How to Calculate Bra Size (Band + Cup)

A correct bra size needs just two tape measurements and a little simple math. Fitters have used the band-and-cup system for generations, and understanding it demystifies what can otherwise feel like an intimidating maze of letters and numbers.

The system divides a bra size into two independent parts: a band number (your ribcage circumference) and a cup letter (how much larger your bust is than your band). Get the band right and the cup follows, and you'll be in the right region of the size chart — though, as we'll see, the final word always belongs to how the garment actually fits your body, not the number on the label.

Step 1 — Measure

You need two measurements, both taken with a soft tape:

• Underbust — snugly around the ribcage, directly under the bust. This determines the band.
• Bust — around the fullest part of the bust, keeping the tape level. This drives the cup.

Measure in inches (or centimetres and divide by 2.54 to convert). Take the measurements over bare skin or a lightly fitted top — a padded bra will skew the bust measurement.

Step 2 — Find the band

There are two schools of thought, and they can give different answers:

Modern method — round the underbust to the nearest even number:

Underbust 31" → band 32
Underbust 30" → band 30

Traditional (+4) method — add 4 (or 5 if the underbust is an odd number) to get the band:

Underbust 31" → 31 + 5 = 36

The traditional approach dates back to an era of stiffer, less stretchy fabric that needed more room to be comfortable. Modern, stretchier bands measure closer to your true underbust, which is why most current sizing guides favour the modern (add-a-little-or-nothing) method.

Step 3 — Find the cup

Subtract the band from the bust, and map the difference to a letter. Each inch of difference roughly equals one cup letter:

Difference	Cup size
1"	A
2"	B
3"	C
4"	D
5"	DD
6"	DDD / F

So a 1-inch difference is an A cup, a 4-inch difference a D cup, and so on.

Worked example

Underbust 31", bust 36", using the modern method:

Band = 32
Difference = 36 − 32 = 4" → D cup
Size = 32D

Now try the traditional method on the same numbers: band becomes 36, and 36 − 36 = 0" → a 36AA or 36A. The two methods can land you two or more letter-cups apart — which is exactly why fit-in-store beats any formula alone.

Sister sizes

"Sister sizes" are different combinations of band and cup that hold the same cup volume:

32D = 34C = 30DD

The trick is that when you change the band by one step, you change the cup letter in the opposite direction by one to keep the same volume. This is a genuinely useful tool: if a bra is too snug in the band but too loose in the cup, try the sister size (a larger band, smaller cup, same volume). It's how a slightly off size can often be rescued into a good fit.

Why fit beats the math

Every body is different, and sizing varies by brand, fabric and style. The calculated size is an excellent starting point — not a verdict. A firm, comfortable band and a smooth, supportive cup are the real test. Many women wear a size that feels right rather than the one a formula returns, and that's both normal and correct.

Key takeaways

• Measure underbust and bust; the band comes from the underbust, the cup from the difference.
• Modern method rounds the underbust to even; the traditional +4 era method adds 4–5.
• Sister sizes share volume — change the band and the cup letter in step with it.
• Use the math to find where to start, then let the fit on your body decide.

FAQ

Why do my measurements give a different size than a store chart? Retailers, manufacturers and fitting methods differ. Always start from your own two measurements and adjust by how it feels, not by what the label promises.

What is the +4 method? It's an older approach that adds 4 (even) or 5 (odd) inches to the underbust. It suited old stiff fabrics; modern stretchy bands typically fit closer to your true measurement.

How do sister sizes work? Sister sizes hold the same cup volume — e.g., 32D = 34C = 30DD. When you change the band, shift the cup letter the opposite way by the same number of steps.

Should I round up or down? Round the underbust to the nearest even number for the band (modern method) and the cup difference to the nearest whole inch. When between sizes, try both and keep the one that fits better.

Get your number with the Bra Size Calculator, and pair it with the BMI Calculator or the Ideal Weight Calculator for more of your health metrics.