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 princess seam FBA works
Princess seam patterns have no bust darts — instead, the front is split into a center front panel and a side front panel joined by a curved seam that passes through the bust apex. This seam does the work of shaping; a larger bust needs more width and curve at that seam.
The princess FBA uses the same adjustment amount as a darted FBA but applies it differently: the side front is slashed and spread to add width, and the center front curve is deepened to match. Both changes are centered at the bust point.
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 princess seam FBA, step by step
- Trace both front pieces — center front and side front — in your high bust size, and mark the apex on the side front where the seam curve is deepest.
- Draw a vertical line on the side front, parallel to the grainline, running from the hem up through the apex to the armhole.
- Draw a horizontal line from that vertical line across to the princess seam at apex level.
- Slash and spread the vertical line by the calculated amount, hinging at the armhole. Tape paper behind the opening.
- Add length at the horizontal line if you need vertical room as well as width — a larger bust projects forward, which eats length as well as width.
- Deepen the center front curve to match, so the two seam lines still describe the same curve. Blend smoothly above and below the apex rather than creating a corner.
- Walk the princess seam — measure the center front seam against the side front seam. The side front is now slightly longer through the bust; that extra is deliberate ease and gets eased in when sewing, not trimmed away.
- Level the hem and true the side seam against the back piece.
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
Pattern pulls across the bust? Calculate the exact slash-and-spread amount for darted bodices.
Overlap the side front and flatten the CF curve for smaller busts on princess-seam patterns.
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.