Bra Size Calculator — Your True Size, From Two Measurements

The US sizing system is refreshingly simple. Your band is your underbust rounded to the nearest even number, and your cup is determined entirely by the difference between bust and band:

Every bra size is a pair: a number and a letter. The number is the band — the measurement around your ribcage, directly under the bust, rounded to the nearest even number. The letter is the cup, and it is not a measurement of the bust itself. It is the difference between your bust measurement and your band: one inch of difference is an A cup, two inches a B, three a C, four a D, and so on up the chart. That is the whole system this bra calculator runs on — the band half reads your underbust, and the letter half works as a cup size calculator that converts the gap between the two tape readings into a letter.

Because the cup is a difference, the letter alone tells you almost nothing. A D cup on a 30 band and a D cup on a 38 band are both “four inches of difference”, but they are not the same cup: the 38D is built for a 42-inch bust, the 30D for a 34-inch bust, and the 38D holds roughly the volume of a 34DDD. Bra cup sizes are relative to the band they sit on, which is why two people can both be “a D” and need completely different bras — and why a size is only ever the full pair, number and letter together.

It also means small measuring errors move the letter fast. One extra inch on the bust reading adds a cup size; one extra inch on the underbust can move the band up and the letter down at the same time. The next section is the measuring protocol this page’s numbers assume, because the calculator can only be as honest as the two numbers fed into it.

What you need: a soft fabric tape measure, a mirror, and either no bra or a thin, non-padded one. Padded and push-up bras add their own thickness to the bust reading — up to an inch — and an inch is a whole cup size. Measure standing, in front of the mirror, at a normal point in your day; bodies measure slightly differently first thing in the morning and after a large meal, and the honest middle is what you want.

Step 1 — the underbust. Wrap the tape around your ribcage directly under the bust, exactly where the band of a bra sits. Pull it snug — firmer than feels polite, but not digging in or denting the skin. Breathe out normally and read the tape at the end of the exhale; a held-in breath can shrink the reading, a big inhale can stretch it. Check the mirror: the tape must be level all the way around, parallel to the floor. This is your band measurement, and it is the reading everything else hangs on.

Step 2 — the bust. This is how to measure bust size correctly: move the tape up to the fullest point of the bust, usually across the nipple line, keeping it level around your back at the same height. This time the tape should rest on the bust, not compress it — snug at the back, flat but floating at the front. Do not pull it tight to “be accurate”; a compressed bust measurement reads small and hands you a cup that will spill by lunchtime.

Step 3 — round once, at the end. Write both readings down to the nearest quarter inch before you round anything. The calculator then does the only rounding the system needs: the underbust rounds to the nearest even band (33 inches becomes a 34 band), and the cup is the bust minus the band, rounded to the nearest inch. Rounding early — or rounding both numbers in the same direction — is how careful measuring still produces the wrong size.

Step 4 — measure twice. Take both readings a second time. If the two rounds agree within a quarter inch, you have your numbers. If they disagree, measure a third time and use the middle value. Two minutes of this beats years of wearing the compromise size.

The errors that actually move sizes. Most wrong sizes are not body problems, they are tape problems:

Notice the pattern in the table: underbust errors are the expensive ones, because the band appears in the size twice — once as the number, and once inside the cup difference. A 2-inch underbust error moved the example two band sizes up and two cup letters down at once. If you only double-check one reading, double-check the underbust.

The calculator is quick, but the chart underneath it is worth seeing once, because it shows exactly what the tool is doing. Find your band down the left side — your underbust rounded to the nearest even number — then read across to the bust measurement inside each cell. The column you land in is your cup. This one grid is the bra size chart, the cup size chart and the breast size chart at once: three names for the same table, because the system really is just band plus difference.

Reading it by hand: an underbust of 33 inches rounds to band 34. A bust of 38 inches sits in the 34 row under the D column — 34D, the same answer the calculator gives, because the chart’s cells are simply band + difference (34 + 4 = 38). A bust of 39 inches in the same row lands under DD: 34DD. Nothing else is hiding in the system — if a result ever surprises you, this table is where to check it, cell by cell.

