FBA/SBA Bust Adjustment Calculator
Calculate a full bust or small bust adjustment amount from your high bust and full bust measurements.
Reviewed by the ToolNestr Editorial Team — July 2026
How the FBA/SBA amount is calculated
Most commercial patterns are drafted assuming roughly a 1-inch difference between high bust and full bust, corresponding to a "B cup" proportion. This calculator subtracts high bust from full bust to find your actual difference, then subtracts that standard 1-inch assumption — if the result is positive, that's your full bust adjustment (FBA) amount; if negative, that's your small bust adjustment (SBA) amount (as a positive number to remove).
Because a bodice front is typically cut as two mirrored halves (left and right), the total adjustment amount is generally split evenly between both sides, so the per-side adjustment applied to each pattern half via the standard pivot-and-slide method is roughly half the total calculated amount.
The 1-inch "B cup" assumption reflects the standard convention referenced across pattern-fitting and dressmaking adjustment guides; this calculator estimates the adjustment amount, not the full pattern-alteration technique itself.
Worked example
A high bust of 34 inches and a full bust of 38 inches.
Real-world context
Sewists whose full bust and high bust measurements point to two different pattern sizes run into this constantly — sizing to the full bust measurement alone often results in a bodice that fits well at the bust but is oversized at the shoulders and neckline, which is exactly the mismatch an FBA is designed to fix without sizing up the whole garment.
A well-executed FBA or SBA is one of the most commonly cited "fit game changer" adjustments in home sewing, since it directly addresses a mismatch between commercial pattern assumptions and individual variation that off-the-rack clothing sizing simply doesn't accommodate at all.
Common misconceptions
"An FBA amount should be added entirely to one side of the pattern." It's typically split. Since the bodice front is symmetric, the total adjustment is usually divided between the two mirrored pattern halves, not applied entirely to a single piece.
"Every pattern assumes the same bust size relationship." Mostly consistent, but check. The 1-inch B-cup assumption is a common commercial-pattern default, but some pattern companies, especially those drafting for a wider size range, may assume a different base cup size — check the pattern's own fitting notes when available.
Related tools
Frequently asked questions
What is high bust versus full bust?
High bust is measured across the chest, above the bust and under the arms, close to the underarm crease. Full bust is measured around the fullest part of the bust. The difference between the two is what pattern sizing is typically based on.
Why do pattern companies assume a 1-inch difference?
Most commercial patterns are drafted assuming a "B cup" proportion, which corresponds to roughly a 1-inch difference between high bust and full bust — if your actual difference is larger or smaller than that, the pattern's bust area will not match your shape without adjustment.
What do I do with the calculated adjustment amount?
For an FBA, that amount is typically added by pivoting and slashing the front bodice pattern piece to add width and length at the bust point, split between the two pattern halves. For an SBA, fabric is instead removed using a similar pivot method. Follow a dedicated FBA/SBA tutorial for the exact pattern-alteration technique.
Should I size my pattern using high bust or full bust?
Many fitting guides recommend choosing your pattern size based on high bust measurement, then adding an FBA if needed, rather than sizing up based on full bust — sizing up by full bust alone can result in an oversized fit everywhere except the bust.