Our Methodology
The exact formulas Shape Finder uses for band, cup, and sister sizes, with US/UK/EU mapping and named sources. Last verified June 2026.
Usage Guide
How the calculator works
Shape Finder uses two measurements, both taken in inches, snug but not tight, with an unpadded bra or none at all:
- Underbust — measured directly under the bust, around the ribcage.
- Bust — measured around the fullest part of the chest.
From those two numbers, everything else is arithmetic. We show you each step so the result is never a black box.
Band size
Your band is your underbust measurement rounded to the nearest whole inch. If you measure 31.4″, the band is 31; if you measure 31.6″, the band is 32. We use US sizing as our primary system and cross-reference UK and EU. This is a deliberately simple rule, and it is also why fit can vary between brands — band construction and stretch differ from one maker to the next.
Cup size
Your cup comes from the difference between your bust and your band: cup = bust − band. Each full inch of difference is roughly one cup letter. The table below shows the mapping we use.
| Bust − band (inches) | US cup |
|---|---|
| 1″ | A |
| 2″ | B |
| 3″ | C |
| 4″ | D |
| 5″ | DD / E |
| 6″ | DDD / F |
Cup letters are approximate by nature: an inch is a coarse unit, and a half-inch can sit you between letters. The calculator rounds to the nearest whole inch and tells you the letter that follows.
Sister sizes
A sister size is a different band-and-cup label that holds about the same cup volume. The rule is simple: go up one band and down one cup, or down one band and up one cup. Because the cup volume is preserved, sizes like 34C, 36B, and 32D all hold roughly the same amount — they differ mainly in how tightly the band sits. Sister sizes are useful when a band feels too loose or too tight but the cups otherwise fit.
US, UK, and EU
We treat US sizing as the primary output and show UK and EU equivalents alongside it for cross-reference. Letter conventions diverge between systems, especially above a D cup, which is why a single garment can carry two or three different labels. The calculator maps between them so you are not left converting by hand.
Sources we rely on
We try to make every claim traceable. The sources that inform our sizing content and our guidance include:
- Wood, Cameron & Fitzgerald, Chiropractic & Manual Therapies (2008) — the often-quoted finding that around 80% of women may be wearing the wrong size. We treat this as illustrative rather than definitive: it was a small study, and we say so.
- Dr. Tracy Pfeifer’s fitting guide — the principle that the band, not the straps, provides roughly 80–90% of a bra’s support, which is why band fit matters so much.
- Cleveland Clinic — guidance on waist trainers: they do not permanently reshape the body, wear should be limited, and you should consult a professional.
- American Cancer Society — the clear position that bras do not cause breast cancer.
- Mordor Intelligence — market-size figures we cite for context on the shapewear industry.
Accuracy and corrections
This methodology is maintained by the Shape Finder editorial team. Last verified: June 2026. If you spot an error in our formulas, our table, or a citation, you can flag it for review; corrections are checked against the sources above, and this page is updated when something changes. We would rather be corrected than be wrong.