Two honest limits of any printed chart: it assumes the direct band convention (underbust rounded to even, covered in its own section below), and it stops being the whole story at the edges — very small bands, very large cups and in-between half inches are where sister sizes and the fit check earn their keep.

Sister sizes are the sizes that hold the same cup volume on a different band. The rule is one step each way: go up one band size and down one cup letter, or down one band and up one cup letter. A 34D’s sisters are 32DD and 36C — same cup, three different ribcages. Treat this section as a bra sister size calculator: find your size in its row below and every other size in that row carries the same volume.

When a sister size is the right fix. Sisters solve band problems, not cup problems. If the cups fit but the band digs in, move to the sister with the bigger band (34D → 36C). If the cups fit but the band rides up or shifts when you move, take the sister with the smaller band (34D → 32DD) — the tighter band also pulls the cups closer to the body, which is usually the real fix. If the cups themselves are wrong — spilling or gaping — a sister move keeps the same wrong volume; change the letter on your own band instead (34D → 34DD for spillage).

Sisters are also the practical answer to stock and brand variation: when your exact size is sold out, or a brand cuts its bands tight, the sister in the same row is the first thing to try. What sisters cannot do is fix a size that was measured wrong — if both the band and the cup feel off, re-measure before you sister-hop.

Ask two fitting sources for your band and you may get two answers, because the industry quietly runs two conventions. The direct method — the one this calculator uses — takes your underbust as it is and rounds it to the nearest even number: a 33-inch underbust is a 34 band. The +4 method, the older convention from the era of less elastic fabrics, adds four inches to the underbust first (sometimes five for larger measurements, in older charts still in circulation) and then rounds: the same 33-inch underbust becomes a 38 band.

Neither row is an arithmetic error — they are different answers to the question “what number should the band be?”, and the cup letter then shifts to match, because the letter is measured from the band. That is the whole reason two honest calculators can disagree by a band size and several letters on identical measurements.

Neither convention is a law, and this page will not sell one as the truth. Modern bands are elastic and are cut to be worn at roughly their stated size, which is why most current fitting guidance — and this calculator — uses the direct method. But some brands still grade their patterns on +4 assumptions, which is why the fit check below outranks any number: if a direct-method size feels loose in the band from a +4-graded brand, the sister size with the smaller band is the bridge between the two systems.

A size is a starting point; fit is the verdict. Run these five checks in a mirror with your best-fitting bra on its loosest hook (new bras should fit there — the tighter hooks are for later, as the elastic relaxes).

Work through the signs in order — band first, then cups, then straps. Most “cup problems” turn out to be band problems in disguise: a band two sizes too big makes cups look simultaneously too small (spillage at the front, where tissue escapes the floating cups) and too big (gaping when you lean). Fix the band, re-check, and only then touch the letter. And if a size passes all five checks in the morning and fails by evening, that is not a sizing mystery — it is a sign to re-measure, because bodies and elastic both change.

The calculator’s results panel already converts your size into eight systems, which makes the results a bra calculator US, UK, EU and AU shoppers can all read. Here is the arithmetic it is doing, so you can convert any size by hand. Bands first: the UK keeps the US number; the EU (and German) band is the US band × 2.5, minus 10; French and Spanish sizes add 15 to the EU number; Italian sizes count up from 1; Australian dress sizes are the US band minus 22.

Cups are where systems genuinely diverge. All three systems below start from the same bust-minus-band difference, but the UK inserts double letters differently (DD, then E, then FF) while the EU runs single letters straight through:

That is why a US 34DD is a UK 34DD but an EU 75E — and why a US DDD (6 inches) becomes a UK E, not an F. Convert the band by formula and the cup by table, never the letter by alphabet alone. The full conversion guide below tables every band and cup side by side, including the systems this summary skips.