hook. Bands stretch with wear, and the tighter hooks are where you go in a year’s time. If a new bra only fits on the tightest hook, it is already too big.
Why your bra size changed after 50
If a size you wore comfortably for twenty years has stopped working, nothing has gone wrong. Several things change at once, and only one of them is about weight.
- The ribcage widens. Slowly, and permanently. This alone pushes band size up over the years and is the most common reason a long-trusted size stops fitting.
- Tissue composition changes. After menopause, firmer glandular tissue gives way to softer fatty tissue. Volume may go either way, but the shape almost always changes — typically fuller at the bottom of the cup and emptier at the top.
- Shape matters more than size now. This is the part most advice misses. If cups gape at the top but fit everywhere else, the answer is a different cut — a balconette, or a seamed cup — not a smaller cup size. Dropping a size will just make it too tight elsewhere.
- Skin thins. Wider straps and a wider band spread the load and are noticeably more comfortable.
- HRT moves things. If you have started or changed HRT, expect your size to shift for a few months, then re-measure once it settles.
Re-measure every six to twelve months, and after any weight change of about ten pounds. Our body shape calculator covers the same territory for the rest of your wardrobe, and the change in bust measurement often shows up there too.
A calculator is for fit, not for health. Gradual change in both breasts over years is ordinary. A change in one breast and not the other, a lump, or any change to the skin or nipple is worth mentioning to your doctor — not because it is likely to be serious, but because that is a question a website cannot answer and a GP can.
Frequently asked questions
Which method should I trust, +4 or no-add?
The modern no-add method, for any bra made in the last few decades. The +4 rule exists because 1940s bands had no elastic. If the modern result looks startlingly different from what you have been wearing — a much smaller band and a much bigger cup letter — that is expected, and it is the point. Try it before you dismiss it.
The calculator says I am a G cup. That cannot be right.
It probably is. Cup letters are relative to the band, not absolute, so a G on a 30 band is nothing like a G on a 42 band. Because the old method deflated cup letters by four or five, most women have spent their lives being told a letter far below their actual one. The letter is not a judgement; it is a subtraction.
Is DD bigger than D?
Yes, by exactly one cup size. In the US, DD is sometimes printed as E, and DDD as F, for the same cup. In the UK the sequence runs D, DD, E, F, FF, G. In Europe there is no DD at all — it goes straight from D to E.
What is a sister size?
A different band and cup combination holding the same cup volume. Go down a band and up a cup, or up a band and down a cup: 34C, 32D and 36B are all the same volume. Useful when your size is out of stock, or when a brand’s band runs tight or loose.
Why do two bras in my size fit completely differently?
Because cup shape varies enormously between brands and styles even at identical sizes, and there is no standard. A balconette, a plunge and a full cup in the same 34DD are three different garments. If nothing in your calculated size fits, the shape is the variable to change, not the number.
How often should I re-measure?
Every six to twelve months, and after any weight change of around ten pounds. Bands also stretch out with wear, so a bra that fits on the tightest hook has reached the end of its life regardless of what the label says.
My size seems impossible to buy anywhere.
High-street shops typically stop around a DD or DDD and around a 40 band, which leaves a great many perfectly ordinary sizes unstocked. Your size being hard to find is not evidence you measured wrong. Specialist fitters and online lingerie retailers carry the full range, and the calculator flags when your result falls outside the usual high-street stock.
Are my measurements saved?
No. They are sent to our server to be calculated and used only to produce the result on screen. Nothing is stored, logged against you or shared. This calculator also has no shareable link, deliberately — unlike our shoe and dress size tools, a bra size is not something that should end up in a URL.
