Full Bust Adjustment (FBA) Guide for Sewing Patterns

The Full Bust Adjustment, or FBA, is the most consequential pattern alteration most full-busted sewers will learn. Without it, a pattern drafted for a B cup either pulls and gapes at the bust (if you cut your high-bust size) or hangs loose elsewhere (if you cut a larger size to accommodate the bust). With it, the same pattern fits properly across the bust without distorting the rest of the garment.

The FBA is intermediate-level pattern work — more involved than length adjustments, less than from-scratch drafting. Once you understand it, you’ll do it without thinking on every fitted top, dress, or jacket pattern.

What an FBA actually does

Most sewing patterns are drafted assuming a B cup — meaning the difference between high bust and full bust is about 5 cm. If your difference is bigger (C cup is ~7.5 cm difference, D is ~10, DD is ~12.5, etc.), the pattern doesn’t accommodate your bust shape.

The naive solution — “just cut a larger size” — works only at the bust. The shoulders, back, neckline, and waist all become too big because you sized up for the bust alone.

The FBA is the surgical solution: it adds width and length only at the bust, leaving the shoulders and upper body sized for your high bust measurement. The dart angles and bust apex position adjust accordingly.

When you need an FBA

You need an FBA if your full bust measurement is more than 5 cm larger than your high bust measurement. Specifically:

B cup or smaller (≤5 cm difference): no FBA. Cut your bust size.
C cup (~7 cm difference): consider an FBA, especially on fitted bodices.
D cup (~9 cm difference): FBA recommended on most fitted patterns.
DD/E and larger (>10 cm difference): FBA essential on fitted patterns.

For more on taking these measurements correctly, see our body measurements guide.

Some patterns are drafted for larger cup sizes — Cashmerette, Muna and Broad, and a few others publish patterns drafted for C or D cup defaults. For these, you may need a smaller FBA or none at all. Always check the pattern’s documentation.

For loose-fitting and unstructured garments (gathered tops, oversized boxy tops, kimono-sleeve garments), an FBA may not be needed even at larger cup sizes — the looseness accommodates the bust without specific shaping.

Calculating how much to add

The FBA adds width to one side of the pattern (which doubles when cut on the fold or matched to the other side, giving total width across the bust).

Formula

FBA amount per side = (Full bust − High bust − 5 cm) ÷ 2

Example: Full bust 102 cm, High bust 90 cm. 102 − 90 − 5 = 7 cm total to add at bust. 7 ÷ 2 = 3.5 cm to add to one side of the pattern.

(The ”÷ 2” is because the pattern piece is one side; the adjustment doubles when both sides are sewn together.)

Or for imperial measurements: Full bust 40”, High bust 36”. 40 − 36 − 2 = 2” to add at bust. 2 ÷ 2 = 1” to add to one side of the pattern.

Choose your size first

Choose the pattern size based on your high bust measurement, not your full bust. The high bust gives a size that fits the shoulders and upper body correctly. The FBA then adds the required bust accommodation.

This is counterintuitive — you might be a “size 14” by full bust but a “size 12” by high bust; you cut the size 12 and FBA up to a size 14 bust width. The shoulders fit; the bust fits; nothing is too big elsewhere.

The slash-and-spread method (standard FBA)

This is the classic FBA method — the slash-and-spread. It works on bodices with a standard bust dart.

Tools

The pattern piece (front bodice; back bodice typically doesn’t need an FBA).
Pattern tracing paper.
Scissors.
Tape (paper or scotch).
Ruler and pencil.

Step 1: Trace the pattern

Always trace and adjust on a copy. Don’t cut the original.

Step 2: Mark the bust apex

Find the point that corresponds to your bust apex on the pattern. The apex is the most-forward point of the bust. On the pattern, it’s typically at the converging point of the bust dart, or you can mark it by:

Drawing a line from the centre of the side bust dart toward the centre front.
Where this line meets a vertical line from your bust point (typically marked or implied near the centre of the bodice front), that’s the apex.

Step 3: Draw the slash lines

Three lines, all converging at the apex point:

Line 1: From the bust apex straight down to the waist (or hem of the pattern).
Line 2: From the bust apex through the centre of the bust dart, extending to the side seam.
Line 3: From the bust apex up to the armhole, ending at the lower notch (typically about 5 cm down the armhole curve from the side seam).

These three lines form a Y-shape converging at the apex, with one leg going down, one going out to the side, and one going up to the armhole.

Step 4: Cut along the slash lines

Cut along all three lines, but stop just short of each endpoint:

Don’t cut all the way through the side seam — leave a paper hinge of about 1 mm.
Don’t cut all the way through the armhole edge — leave a paper hinge.
The bottom edge (waist or hem) you can cut all the way through, since you’ll spread the pattern there.

