Full Bust Adjustment
Your pattern pulls across the bust or the side darts don't lie flat? You probably need a Full Bust Adjustment. Enter your high bust and full bust measurements — we'll calculate the exact slash-and-spread amount for your pattern type.
How the darted FBA works
Commercial sewing patterns are drafted for a B cup — a bust measuring about 2 inches larger than the high bust. If your cup is C or above, the front bodice will be too small, causing pulling, gaping buttons, or a hemline that rides up.
The darted FBA adds width to the front piece using a slash-and-spread technique: two cuts are made through the bust point and the piece is opened by the FBA amount, creating a new side dart. The back, sleeves, and collar are unchanged. Pattern size is set by your high bust, not full bust.
How to take these measurements
High Bust
Wrap the tape directly under your armpits, above the bust. Keep it parallel to the floor, snug but not tight. This sets your pattern size.
Full Bust
Wrap the tape at the fullest part of your bust, parallel to the floor. Wear a well-fitting, non-padded bra. Don't compress — the tape should rest against your body. This sets your adjustment amount.
What your measurement difference means
Pattern cup size is not bra cup size. It is simply the gap between your high bust and full bust, and it is the only number that decides how much you add:
| Full bust minus high bust | Pattern cup | Typical FBA |
|---|---|---|
| 2 in / 5 cm | B — pattern default | None needed |
| 3 in / 7.5 cm | C | 1/2 in |
| 4 in / 10 cm | D | 1 in |
| 5 in / 12.5 cm | DD / E | 1 1/2 in |
| 6 in / 15 cm | F | 2 in |
| 7 in / 18 cm | G | 2 1/2 in |
A 34F bra and a 42B bra can need the same adjustment, or none at all — what matters is the difference between the two measurements, not the letter on your bra label. The calculator above works from your actual numbers, including patterns already drafted for a C or D cup.
How to do a darted FBA, step by step
- Trace the front bodice in your high bust size and mark the bust apex — the point the existing side dart aims at, usually about 1 in beyond the printed dart point.
- Draw line 1: from the hem, parallel to the grainline, straight up through the apex, then angle out to the armhole about a third of the way up from the underarm.
- Draw line 2: from the apex into the middle of the side dart.
- Draw line 3: horizontally across the lower front, from line 1 out to the side seam, below the dart.
- Cut lines 1 and 2, stopping a thread's width short at the armhole and at the dart tip. Those uncut slivers are hinges — they let the pattern pivot without detaching.
- Spread by the calculated amount, keeping the two edges parallel. Tape paper underneath. The side dart opens up as you spread — that is the new bust room.
- Level the hem. The lower section has dropped. Cut line 3 and raise that piece until the hem is straight again, filling the gap with paper.
- True the dart. Redraw both dart legs to a new point that stops 1 to 1 1/2 in short of the apex, then fold the dart closed and cut the side seam while folded so the seam edge is correct once sewn.
- Walk the seams against the back piece. Side seams should match; the armhole should still be smooth.
Common FBA mistakes
Choosing your size by full bust. This is the single most common error. Sizing by full bust gives you a bodice that fits the bust but is far too wide at the shoulders and gapes at the armhole and neckline — and those are much harder to fix than bust width. Size by high bust, then add the bust room back.
Skipping the hem correction. Spreading the pattern drops the lower front. If you do not level it, the finished garment hangs longer in the front than the back.
Putting the dart point on the apex. A dart that reaches the apex produces a cone. Stop it 1 to 1 1/2 in short — further for a larger cup, since the fullness needs to spread over a wider area.
Forgetting to move the apex. Patterns place the apex for a B cup at that size. A fuller bust usually sits slightly lower and wider, so check the apex position on your muslin before committing.
Adding it all in one place. Above roughly 2 in, an FBA concentrated in a single side dart creates a large, obvious dart. Rotate some of it into a waist dart, a French dart, or gathers instead.
After the adjustment
An FBA changes only the front bodice. The back, sleeves, collar, and facings stay as drafted — but a few pieces need to be re-checked against the new front: any facing or lining cut from the front piece must be re-traced from the adjusted pattern, and button placement should be re-spaced with a button positioned at the fullest point of the bust to stop gaping.
Sew a muslin before cutting your fashion fabric. An FBA is a good adjustment but it is rarely the only one — a full bust frequently travels with a narrow upper back, a forward shoulder, or a sway back, and those are far easier to diagnose on a calico test garment than on the finished piece.
Related tools
Same FBA calculation for princess-seam patterns — spread the side front without darts.
Excess fabric at the bust? Calculate the overlap amount for A cup and smaller.
Is your pattern too tight or too loose? Compare your body to the pattern to check ease by fit style.
Enter bust, waist, and hip — find your size across 11 brands with size charts. Sewing sizes ≠ store sizes.