Bra Size Rounding Chart: Convert Your Rib Cage Measurement to Band Size
To convert a rib cage measurement to a band size, round to the nearest even number: 27 or 28 inches rounds to a 28 band, 29 or 30 inches rounds to a 30 band, and so on in 2-inch steps up through the full range. No inches are added on top of the real measurement — the rounded rib cage number is the band size, full stop. Here's the complete chart, plus the fit checks and edge cases that a simple calculator can't answer for you.
Rib Cage to Band Size Chart
| Rib cage measurement | Starting band size |
|---|---|
| 27 inches | 28 |
| 28 inches | 28 |
| 29 inches | 30 |
| 30 inches | 30 |
| 31 inches | 32 |
| 32 inches | 32 |
| 33 inches | 34 |
| 34 inches | 34 |
| 35 inches | 36 |
| 36 inches | 36 |
| 37 inches | 38 |
| 38 inches | 38 |
If the rib cage already measures an even number, that number is the starting band size directly. If it measures an odd number, round up to the next even number. This is rounding, not adding — for the full explanation of why that distinction matters, see Band Size Explained: Rib Cage vs. Plus-4 vs. Armpit Method.
Worked Example: One Body, Three Methods
Consider a 28-inch rib cage and a 33-inch fullest-bust measurement:
| Method | Starting result | What happened |
|---|---|---|
| Rib cage method (BRABAR) | 28DD/E | Uses the 28-inch rib cage directly and the 5-inch bust difference |
| Plus-4 method | Approximately 32A | Adds 4 inches to the rib cage, which shrinks the cup difference |
| Above-bust (armpit) method | Varies by upper-chest measurement | Uses a measurement taken where the band doesn't actually sit |
Same body, three different results, because only the methodology changed. A DD/E cup on a 28 band is not an unusually large size — it simply reflects a 5-inch difference between a small rib cage and a fuller bust. For the full breakdown of why these methods diverge, see Bra Measuring Methodologies Explained.
The Strapless-Fit Test
One of the clearest ways to check whether a band is actually doing its job: lower or loosen the straps temporarily and see what happens.
- Does the band stay in place on its own?
- Does it remain level across the back?
- Does the bra still support the bust without the straps holding it up?
- Does breast tissue slip beneath the band?
If the bra immediately loses support the moment the straps are loosened, the band is too loose — and tightening the straps further only transfers the band's job onto the shoulders rather than fixing the actual problem. For the full list of related symptoms, see Signs You Are Wearing the Wrong Bra Size.
Why a Bra-Size Calculator Isn't the Final Answer
A calculator using the rib cage method will give a more accurate starting point than one using plus-4 or armpit measuring — but it's still a starting point, not a guarantee. Two people with identical rib cage and bust measurements can need different bras because of differences in breast shape, tissue distribution, spacing, and how a specific style is cut. Use the calculated size to narrow the search, then confirm fit on the body: band level and secure, cups containing tissue without gaping or spilling, straps stable without digging in.
Frequently Asked Questions
What band size is a 28-inch rib cage?
A 28-inch rib cage rounds to a 28 band under the rib cage method, since it's already an even number and gets no inches added on top.
What band size is a 31-inch rib cage?
A 31-inch rib cage rounds up to a 32 band — odd measurements round up to the next even number.
Should I measure with or without a bra?
Either works, as long as the bra you're wearing (if any) is unpadded and isn't compressing or reshaping the breast tissue. A padded or underwire bra will distort the measurement.
What if one breast is larger than the other?
Breast asymmetry is common. Fit the cup to the fuller breast, then adjust the strap on the smaller side if needed. A flexible or lightly padded cup can also help accommodate the natural difference.
Is a bra-size calculator always accurate?
It gives an accurate starting point when it's built on the rib cage method, but it can't account for breast shape, tissue distribution, or how an individual style is cut. Two people with the same measurements can still need different bras — always confirm fit on the body after using a calculator.
Why does my band ride up even though I measured correctly?
A rising band usually means it's too loose despite the measurement, or the straps have been overtightened to compensate for a band that isn't holding on its own. Try the strapless-fit test above to isolate the actual cause.
Find your size with BRABAR's Fit Guide and Bra Size Calculator, built on the rib cage method with no inches added, and explore the full range of band and cup sizes.
