What is a Full Bust Adjustment (FBA)?

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).

When do I need an FBA?

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.

Does this affect dart size?

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.

SewCalcs

Bust Adjustment Calculator — FBA & SBA

Calculate full bust adjustment (FBA) or small bust adjustment (SBA) for bodice patterns.

Your Bust Measurement

Full bust circumference

Pattern Bust Measurement

Bust on the pattern size chart

Adjustment TypeFBA adds bust room (body larger than pattern); SBA removes it (pattern larger than body)

How We Calculate This

The bust adjustment is calculated from the difference between your body bust and the pattern bust measurement. Because you alter only the bodice front (cut on the fold, i.e. half the body), you apply half the total difference to that one pattern piece using the slash-and-spread method.

The formula

Difference = Body bust − Pattern bust

Spread on the bodice front = Difference ÷ 2

Dart increase = the spread (Difference ÷ 2)

When you open the pattern by the spread amount, that same opening widens the bust dart at the side seam — so the new dart grows by the spread, not a quarter of the difference. If the resulting dart looks too wide, split the extra between the bust dart and a new waist dart.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify quantities before cutting or purchasing fabric.