Bra Size Calculator · Practical guide
What Bra Size Is a 34 Band and 38 Bust?
A 34-inch under-bust and 38-inch bust work out to a 34D — here is the math and what to try next.
At a glance
The Short Answer: 34D
With an under-bust measurement of 34 inches and a fullest-bust measurement of 38 inches, your estimated bra size is 34D in both US and UK sizing. The band is already an even number, so it stays 34, and the 4-inch difference between bust and band maps directly to a D cup.
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The Math Behind It
Cup size comes from one simple subtraction: bust minus band. A 4-inch gap corresponds to the fourth step in the cup scale — 1 inch is A, 2 is B, 3 is C, 4 is D. Because 38 − 34 = 4, the cup is D. If your tape actually read 33.5 inches underneath, the band would round up to 34 anyway, so the answer stays the same.
Nearby Sizes for a 34 Band
| Bust (inches) | Difference | Size |
|---|---|---|
| 35 | 1 in | 34A |
| 36 | 2 in | 34B |
| 37 | 3 in | 34C |
| 38 | 4 in | 34D |
| 39 | 5 in | 34DD |
| 40 | 6 in | 34E |
If the 34D Does Not Feel Right
Band too tight? Try a 36C — same cup volume on a looser band. Band riding up? Try a 32DD, the sister size on a firmer band. Gaping at the top of the cup suggests dropping to a 34C, while spillage over the cup edge points toward a 34DD. Sizing up or down one step at a time is the fastest way to dial in the fit without a professional fitting.
Frequently Asked Questions
Is a 34D the same in US and UK sizing?
Yes. US and UK band sizes are identical, and cup letters match through DD in the standard conversion this calculator uses, so a 34D is a 34D in both systems.
Is 34D a big cup size?
Cup letters are relative to the band. A 34D holds the same cup volume as a 32DD or a 36C, so the letter alone says nothing about absolute size.
What if my bust measurement is 38.5 inches?
The difference rounds to the nearest whole inch, so 38.5 minus 34 rounds to a 5-inch gap — a 34DD. When you fall between letters, trying both is the practical answer.
Why do I wear a different size in some brands?
Brands cut their bands and cups differently, and some use vanity sizing. Your measured 34D is the baseline; expect to shift a cup or band size between labels.