The pattern is now divided into sections that hinge at the side seam and armhole, with a free piece at the waist.

Step 5: Spread the pattern

Place the pattern over a piece of plain paper (or pattern tracing paper). Spread the cut sections so that:

The bust dart opens up by your calculated FBA amount (e.g., 3.5 cm).
The vertical slash from apex to waist opens by the same amount.

Tape the pieces in their new positions to the underlying paper.

The result: the pattern is wider at the bust by 3.5 cm, the dart is bigger (because more fabric is being draped over a larger bust), and the waist is wider by 3.5 cm.

Step 6: Add length below the bust (if needed)

The FBA adds width, but a larger bust also needs more vertical fabric to drape over the apex. To add length:

The vertical slash from apex to waist may have already opened up at the bottom — that adds length naturally.
If not, you can add 1–3 cm of length at the bottom of the pattern, blending into the side seam.

For very large cup sizes (DD+), you may need to add length at a horizontal slash through the bust as well. This is a more complex adjustment.

Step 7: Re-draw the dart

The bust dart is now bigger. Re-mark its position with new dart legs, narrowing the dart point about 2.5 cm from the bust apex (you don’t want the dart point landing exactly at the apex — leave space).

Sometimes the FBA produces a dart that’s too large for one dart to handle (15+ degrees of dart angle). In that case, split the dart into two darts (a side bust dart and a French dart, or a side dart and a waist dart) — each one smaller than the original.

Step 8: Even out the bottom hem

The slash and spread will have made the bottom (waist or hem) slightly uneven. Re-draw it as a smooth line, blending across the gap.

Step 9: True up the seam allowances

The side seam and armhole may have small jogs where the cut hinges allowed the pattern to flex. Re-draw these as smooth curves.

Verifying the FBA

Make a muslin. The FBA is one of the alterations where seeing it on your body is the only true test. See our muslin guide for the process.

Specific things to check:

Is the bust dart pointing at your bust apex? If not, adjust dart position.
Is the bust roomy without gaping? If gaping at the neckline, the FBA may be too large or you need a separate gape adjustment.
Do the side seams hang vertically without tension? If the side seams swing forward, the FBA needs more adjustment in the lower torso.

Variations of the FBA

FBA on princess seams

Patterns with princess seams (curved seams over the bust instead of a separate dart) need a different adjustment. The princess seam is widened to add bust room, and length is added to the curved seam. Detailed FBA tutorials specifically for princess seams are widely available.

FBA on knit patterns

Knit fabric stretches, so the FBA is sometimes unnecessary or smaller. The negative ease in knit patterns can compensate for cup size differences. Test with a low-stakes muslin.

FBA on shirts with darts

Same process as a basic dress bodice. The shirt’s bust dart is widened and the seam allowances re-trued.

Small Bust Adjustment (SBA)

The mirror process. If your full bust measurement is less than 5 cm larger than your high bust (i.e., you wear an A cup), an SBA removes width from the bust. Slash and overlap (instead of slash and spread). Less common because most patterns assume a B cup is the smallest you’d cut.

Common FBA mistakes

Cutting the wrong starting size

Choose by high bust, not full bust. Choosing by full bust skips the entire reason for the FBA.

Forgetting to re-trace the dart

The dart is bigger after FBA. The dart point should still land 2–3 cm short of the bust apex. If you sew the dart at the original dart angle, the new wider dart pulls oddly at the bust.

Not adding length when needed

Larger busts need more vertical fabric. The FBA’s vertical slash from apex to hem typically opens up enough length, but very large cups may need additional length added.

Trying to FBA a pattern not drafted for a fitted bust

Loose tops, kimono-sleeve garments, and very oversized blouses don’t really benefit from an FBA — the looseness accommodates any bust size. FBA is for fitted styles.

When to learn the FBA

The FBA is intermediate-level pattern work. Most sewers learn it after they’ve made a handful of fitted garments and noticed the same fit problems happening repeatedly. If you’re at the point where you’ve made several blouses or dresses and they all gape at the bust, pull across the chest, or hang oddly elsewhere — it’s time to learn the FBA.

For where pattern adjustment fits in the broader sewing journey, our understanding sewing patterns guide and 25 beginner projects path cover the surrounding context.

Closing thought

The FBA is the single most-needed pattern alteration for full-busted sewers, and the absence of one is the reason most home-sewn clothes don’t fit at the bust. Learning it once is a permanent upgrade — every fitted pattern from then on can be properly adjusted to your body.

It looks complicated when you’re staring at the slash-and-spread instructions for the first time. By the third or fourth FBA, it’s a 15-minute task. By the tenth, it’s automatic.

The first FBA you do will probably go imperfectly. The fix is the same as for any new technique: test on a cheap muslin, observe the result, adjust, repeat.