Full / Small Bust Adjustment
Turn high-bust vs full-bust into a pattern change.
Snug, under the arms and above the nipples.
Loosest, across the fullest point.
2 in add
Estimated cup size D against a B-cup base.
- High to full gap
- 4 in
- Total to change
- 2 in
- Per side seam
- 1 in
Draft from the high bust, then split the total change across the two side seams and a torso slash so the shoulder and armhole stay put. Add length from the apex when the cup is deep, since width alone will not clear a full cup.
About This Tool
The Full Bust and Small Bust Adjustment calculator turns the gap between your high bust and full bust into the exact inches to add or remove from a pattern, so the bodice fits the chest instead of the model it was drafted for.
You enter a snug high bust and a full bust at the widest point.
It subtracts the two-inch B-cup drafting base and shows the total change plus the amount per side seam.
The answer ends the diagonal pull lines and gaping armholes that plague a too-small bodice.
Whether you are sewing a fitted shell or a button-front shirt, one number tells you how far to slash.
It also estimates your cup letter so you know how much of the change is depth rather than width.
Learning this one adjustment makes a shelf of store patterns suddenly wearable.
Why Full / Small Bust Adjustment Math Matters
Standard patterns are drafted on a B cup, so any fuller figure runs short across the front.
Choosing a size by the full bust alone makes the shoulders and armholes swallow you.
A correct FBA adds room only at the bust while leaving the frame that already fits untouched.
Without it, the button placket springs open and the hem swings up over a short front.
The reverse, a small bust adjustment, removes pooling fabric that no amount of pinning will tame.
Splitting the change across side seams and a torso slash keeps the shoulder seam in place.
Learning this one adjustment unlocks an entire shelf of patterns that never seemed to fit.
A bodice fitted this way sits flat and hangs straight instead of pulling toward the floor.
The Formula
The total adjustment equals the full bust minus the high bust, less the two-inch B-cup base the pattern already holds, and that total is divided by two to give the extra to add at each side of the bodice.
- High bust —
- the snug measure above the bust that reflects your true frame size
- Full bust —
- the loosest measure across the fullest point of the chest
- B-cup base —
- the roughly two-inch difference the standard pattern block already accounts for
A positive result means a full bust adjustment to add fabric, a negative result means a small bust adjustment to remove it, and the gap also maps to an approximate cup letter.
Worked Example: Step by Step
- 1Your high bust measures 34 and your full bust measures 40 inches.
- 2The gap between them is six inches, well beyond the two-inch B-cup base.
- 3Subtracting that base leaves four inches of extra fullness the pattern does not hold.
- 4The tool divides the four by two and returns two inches to add at each side.
- 5It estimates your cup near a DD from the six-inch difference.
- 6You slash the pattern from shoulder to hem and spread the front by the measured amount.
- 7The finished bodice now lies flat at the bust without straining the shoulders or armhole.
- 8Because you kept the frame size, the neckline and sleeves still sit where they should.
How to Use This Calculator
- Measure the high bust snugly with the tape parallel to the floor.
- Measure the full bust loosely across the fullest point.
- Enter both numbers and read the total adjustment.
- Note the per-side figure, which is what you slash and spread.
- Choose your base pattern size from the high bust, not the full.
- Add length from the apex if the cup is deep as well as wide.
Key Terms
- FBA
- a full bust adjustment, the method of adding room to a bodice for a cup larger than the drafting base
- SBA
- a small bust adjustment, the reverse change that removes excess fabric for a flatter chest
- High bust
- the snug measurement above the bust used to pick a frame size
- Apex
- the fullest point of the bust where the pattern is usually slashed
- Slash and spread
- cutting the pattern along marked lines and opening it to add fullness
- B-cup base
- the roughly two-inch bust-to-frame difference standard blocks are drafted to assume
- Hinge
- the thin uncut strip that keeps slashed pattern pieces aligned as they open
Full / Small Bust Adjustment Reference Table
| High-to-full gap | Approx cup | Typical change |
|---|---|---|
| 1 in | A | Slightly small |
| 2 in | B | No adjustment |
| 3 in | C | Add 1 in total |
| 4 in | D | Add 2 in total |
| 6 in | DD or E | Add 4 in total |
Cup letters are rough guides because bra sizing and pattern drafting measure them differently, so trust the inch result over the letter, and if you sit between rows it is wiser to add slightly less and refine it on a cheap muslin shell before cutting the good fabric.
Common Mistakes & Pro Tips
- Picking the pattern size from the full bust and then getting a bodice far too big in the shoulders.
- Adding the entire adjustment at one side seam, which shifts the center front off the body.
- Adding width but forgetting length, so a deep cup still pulls the hem up at the front.
- Slashing all the way to the edge and losing the hinge that keeps the pieces aligned.
- Skipping the muslin and cutting expensive fabric straight from an untested adjusted pattern.
- Confusing bra cup letters with pattern cups and expecting one number to serve both.
- Spreading the bust but never truing the armhole, leaving a notch that no longer matches.
Real-World Use Cases
- Fitting a woven button-front shirt across a full bust.
- Adjusting a favorite dress block that always gaps at the armhole.
- Turning a too-large pattern down for a small bust.
- Drafting a costume bodice to match real body measurements.
- Fixing a jacket that pulls horizontally over the chest.
- Teaching the slash-and-spread method on a practice tissue.
Full / Small Bust Adjustment: Frequently Asked Questions
Why size by the high bust instead of the full bust?
The high bust tracks shoulder, ribcage and armhole better than the full bust, which soft tissue inflates. Drafting from it keeps the frame right, and the FBA adds room only across the chest.
How do I split the total between the two sides?
Divide the total adjustment by two, since a bodice has a left and right half each needing the same addition. Equal halves keep the center front on the fold and the garment hanging straight.
Do I ever need to add length too?
Yes, when the cup is deep as well as wide. Extra projection pushes fabric outward, so a spread that only widens still rides up at the front and needs a short length from the apex.
What is the two-inch base doing in the formula?
It represents the fullness the standard block already assumes for a B cup, so subtracting it prevents adding room the pattern has built in. Ignoring it means over-spreading a double-counted allowance.
Can I do this adjustment on a finished garment?
A true FBA belongs on pattern tissue, not a sewn piece, because darts, armhole and side seam must be redrawn together. On a finished garment you can only let out a seam, so adjust the pattern.
This page explains how the full / small bust adjustment works, what each input means and where the numbers come from, so you can plan your next make with confidence.
Written by Bobbin & Form Editorial Team
Home sewists & machine-embroidery practitioners
Reviewed by Marie Delacroix
Custom sewist & machine-embroidery technician, 15 years at the machine
Formulas follow standard sewing and needlework references. Results are estimates — always confirm against your pattern and a pre-washed swatch.
Sources & further reading
- Threads Magazine — fabric cutting & yardage guides
- ISO 13934 — textile tensile & fabric property standards
- Your local fabric store's bolt-width charts