A cup letter is not a size. It is a difference — the number of inches between your bust measurement and your band, wearing a letter as a label. One inch is A, two is B, three is C, four is D. Everything confusing about cup sizes — why a D can be small, why letters seem to grow on bigger bands, why a DD is not “two D’s” — dissolves once that single fact is in view. This is the cup size chart with the arithmetic left in.

The difference table

US sizing assigns one letter per inch, in a fixed order. This bra cup size chart shows the full everyday range with the bust each letter implies on one example band:

Read one row slowly: a D cup is a 4-inch difference. On a 34 band that is a 38-inch bust — the 34D this site’s calculator uses as its worked example. On a 30 band the same D is a 34-inch bust. The breast cup sizes in the table are not volumes; they are gaps between two tape readings, and the volume only appears when the band joins the letter.

Letter by letter: A to DDD

An A cup size is a 1-inch difference — the smallest step on the chart, a cup that adds an inch of projection over the band. An a cup bra size on a 32 band fits a 33-inch bust; on a 38 band it fits a 39-inch bust, and those two A cups are not the same garment at all.

A B cup is a 2-inch difference. A C cup is 3 inches — historically the “average” of marketing copy, a title with no anatomical meaning, since the letter describes a gap, not a body. A D cup is 4 inches, and it is where the folklore starts: a cup size D is treated as enormous in everyday speech and is, on a small band, nothing of the kind.

DD is a 5-inch difference and DDD is 6 inches — single steps, not multiples. A DD is not “a D twice”; it is a D plus one inch. (UK sizing calls the 6-inch step E instead of DDD, which is why the same body collects different letters in different shops.) Beyond DDD the US chart continues G, H, I, J, K at 7 through 11 inches — breast cup size chart territory most high-street ranges stop printing, but the arithmetic never changes.

The fact that fixes the folklore: volume rides on the band

Sister sizes prove it. These sizes all hold the same cup volume: 30DDD, 32DD, 34D, 36C, 38B, 40A. Read the row both ways. A 34D and a 38B are the same volume wearing different letters — and a 40A, the “smallest” letter on the chart, holds exactly as much as the “big” 34D. One row further: a 38A sits in the next volume row up (30DD, 32D, 34C, 36B, 38A), which means a 38A holds more than a 30D. Boob cup sizes compared across bands without the volume rows are meaningless — the letter was never the amount.

This is also the practical answer when a cup size comparison surprises you in a fitting room: if the letter feels wrong but the volume feels right, you do not need a different volume — you need the same volume on your band, one row over in the sister table.

Cup sizes across bands: the comparison table

Cup sizes: bra labels A to DDD, translated into the bust measurement each one means at three common bands. Bra cups sizes at a glance:

Every cell is band + difference — the same sum the difference table runs on. That is how to measure cup size bra fitters’ way: bust minus band, counted in inches, named by the table. Measure both numbers honestly (snug underbust, resting bust tape) and the letter is never a mystery again: it is the gap between your two measurements, and nothing else.

Past DDD: the G-to-K range

The chart does not stop at DDD. The same one-inch ladder continues — G at 7 inches of difference, H at 8, I at 9, J at 10, K at 11 — and the cup sizes bra brands print at the fuller end of the market follow it exactly. Read the cup sizes chart as a ladder with no ceiling: a 34 band with a 43-inch bust is a 9-inch difference, a 34I. Two things change at these letters, and neither is the arithmetic. Availability narrows — many ranges stop at G — and the UK alphabet separates further from the US one (the UK inserts FF, GG and HH double steps, so a US H and a UK H are different differences). The difference table at the top of this page is the safe ground: convert by inches, never by the letter’s position in the alphabet.

One letter, every band: the D-cup table

To make the relativity impossible to unsee, here is a single letter — D, the folklore’s “huge” cup — at every common band. Each cell is the bust that letter means there:

The smallest D on the table belongs to a 34-inch bust; the largest, to a 48-inch bust. They share a letter and nothing else — not volume, not garment, not wearer. And the mirror image holds at the small end: an A cup breast size on a 44 band means a 45-inch bust, a measurement larger than the bust of the smallest D on this page. Letters are gaps. Bands are sizes. The size is the pair.

There is no half cup — and that is fine

Shoppers between letters often ask for the half step — a C-and-a-half. The US system has no half letters: on your own band, the letter jumps in whole inches, and no labelled size holds a volume halfway between a 34C and a 34D. If a 34C feels small and a 34D feels big, the resolution is one of three honest moves: a sister size — the same volume class reached from a neighbouring band can sit differently enough to settle the question — a different cup shape at the same size, or the nearer letter judged by the fit checks rather than the tape. The alphabet’s coarseness is a labelling limit, not a fitting limit.

What changes your letter over time is equally plain: anything that moves the bust more than the band — weight change, cycle, pregnancy, training — moves the difference, and the letter follows the difference. Re-measure, recount the inches, and let the ladder relabel you. It was never describing you. It was describing a subtraction.

Beyond K, and reading the label you already own

The ladder does not run out at K. Extended ranges continue the same one-inch steps alphabetically — L at a 12-inch difference, M at 13 — which is the rule the calculator’s own conversion engine uses when it spells cups past the everyday chart (its display caps at “K+” because labels beyond K are printed inconsistently across brands, but the differences keep counting regardless of who prints them). If your difference lands past 11 inches, the honest output is the number first: your band, and your difference in inches, convert cleanly into any extended range’s alphabet.

Finally, the label in the bra you are wearing can be read backwards, and it is worth doing once. Its number is the underbust it was cut for; its letter is the difference it was cut for; together they reconstruct the measurements its maker assumed — which you can hold against the two numbers your tape gave you. When the tag’s implied bust (band plus difference) sits two inches from your measured bust, you have found, in arithmetic, why it never fit.

Rounding: the letter at the half inch

Differences are not always whole inches. A bust of 38.4 inches on a 34 band is a 4.4-inch difference, and the method rounds it to the nearest inch: 4, a D cup — the same rounding the calculator applies, and the reason two careful measurers occasionally name neighbouring letters for the same body. At exactly the half inch (a 4.5-inch difference), the letter is genuinely undecided by arithmetic: round up to DD by the numbers, but let the fit checks arbitrate, because a half-inch of difference distributed over two cups is a quarter inch each — well inside brand-to-brand cutting variation. The practical rule is the one used throughout this site: take the rounded letter as your starting size, and if its checks fail in the “too small” direction only, the next letter up is a legitimate size, not a vanity move.

Why some tags print DD as E and DDD as F

One last source of alphabet confusion is printed on tags themselves. Some US brands label the 5-inch step E instead of DD, and the 6-inch step F instead of DDD — the main calculator page’s own chart shows both spellings side by side (DD (E), DDD (F)) for exactly this reason. Nothing has been converted and nothing has changed size: it is the single-letter alphabet worn by the double-letter steps. The UK adds one more wrinkle — its E is the 6-inch step, so a UK E and a US-brand E are different differences — which is why every reliable chart, including this one, routes through the inches. When two tags disagree about your letter, ask both what difference they mean. The inches never disagree.