Bra Size Calculator · Practical guide
Cup Size Difference Chart in Inches
Your cup letter is one subtraction away: fullest bust minus under-bust band, in inches.
At a glance
How Cup Size Is Calculated
Cup size comes from a single measurement: the difference in inches between your fullest bust and your under-bust band. A 1-inch difference is an A cup, 2 inches a B, 3 inches a C, 4 inches a D, and so on up the scale. The band itself is your under-bust measurement rounded to the nearest even number — so the full size always pairs a number with a letter, like 34C.
Interactive tool
Bra Size Calculator
Enter your own values, then compare the result with the examples below.
Results
The Full Difference Chart
| Bust − Band (inches) | Cup Size | Example (34 band) |
|---|---|---|
| 0 | AA | 34 bust → 34AA |
| 1 | A | 35 bust → 34A |
| 2 | B | 36 bust → 34B |
| 3 | C | 37 bust → 34C |
| 4 | D | 38 bust → 34D |
| 5 | DD | 39 bust → 34DD |
| 6 | E | 40 bust → 34E |
| 7 | F | 41 bust → 34F |
| 8 | G | 42 bust → 34G |
Rounding Rules That Move the Letter
Two roundings happen before the chart applies. First, the band rounds to the nearest even number — a 33-inch under-bust becomes a 34 band. Second, the bust-minus-band difference rounds to the nearest whole inch, so a 3.5-inch gap counts as 4 and lands on a D. Worked example: under-bust 33 inches, bust 37 inches. Band rounds to 34, the difference is 3 inches, and the result is 34C. A half inch either way can flip the letter, which is why careful measuring matters.
Why the Letter Is Not a Fixed Volume
A D cup on a 32 band holds less volume than a D cup on a 38 band — the letter only encodes the difference, not an absolute size. That is why sister sizes like 34C and 36B share the same cup volume on different bands, and why going down a band usually means going up a letter to keep the same fit through the cup.
Frequently Asked Questions
What if my difference is more than 8 inches?
This calculator's standard US/UK scale tops out at G (8 inches), so larger differences report as G. Fuller-bust brands extend the scale with letters like GG, H, and beyond.
What does a 0-inch difference mean?
A bust that matches the band measurement maps to an AA cup — the smallest cup on the standard scale, common for very petite or athletic frames.
Why did I get a different letter than I expected?
Usually because of band rounding. If your under-bust is odd, the band rounds up an inch, which shrinks the bust-minus-band difference by one and drops the letter by one step.
Is a DD bigger than a D?
Yes. Each letter step is one additional inch of bust-over-band difference, so DD is one inch fuller than D, and E is one inch fuller than DD.