To measure bra cup size accurately, you need two main numbers: your band size and your bust measurement, then you convert the difference into a cup letter.

Step 1: Get ready to measure

  • Wear a thin, non-padded, well-fitting bra (not a sports bra) so your shape is natural but supported.
  • Stand straight, breathe normally, and use a soft tape measure.
  • Do not pull the tape so tight that it digs in, but don’t let it hang loose either.

Step 2: Measure your band size

  1. Wrap the tape snugly around your ribcage, directly under your breasts, level all the way around.
  1. Note this underbust measurement in inches.
  2. Many traditional methods add 3–5 inches to get a band size (for example, 31 underbust → 34 band).
  1. Round to the nearest even number if needed, because many bands are sold in even sizes (30, 32, 34, etc.).

Example: Underbust is 31 inches → 31 + 3 ≈ 34, so start with a 34 band.

Step 3: Measure your bust

  1. Wrap the tape loosely around the fullest part of your bust, usually over the nipples, keeping it level.
  1. Stand naturally; don’t inhale to “boost” the number.
  2. Write down this bust measurement.

Example: Bust is 37 inches.

Step 4: Convert difference to cup size

Now subtract your (calculated) band size from your bust measurement.

  • Difference = Bust − Band.

Typical inch-based guide (can vary a bit by brand):

  • 0 inch difference → AA
  • 1 inch → A
  • 2 inches → B
  • 3 inches → C
  • 4 inches → D
  • 5 inches → DD / E
  • 6 inches → DDD / F

Example: Bust 37 − Band 34 = 3 inch difference → C cup → approx. 34C.

Some systems use slightly different mappings (e.g., AAA, AA, A, B, C, D with 1-inch steps), but the core idea is the same: each extra inch of difference usually means one cup size up.

Mini table: difference → cup (typical)

Difference (inches)Common cup label
0AA
1A
2B
3C
4D
5DD / E
6DDD / F
[1][10]

Step 5: Check the actual fit

Math gives a starting point; comfort confirms the size.

  • Cups too small: spilling over top or sides, “double boob,” center gore not lying flat.
  • Cups too big: gaping at the top, wrinkles or empty space in the cup.
  • Band too big: band rides up your back, you must tighten straps all the way, support feels weak.
  • Band too small: band digs in, feels painful or you can’t wait to take it off.

If the band is right but the cup is off, adjust cup size only; if the band is wrong but cup volume feels right, try a “sister size” (up a band/down a cup, or down a band/up a cup).

Online tools and forum wisdom

  • Online bra-size calculators let you plug in underbust and bust measurements and suggest a starting size; examples include various bra calculators and community tools.
  • Forum communities (like bra-fitting subforums) often suggest taking several measurements and then trying a few nearby sizes because brands and styles vary a lot.

Special note: sewing vs. bra sizing

If you are sewing clothes or bras, “cup size” in sewing patterns may be calculated differently (often based on high bust vs. full bust rather than actual bra size), so always follow the specific pattern’s instructions.

TL;DR: Measure snug underbust for band, measure loosely around fullest bust, subtract band from bust, then use the inch difference to pick a cup letter, and always confirm with how the bra actually feels.

Information gathered from public forums or data available on the internet and portrayed here.