Full Bust Adjustment Calculator
FBA or SBA amount, and which size to start from.
Calculate the full or small bust adjustment a pattern needs from your high bust and full bust, and how much to add to the pattern piece.
Above the bust, under the arms.
Full bust adjustment, on the piece
1.00"
4.00" between high and full bust
Choose the size by high bust, not full bust. That gets the shoulders, neckline and armholes right — which are difficult to alter — and the adjustment then adds the bust room, which is straightforward. Note the adjustment lengthens the front side seam; the extra has to go into a dart or be eased.
How the Full Bust Adjustment Calculator works
Commercial patterns are drafted for a B cup — a two inch difference between the high bust and the full bust. If yours is larger, the pattern is too small across the bust and too large everywhere else, which is why sizing up to fit the bust gives a garment that gapes at the neck and shoulders. The fix is to choose the size by high bust and add the difference back at the bust only.
Also known as: FBA calculator · SBA calculator · bust adjustment sewing calculator
Why sizing up does not work
Commercial patterns are drafted for a B cup — a two inch difference between the high bust, measured above the bust and under the arms, and the full bust. That draft assumption is fixed regardless of the size.
If your difference is larger, choosing the size by full bust gives a pattern that fits at the bust and is too large everywhere else: the shoulders drop, the neckline gapes, the armholes are too deep. Those are the hardest areas of a garment to alter.
The fix is to choose the size by high bust, which gets the frame right, and then add the missing width at the bust only. The amount is your difference less the draft allowance, and half of it goes onto the half-front piece — the fraction that catches people.
A worked example
A high bust of 36 and a full bust of 40 is a four inch difference. Against the pattern's two inch draft, two inches are missing around the body, so one inch is added to the half-front piece.
Cut the size for a 36 inch bust, not a 40. Everything above the bust then fits as drafted, and the adjustment adds exactly the width that was missing where it was missing.
Where the answer misleads
The adjustment lengthens the front side seam as well as widening the piece, and the extra length has to go somewhere — into a bust dart, a deeper waist dart, or eased into the back. Ignoring it leaves the front hem hanging lower than the back, which is the usual sign of an FBA done halfway.
The amount also depends on where the fullness sits, not just how much there is. Two people with the same measurements and different bust shapes need the same total adjustment distributed differently, which is why a toile is worth making.
Finally, some patterns are drafted for a C or D cup and say so. Using the standard two inch assumption on those over-adjusts, so check the pattern's own stated draft before calculating.
Frequently asked questions
How do I know if I need a full bust adjustment?
Measure the high bust, above the bust and under the arms, and the full bust at the fullest point. If the difference is more than about two inches, the pattern needs an FBA; less than that, a small bust adjustment.
How much do I add?
The difference between your measurements, less the pattern's two inch draft allowance, gives the amount needed around the body. Half of that goes onto the half-front pattern piece — which is the step people most often skip.
Which size should I cut?
The size matching your high bust, not your full bust. That gets the shoulders, neckline and armholes right, which are difficult to alter, and the FBA then adds the bust room, which is straightforward.
Does an FBA change the side seam length?
Yes, it lengthens the front side seam, and the extra has to go somewhere — usually into a dart, a slightly deeper waist dart, or eased into the back. Ignoring it leaves the front hem hanging lower than the back.