Band Size Explained: Rib Cage vs. Plus-4 vs. Armpit Method

There are three common ways to measure a bra band size in the US, and they produce three different results for the exact same body: the rib cage method (measure directly under the bust, no added inches), the plus-4 method (add four inches to the underbust measurement), and the armpit method (measure higher on the torso, near the strap line). Only the rib cage method measures the band where it actually sits.

The Rib Cage Method

Wrap a soft measuring tape directly around your rib cage, just under your bust, keeping it level. Round to the nearest even whole number. That number is your band size, with no further adjustment. This is BRABAR's EZ-fit method and the standard used throughout this guide.

The Plus-4 Method

This method measures the underbust, then adds four inches before calling the result a band size. It is still built into sizing calculators from brands including SKIMS and Aerie. It dates back to an era of stiffer, less flexible bra fabrics, when a looser band was considered more comfortable. With today's stretch materials, it mostly produces a band too loose to support anything.

The Armpit Method

This method measures higher on the torso, near where the straps meet the cup, rather than at the rib cage. Because the torso is wider up near the bust than underneath it, this method produces a larger band number by design. It is the method described in Victoria's Secret's own fitting instructions.

Side-by-Side Comparison

Same body, different starting measurement, different final size. Consider a 28-inch rib cage and a 34-inch bust:

• Rib cage method: Band = 28. Cup = 34 minus 28 = 6 inches = DDD/F. Final size: 28DDD/F.
• Plus-4 method: Band = 28 + 4 = 32. Cup = 34 minus 32 = 2 inches = B. Final size: 32B.

Same person, two dramatically different sizes, depending only on which method was used to find the band number.

Why the Band Matters More Than the Cup

About 80 percent of a bra's support comes from the band, not the straps. When the band is too loose (as both the plus-4 and armpit methods tend to produce), the straps take on weight they were never designed to carry — which is the actual mechanism behind digging straps, bands that ride up, and bras that never feel quite right no matter how many times you re-buy the same labeled size.

Frequently Asked Questions

Which bra band measuring method is most accurate?

The rib cage method, which measures the band directly where it sits on the body and does not add extra inches or shift the measurement point higher on the torso.

Why do SKIMS and Aerie use the plus-4 method?

It is a legacy shortcut from an earlier era of less stretchy bra fabric. Several brands still use it in their online sizing calculators, though it tends to produce a looser band than the rib cage method.

Why does Victoria's Secret give a different size than other brands?

Their fitting instructions use the armpit method, measuring higher on the torso near the strap line rather than at the rib cage, which produces a larger band number for the same body.

Can I use the same cup math no matter which band method I use?

Yes, the cup calculation works the same way regardless of method, but a different band number changes the resulting cup letter, which is why the three methods produce different final sizes.

Find your accurate size with the Fit Guide and Bra Size Calculator, and explore BRABAR's full range of band and cup sizes, built around the rib cage method.