Bust Adjustment Calculator — FBA & SBA

An FBA adds width at the bust point of a bodice pattern without changing the shoulders, armhole or waist. It is needed when your bust measurement is larger than the pattern size you selected (based on your high bust measurement).

Most patterns are drafted for a B cup — a full-bust-minus-high-bust difference of about 5cm (2″). Each extra 2.5cm (1″) of that difference equals one more cup size, so if your cup runs larger than the pattern’s drafted cup you need an FBA. In practice you should adjust once your full bust sits roughly 2.5cm or more beyond the pattern’s drafted full bust for the size you picked by high bust. The adjustment adds room over the bust and takes it back out at the waist with a larger dart.

A Small Bust Adjustment is the reverse — removing excess bust room when the pattern bust is larger than your body. This is less common but needed when choosing a larger size for shoulder width.

Yes. When you slash and spread the bodice front by the per-side amount (the total bust difference ÷ 2), the same opening forms a wider bust dart at the side seam. So the dart widens by the spread amount — about half the total bust difference — not a quarter of it.