How to Calculate Your Bra Size: Real Measurement Scenarios
The rib cage method comes down to two numbers and one subtraction: measure your band (rib cage, directly under the bust, nothing added), measure your bust (fullest part), then subtract band from bust. The difference in inches gives your cup letter. Here's exactly how that plays out across a wide range of real measurements.
The Formula, One More Time
Band size = rib cage measurement, rounded to the nearest even number.
Cup size = bust measurement minus band measurement, in inches, mapped to a letter:
- 1/3 inch = AAA
- 1/2 inch = AA
- 1 inch = A
- 2 inches = B
- 3 inches = C
- 4 inches = D
- 5 inches = DD/E
- 6 inches = DDD/F
For the full explanation of why these steps exist, see Bra Cup Sizes Explained: A to DDD/F.
Worked Scenarios: Band 28
Rib cage 28, bust 31 (3-inch difference)
Band rounds to 28. Bust minus band = 3 inches = C cup. Result: 28C.
Rib cage 28, bust 33 (5-inch difference)
Band rounds to 28. Bust minus band = 5 inches = DD/E cup. Result: 28DD. This is one of the sizes most mainstream brands don't carry — see BRABAR's 28DD collection.
Rib cage 28, bust 34 (6-inch difference)
Band rounds to 28. Bust minus band = 6 inches = DDD/F cup. Result: 28DDD/F — a small band paired with a full cup, more common than most people realize. See Small Band, Full Cup Bras.
Worked Scenarios: Band 30
Rib cage 30, bust 31 (1-inch difference)
Result: 30A.
Rib cage 30, bust 32 (2-inch difference)
Result: 30B.
Rib cage 30, bust 34 (4-inch difference)
Result: 30D.
Worked Scenarios: Band 32
Rib cage 32, bust 33 (1-inch difference)
Result: 32A.
Rib cage 32, bust 35 (3-inch difference)
Result: 32C.
Rib cage 32, bust 37 (5-inch difference)
Result: 32DD/E.
Worked Scenarios: Band 34
Rib cage 34, bust 35 (1-inch difference)
Result: 34A.
Rib cage 34, bust 38 (4-inch difference)
Result: 34D. This is the example used throughout BRABAR's Fit Guide.
Rib cage 34, bust 40 (6-inch difference)
Result: 34DDD/F.
Worked Scenarios: Band 36
Rib cage 36, bust 38 (2-inch difference)
Result: 36B.
Rib cage 36, bust 41 (5-inch difference)
Result: 36DD/E.
Worked Scenarios: Band 38
Rib cage 38, bust 39 (1-inch difference)
Result: 38A.
Rib cage 38, bust 42 (4-inch difference)
Result: 38D.
What if My Measurement Falls Between Two Even Numbers?
Round the rib cage measurement to the nearest even number first, then calculate cup size from there. A 33-inch rib cage rounds to a 34 band. A 29-inch rib cage rounds to a 28 or 30 depending on which is closer — 29 is exactly between, so either works, though sizing down to 28 usually gives a snugger, more supportive fit. See the full rounding chart for every inch-by-inch case.
Frequently Asked Questions
What size am I if my rib cage is 28 inches and my bust is 33 inches?
A 28-inch band with a 5-inch difference to a 33-inch bust is a 28DD/E.
What size am I if my rib cage is 34 inches and my bust is 38 inches?
A 34-inch band with a 4-inch difference to a 38-inch bust is a 34D.
What size am I if my rib cage is 30 inches and my bust is 34 inches?
A 30-inch band with a 4-inch difference to a 34-inch bust is a 30D.
Do I round my rib cage measurement before or after calculating cup size?
Before. Round the rib cage measurement to the nearest even number first to get your band size, then subtract that rounded band number from your bust measurement to find your cup size.
Use BRABAR's Fit Guide and Bra Size Calculator to run your own exact measurements, or explore the full Ask BRABAR FAQ for more sizing questions.
