Estimate your bra band size and cup size from your band measurement (underbust) and bust measurement, using the standard sister-size method.
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Estimating bra size starts with two measurements: the band size (measured snugly around the ribcage directly under the bust, then commonly rounded to the nearest even number) and the bust measurement (around the fullest part of the bust). The difference between these two numbers, in inches, is what determines the cup letter — a common industry convention maps each inch of difference to one cup size step (0 inches difference is an AA cup, 1 inch is an A cup, 2 inches is a B cup, and so on upward). This "sister size" method is a widely used general approximation across the lingerie industry, though exact best fit can still vary meaningfully between individual brands, styles, and body shapes, since there is no single international regulatory standard governing bra sizing the way there is for some other clothing measurements. Measuring accurately, snugly but not tightly, while wearing a non-padded bra gives the most representative starting point for this calculation.
Bra sizing is notoriously inconsistent from brand to brand, which makes a quick, formula-based estimate a genuinely useful starting point before trying anything on. Rather than guessing at a size or relying on whatever number you last wore (bodies and brands both change), this calculator uses the two measurements every standard bra-fitting method is built on: your band measurement and your bust measurement.
The band number comes first. It's measured snugly — not tight — directly under the bust, around the ribcage, and is the foundation the rest of the size is built on. Because bra bands are manufactured in even-numbered sizes as a long-standing industry convention (32, 34, 36, and so on), an odd measurement is rounded up to the next even number, which is why you'll sometimes see a measured 33 inches translate to a 34 band.
The cup size is where the second measurement comes in. Bust measurement is taken around the fullest part of the chest, and the difference between that number and the band measurement, in inches, determines the cup letter. This is often called the "sister size" method: each additional inch of difference steps up one cup letter, starting from AA at zero inches of difference. It's a simplification of how cup volume actually relates to band size, but it remains the most widely used general-purpose method because it only requires two measurements anyone can take at home with a soft tape measure.
It is worth being upfront about the limits of any formula-based bra size estimate. Unlike shoe sizing or clothing sizing in some countries, there is no single, universally enforced sizing standard across the lingerie industry — different brands, different countries, and even different bra styles from the same brand can fit noticeably differently at the "same" calculated size. Treat the result from this calculator as a well-informed starting point for trying on sizes, not a guaranteed final answer. For the most accurate measurement, take both the band and bust measurements while wearing a non-padded, well-fitting bra, with the tape measure level and snug but not compressing the skin.
Because bra band sizes are traditionally manufactured only in even numbers (32, 34, 36, etc.), so an odd measurement is conventionally rounded up to the nearest even band size.
This is a general estimate using a common industry formula; actual best fit varies between brands, styles, and individual body shapes, so treat this as a solid starting point for trying on sizes rather than a guaranteed exact fit.
Measure the band snugly (not tight) directly under the bust while wearing a non-padded bra, and measure the bust around its fullest part in a relaxed, natural position, for the most representative result.
No — unlike some clothing measurements, bra sizing is not governed by a single international standard, which is a major reason the same calculated size can fit differently between brands and styles.