What Is a Bra Size Calculator?
A Bra Size Calculator is a simple tool that estimates your bra size using two body measurements: your band size and your bust size. Instead of guessing or relying on an old, ill-fitting bra, you can use these numbers to find a starting point for the right band number and cup letter. Because sizing systems vary by region, this tool is most accurate for US/UK-style sizing (such as 34B or 36C). Sizes in the EU, France, and Australia use different numbers, so always double-check the brand's own size chart.
How to Use It
Grab a soft measuring tape and take two measurements while wearing a non-padded bra:
- Band measurement: Measure snugly around your ribcage, directly under your bust.
- Bust measurement: Measure around the fullest part of your chest, keeping the tape level.
Enter both numbers into the calculator. It will round your band measurement to the nearest even number and compare your bust to your band to determine your cup size.
The Formula Explained
Bra sizing relies on the difference between your bust and band measurements:
$$\text{Cup Difference} = \text{Bust (in)} - \text{Band}_{\text{rounded}}\,, \quad \text{Band}_{\text{rounded}} = \begin{cases} \text{Band} & \text{if even} \\ \text{Band} + 1 & \text{if odd} \end{cases}$$- Band size: Your under-bust measurement, often rounded to the nearest even inch.
- Cup size: Bust minus band. Each inch of difference equals roughly one cup size.
The general cup guide (in inches): 1" = A, 2" = B, 3" = C, 4" = D, 5" = DD/E, and so on. So if the difference is small, you have a smaller cup; a larger difference means a larger cup.
Worked Example
Suppose your band measures 32 inches and your bust measures 35 inches. The difference is \(35 - 32 = 3\) inches, which corresponds to a C cup. Your estimated size would be 32C.
Cup Size Chart (Bust–Band Difference)
Your cup size is determined by the difference between your bust measurement and your (rounded) band measurement. Measure your bust at the fullest point and your band snugly under the bust; subtract the band from the bust and match the difference (in inches) to the chart below. Where US and UK lettering diverge, both are shown.
| Bust − Band Difference | US Cup | UK Cup |
|---|---|---|
| 0" | AA | AA |
| 1" | A | A |
| 2" | B | B |
| 3" | C | C |
| 4" | D | D |
| 5" | DD / E | DD |
| 6" | DDD / F | E |
| 7" | G | F |
| 8" | H | FF |
Worked example: a bust of 37" with a band measurement of 34" gives a cup difference of 3 inches, which corresponds to a C cup — giving a bra size of 34C. Note that beyond DD the lettering systems diverge: a 5" difference is a US DD/E but a UK DD, so always confirm which regional system a brand uses.
Sister Size Reference Table
Sister sizes share the same cup volume but use a different band. When you change the band by one step, you change the cup letter by one step in the opposite direction to keep the same cup volume. Going down a band size means going up a cup; going up a band size means going down a cup. These are useful when a band feels too tight or too loose but the cup fits well.
| One Band Down (cup up) | Your Size | One Band Up (cup down) |
|---|---|---|
| 30D | 32C | 34B |
| 32D | 34C | 36B |
| 34D | 36C | 38B |
| 32E (DD) | 34D | 36C |
| 34E (DD) | 36D | 38C |
| 34F (DDD) | 36DD | 38D |
For example, if 34C feels right in the cup but the band rides up, try its sister size 32D (smaller, snugger band, slightly larger cup letter for the same volume); if the band digs in, try 36B instead. All three — 32D, 34C and 36B — hold essentially the same cup volume.
Frequently Asked Questions
Is the calculator 100% accurate? No tool can replace a professional fitting. Brands vary, and factors like breast shape and bra style matter. Use the result as a starting point and try a size up or down.
What is sister sizing? If a band feels too tight or loose, you can move to an adjacent band while changing the cup letter to keep the same cup volume — for example, 34B and 36A are sister sizes.
How often should I re-measure? Re-measure after weight changes, pregnancy, or roughly once a year, as your size can shift over time.
