Bra Size Calculator: Get Your Accurate Size in 3 Measurements (US, UK & EU)
Usage Guide
Most women are wearing the wrong bra size — research consistently estimates that up to 80% of women are in an incorrect size. The shapefinder.app calculator uses the same measurement methodology recommended by r/ABraThatFits, the most-cited community standard for accurate bra fitting, not the vanity sizing shortcuts used by major retailers.
Enter your three measurements below to get your size across US, UK, EU, AU, and FR systems — plus a plain-English explanation of how confident we are in that result and why.
How to Measure Yourself Correctly (Snug Band, Loose Band, Bust — and Why Order Matters)
Accurate bra sizing requires three separate measurements, taken in a specific order. Skipping any one of them is the most common reason calculators return the wrong size.
1. Snug Band (underbust, pulled firm) Wrap a soft measuring tape directly under your bust, where the bra band sits. Pull it snug — not tight enough to dig in, but with no slack. Round to the nearest whole inch. This is your snug band measurement.
2. Loose Band (underbust, relaxed) Without moving the tape, let it relax so it sits comfortably against your skin. Record this number too. The difference between snug and loose tells the calculator how much your ribcage expands when you breathe, which affects band size selection.
3. Bust (fullest point, leaning forward) Lean forward at roughly 45 degrees so your breast tissue hangs away from your chest. Measure around the fullest part of your bust, keeping the tape parallel to the floor. This position captures your true projected volume rather than the compressed shape you get when standing upright.
Why order matters: Taking the bust measurement while standing upright — the method most retail calculators instruct — consistently underestimates cup volume. The lean-forward method is the standard recommended by the r/ABraThatFits wiki and is the primary reason community-based sizing differs so dramatically from in-store results.
Why Retail Bra Size Calculators Are Often Wrong (and What We Do Differently)
Most retail calculators were designed around a legacy formula that adds 4–5 inches to the underbust measurement to arrive at a band size. This "add-four" method was developed decades ago when bra fabrics had far less stretch. Today it produces band sizes that are too large and cup sizes that are too small — a combination that looks fine on a hanger but fits poorly on a body.
What retailers gain from this: A 36B is easier to stock and sell than a 32E. Vanity sizing keeps customers in a narrower range of sizes and reduces inventory complexity. It is a business decision, not a fit decision.
What shapefinder.app does differently:
- Uses the no-add method: your snug band measurement maps directly to your band size, with the loose measurement used as a secondary check.
- Calculates cup size from the difference between bust and snug band, not bust and padded band.
- Shows you the math — every result includes the intermediate numbers so you can see exactly how your size was derived.
- Returns a confidence note: if your measurements fall near a boundary between two sizes, we tell you that and explain which to try first and why.
Bra Size Chart: US, UK, EU, AU, and FR Conversions in One Place
Cup sizes use different letters and different reference points across sizing systems. The chart below maps the most common sizes. Note that EU and FR systems use centimeter-based band sizes, while US, UK, and AU use inches (with UK and AU bands typically running one size smaller than US for the same body).
| US | UK | EU | AU/NZ | FR |
|---|---|---|---|---|
| 32A | 32A | 70A | 10A | 85A |
| 34B | 34B | 75B | 12B | 90B |
| 36C | 36C | 80C | 14C | 95C |
| 34D | 34D | 75D | 12D | 90D |
| 32DD/E | 32DD | 70E | 10DD | 85E |
| 34F | 34F | 75F | 12F | 95F |
| 36G | 36G | 80G | 14G | 100G |
Key rule: When converting between systems, always convert band and cup separately. A US 34D and a UK 34D are the same physical bra. A US 34D and an EU 75D are also equivalent. A FR 90D is a US/UK 34D. Confusion usually arises when shoppers assume all systems share the same cup-letter scale — they do not above a D cup.
Sister Sizes Explained: How to Find Alternatives When Your Size Is Out of Stock
Sister sizes are bra sizes that share the same cup volume but use a different band length. When you go up one band size, you go down one cup letter. When you go down one band size, you go up one cup letter.
Example: If your true size is 34D, your sister sizes are:
- 32E (smaller band, same cup volume — fits tighter around the ribcage)
- 36C (larger band, same cup volume — fits looser around the ribcage)
When to use a sister size:
- Your correct size is out of stock
- You are between band sizes and want to test fit before ordering
- A specific bra runs large or small in the band
When not to rely on sister sizes: Sister sizing is a workaround, not a substitute. The band provides roughly 80% of a bra's support. If you consistently need to go up a band to find comfort, your band measurement should be re-checked — you may be sizing into a band that is genuinely too large.
Common Fit Problems Decoded: What Your Current Bra Is Telling You About Your Real Size
| What you notice | What it likely means |
|---|---|
| Band rides up at the back | Band is too large — try one band size down |
| Underwire sits on breast tissue | Cup is too small — go up one or more cup sizes |
| Straps dig in despite adjustment | Band is too large; straps are compensating for lack of band support |
| Cups wrinkle or gap at the top | Cup is too large, or the bra style doesn't match your breast shape |
| Center gore (front panel) floats off chest | Cup is too small — the underwire cannot lie flat |
| Spillage over the top of the cup | Cup is too small — go up at least one cup size |
| Quadding (four-boob effect) | Cup is too small and/or the wrong shape for your breast projection |
The single most common pattern: a band that is too large combined with a cup that is too small. This is the direct result of the add-four method and is why so many women who have always worn a 36B discover — after measuring correctly — that they are a 32E or 30F.
Frequently asked questions
What is the most accurate way to measure bra size at home?
Take three measurements: snug underbust (tape pulled firm), loose underbust (tape relaxed), and bust (leaning forward at 45 degrees so tissue hangs freely). Use the no-add method — your snug band measurement becomes your band size directly. The lean-forward bust measurement is the most important deviation from standard retail instructions and produces a significantly more accurate cup size.
Why does my bra size differ between brands like ThirdLove, Victoria's Secret, and Wacoal?
Each brand uses a different internal sizing formula, different amounts of stretch in their bands, and different cup-shape assumptions. Victoria's Secret has historically used a version of the add-four method, which skews results toward larger bands and smaller cups. Wacoal uses a more European-influenced fit standard. ThirdLove uses half-cup sizes that don't exist in most other systems. Your body measurement does not change — only the label does. Always fit to your measurements, then adjust by one size if a specific brand runs large or small.
What are sister sizes in bras and when should I use them?
Sister sizes share the same cup volume but use a different band length. Going up one band size means going down one cup letter (34D → 36C); going down one band size means going up one cup letter (34D → 32E). Use sister sizes when your correct size is unavailable or when a specific bra runs slightly large or small in the band. Do not use them as a permanent substitute — the band provides most of a bra's support, and a poor band fit affects comfort and posture over time.
How often should I re-measure my bra size?
Re-measure any time your weight changes noticeably, after pregnancy or breastfeeding, after significant changes in fitness or muscle mass, or if your current bras suddenly feel different than they used to. Band size tends to change with weight; cup size can change independently due to hormonal shifts, age, or body composition changes. Many fitters recommend checking measurements once a year as a baseline.