To calculate cup size, measure the band and bust in inches, then subtract the band number from the bust number — each inch of difference equals one cup letter.
The standard US method for working out cup size starts with two measurements taken with a soft tape measure. One wraps around your ribcage, directly under the bust; the other goes around the fullest part of the chest. Subtract the first number from the second, and the difference maps to a letter on a cup-size chart. Whether you need to calculate cup size for a bra fitting at home or just to shop smarter online, the process takes about two minutes — provided you measure the right spots and don’t pull the tape too tight.
The Two Measurements You Need
Wacoal America’s fitting guidance is a reliable baseline here. It calls for a non-padded bra (or no bra), a mirror, and a soft measuring tape. The band measurement is taken by wrapping the tape snugly and level around the underbust, right where the band sits. The bust measurement goes over the fullest part of the chest, with the tape kept parallel to the floor and without compressing any tissue.
Keep the tape level across your back on both readings. If it rides up or dips down, both numbers distort.
Cup Size Chart: Turning Inches Into Letters
After subtracting the band from the bust, the resulting number of inches maps to a cup letter. Vogue’s published chart uses a standard US progression. The most common mapping is consistent across major retailers: zero-inch difference means AA, then each inch adds a letter: A, B, C, D, DD, F, and beyond.
| Bust minus band (inches) | Cup size |
|---|---|
| 0 | AA |
| 1 | A |
| 2 | B |
| 3 | C |
| 4 | D |
| 5 | DD |
| 6 | F |
Why the Same Letters Can Mean Different Sizes
Cup letters are not universal. The inch-to-letter mapping above reflects the US system, but UK, EU, and brand-specific charts can assign a different letter to the same difference. A 5-inch difference reads as DD in the US and often as E in the UK. Before trusting a size, check the chart on the brand’s own site, particularly if you’re ordering from an overseas retailer or a label known for running small or large (some brands run small, so checking reviews helps).
Because bras are so brand-specific, the subtraction method is a starting point, not a guarantee. Wacoal and most fitters treat the calculated size as a base and then adjust for how a specific bra fits — straps that dig in, cups that gap, or a band that rides up all signal a fit problem. Fit the band first, since it carries most of the support, and remember that a poorly fitting cup can cause gapping or spillage even when the number looks right.
Once you have your starting size, a product roundup of well-reviewed bras in your band and cup can narrow the search — our tested best 40B cup bra picks show which styles hold up in that specific size.
What Throws Off the Calculation
The most common errors are easy to avoid. Measuring over a push-up or heavily padded bra inflates the bust number. Letting the tape run diagonally across the back distorts the band reading. Pulling the tape tight over the bust flattens tissue and shrinks the cup estimate. And using the wrong regional chart produces the wrong letter entirely.
One extra note: some calculators round measurements to the nearest whole inch before subtracting, but not all of them use the same rounding rule. If a calculator gives you a letter that seems off, run a second calculator or measure twice — the Vogue bra measurement guide covers the same method with a fuller chart that extends to K cup for larger differences. Since cup letters and sizing systems vary by region and brand, the difference between a 34C and a 34D can come down to the rounding rule, so treat the result as a starting point and confirm against the brand’s size chart, as outlined in the Wikipedia entry on bra size standards.
Final note: The two-measurement method gets you a solid starting size. The final check is always the bra itself — the band should sit level and snug, the cups should hold without gaping or spillage, and the straps should stay put without digging in. If you’re between sizes, size up in the band and down in the cup (or vice versa) to compare both fits.
FAQs
Why does my cup size change between brands?
Different brands use different sizing systems and fit models. One brand’s 34C might run like another’s 34D, especially between US, UK, and European labels. Always check the brand’s own size chart before ordering and be ready to try a bra on, since the cup letter is a relative measurement, not a fixed physical standard.
Can I measure cup size without a tape measure?
You can approximate it using a piece of string or ribbon and a ruler. Wrap the string around your ribcage and bust the same way you would a tape, mark the points, then lay it flat against a ruler. The method works, but a soft tape measure is easier and more accurate.
Should I round my measurements up or down?
It depends on the calculator. Some round to the nearest whole inch, others round up. If you’re between sizes, try both adjacent cup sizes. A bra that fits well is more important than the exact number on the tag, so use the calculated size as a baseline and adjust based on how the bra actually fits.
References & Sources
- Wacoal America. “Bra Fit Calculator.” Official fitting guidance on measuring band and bust.
- Vogue. “How to Measure for a Bra.” Chart mapping inch differences to cup letters.
- Wikipedia. “Bra size.” Overview of sizing systems and regional variations.