Bra Size Calculator: Find Your Perfect Fit Instantly

Finding a well-fitting bra can transform your comfort, posture, and confidence, yet the widely repeated claim that up to 80% of women wear the wrong bra size traces back mainly to a small 2008 study of 30 women with pre-existing back pain, so treat it as a widely cited industry talking point rather than a rigorously established population statistic. What’s genuinely well established is that bra sizing is inconsistent enough between brands and styles that many people are wearing a size that doesn’t fit them optimally, and a proper measurement is a reliable starting point regardless of the exact percentage. This free bra size calculator uses your two key measurements (underbust and bust) to recommend your band size, cup size, sister sizes, and international conversions across US, UK, EU, and AU sizing systems.

How the Bra Size Calculator Formula Works

This calculator measures two separate things from your two inputs: your band size (how snugly the bra sits around your ribcage) and your cup size (how much volume the cup needs to hold, expressed as the difference between bust and band). Both numbers come from the same two measurements, but they’re calculated independently before being combined into a single size.

Underbust is measured around your ribcage directly below your bust, and rounds to the nearest whole number, then up to the nearest even number, to produce the band size. Bust is measured around the fullest part of your chest. Subtracting band size from bust gives the bust-to-band difference, which the calculator rounds to the nearest whole inch and maps to a cup letter, where each additional inch of difference steps up one letter (AA, A, B, C, D, DD, and so on).

Step-by-step calculation walkthrough

Step 1: Identify the inputs. Underbust measurement of 33 inches, bust measurement of 38 inches.

Step 2: Apply the formula. Band size = Underbust rounded to nearest even number. Cup difference = Bust − Band. Cup letter = the letter at that difference’s position in the sequence AA, A, B, C, D, DD…

Step 3: Perform the calculation. 33 rounds to 33, and since 33 is odd, it rounds up to the nearest even number: Band = 34. Cup difference = 38 − 34 = 4.0 inches. A 4-inch difference corresponds to a D cup. Combined size: 34D.

Step 4: Interpret the result. This measurement set recommends a 34D as a starting point. Because band and cup are calculated from the same two measurements but through separate steps, two people with the same bust measurement can end up with completely different cup letters if their underbust measurements differ, and two people with the same band size can have very different cup letters depending on their bust measurement.

Assumptions and limitations: the arithmetic is exact given the two measurements entered, but the resulting size is a starting point for trying bras on, not a guaranteed fit. Bra construction varies enough between brands and styles that the same calculated size can fit differently depending on the specific bra, cup shape, and underwire design. Measurement technique also matters: a loosely held tape, measuring over bulky clothing, or measuring while not standing naturally can shift the result by a full cup size or more.

How Bra Sizes Are Calculated

Bra sizing uses two numbers: the band size (a number, e.g. 32, 34, 36) and the cup size (a letter, e.g. A, B, C, D). Together they form your bra size: for example, 34C or 36DD.

The band size is derived from your underbust measurement (the circumference of your ribcage directly below your breasts). The cup size is determined by the difference between your bust measurement (around the fullest part of your chest) and your band size.

What Are Sister Sizes?

Sister sizes are bra sizes that contain the same cup volume as your calculated size, but with a different band and cup letter combination. The cup volume stays the same when you go up one band size and down one cup letter (or vice versa). For example, if your size is 34C, your sister sizes are 32D (smaller band, larger cup) and 36B (larger band, smaller cup). All three contain the same cup volume, they just sit differently on the body.

Sister sizes are invaluable when shopping: if your exact size is sold out, trying a sister size gives the same cup volume with a slightly different fit. They’re also useful when you’re between sizes, if your band feels slightly loose in 34C, the 32D may feel more supportive with the same cup fit.

Signs You’re Wearing the Wrong Bra Size

International Bra Size Conversions

Australian and New Zealand retailers commonly use their own band numbering, roughly 22 less than the equivalent US/UK band, unlike the direct US/UK/AU match this calculator’s “AU” system option uses internally. If you’re shopping at an AU-specific retailer, use the table above rather than the calculator’s raw AU output for the closest match.

3 Real-Life Examples

Three different situations, calculated the way the tool above does it.

These are illustrative calculations using the same formula the calculator above applies. They’re a starting point for shopping, not a guarantee of fit, actual fit always depends on trying a specific bra on.

Important Notes

• This is a sizing estimate, not a guarantee of fit. The arithmetic is exact given accurate measurements, but bra construction varies enough between brands that the same calculated size can fit differently from one bra to the next.
• Measurement technique affects the result significantly. A loosely held tape, measuring over bulky clothing, or poor posture while measuring can shift the result by a full cup size or more, remeasure if a size seems off.
• The “80% of women wear the wrong size” statistic is widely repeated but methodologically limited. It traces mainly to a small study of women who already had back pain, treat it as a talking point rather than a precise population figure.
• The calculator’s built-in AU/NZ conversion uses the same band numbers as UK/US for simplicity. Genuine Australian retailers commonly use a distinct band numbering (roughly 22 less than the US/UK equivalent), shown in the international conversion table in this article.
• Sister sizes maintain cup volume, not overall fit. They’re worth trying when your exact size is unavailable or when you’re between sizes, but they aren’t automatically a better fit than your calculated size.
• Data privacy. All calculations run in your browser. Your measurements aren’t sent to a server, and the PDF is generated locally on your device.

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Frequently Asked Questions