Full Bust & Small Bust Adjustments: The Guide Most Patterns Skip
If you size up just to fit your chest — and then the shoulders and neck gape — you don't need a bigger size. You need a bust adjustment. Here's how they work.
The fitting problem nobody warns you about
You cut your usual size, sew the bodice, try it on — and it's tight across the bust, but the shoulders are fine. So next time you size up. Now the bust fits, but the shoulders drop, the neckline gapes, and the armholes are cavernous.
You don't need a different size. You need a bust adjustment.
Most commercial patterns are drafted for a B cup — specifically, about a 2.5-inch difference between the high bust and full bust. If your difference is bigger, the pattern doesn't have enough room at the bust (you need a full bust adjustment, or FBA). If it's smaller, the pattern has too much (you need a small bust adjustment, or SBA). Cup size in bra terms isn't the point — the high-bust-to-full-bust difference is.
Step 1: Take the two measurements that decide it
- High bust — around your chest, above the bust, high under the arms.
- Full bust — across the fullest part.
Subtract. The difference tells you what you need:
- Under ~1 inch: small bust adjustment (SBA).
- About 1–2.5 inches: likely no adjustment (you match the B-cup draft).
- Over ~2.5 inches: full bust adjustment (FBA), scaling up the more the difference grows.
Store both numbers in your measurements profile so you're not re-measuring every time you start a fitted pattern.
Step 2: Choose your size by high bust
This is the key move. For any pattern with a bust dart or princess seam, pick your size by your high bust measurement, not your full bust. That gives you shoulders, neckline, and armholes that fit your frame. Then you add the bust room you need with an FBA — without enlarging everything else.
(For garments with no shaping — a boxy tee, a relaxed knit — bust adjustments matter far less, and you can usually just go by full bust. For how sizing works in general, see what size sewing pattern should I cut.)
Step 3: The full bust adjustment, in plain steps
The classic FBA on a darted bodice front:
1. Mark the bust apex (the fullest point) on the pattern.
2. Draw three slash lines: from the hem up through the apex, from the apex to the armhole, and from the apex into the side dart.
3. Slash and spread the front piece along those lines to add width and length at the bust — roughly half your high-bust-to-full-bust difference per side.
4. The dart grows as you spread. That's expected — a bigger bust needs a bigger dart to shape around it.
5. True the lines and re-draw the dart, then add length back to the hem so the front isn't shorter than the back.
A small bust adjustment is the same process in reverse: overlap instead of spread, which shrinks the dart.
Step 4: Always muslin a bust adjustment
An FBA changes the dart position and the side seam length. Sew a quick muslin (test garment) in cheap fabric to check the apex lands where your bust actually is and the dart points toward — not at — the fullest part. Adjust, then cut your real fabric.
Step 5: Save the fix so you never re-solve it
Here's the compounding win: once you've dialed in your FBA on a company's bodice block, it usually transfers to their other patterns with the same block — and a similar amount works across many B-cup-drafted patterns.
In your pattern library, record the adjustment on each pattern: "1-inch FBA, lowered apex 0.5 inch." Log the make in your project journal with a photo of the fitted muslin. The next time you reach for that pattern, you apply a known fix in minutes instead of re-drafting from scratch.
2. Save your high bust and full bust in measurements.
3. Record the FBA/SBA amount on each fitted pattern.
4. Photograph the fitted muslin in the project card.
5. Reuse the fix on the next make and on the same block's other patterns.
Frequently asked questions
How do I know if I need an FBA?
If your full bust exceeds your high bust by more than ~2.5 inches, yes. Under ~1 inch, consider an SBA.
FBA vs. sizing up?
Sizing up enlarges everything and makes shoulders/neck gape. An FBA adds room only at the bust.
High bust or full bust for size?
Choose by high bust for darted/princess styles, then FBA.
How much does an FBA add?
Roughly half your high-bust-to-full-bust difference per side. Always muslin.
Can I reuse a bust adjustment?
Yes, often across the same company's blocks. Save the amount per pattern.
