Full Bust Adjustment (FBA) Calculator
Choosing a size by high bust leaves the pattern's bust smaller than yours. At a 104 cm full bust against a 92 cm pattern bust, the gap is 12 cm. But a front cut on the fold is a quarter of the body, so the spread on the piece is 12 ÷ 4 = 3 cm, and the dart intake grows by the same 3 cm. Spread the full 12 cm and the garment comes out four times too big.
Your numbers
Spread per piece
3cm
Total difference
12cm
Formula
Total difference = Full bust − Pattern bust, Spread per piece = Total difference ÷ 4
The pattern bust has to be the body measurement from the size chart, not the finished bust — the finished figure already contains design ease, so using it either erases the adjustment or leaves the top swimming. A negative gap is not an FBA but its opposite, an SBA, folded out by the same amount. Under about 2.5 cm most sewists skip the adjustment entirely; pressing and wearing ease absorb that much. On a pattern whose front is two pieces rather than cut on the fold — a princess line or a centre-front seam — each piece is half the front, so 12 ÷ 2 = 6 cm is distributed across it.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Full bust (cm)The circumference measured round the fullest part of the bust. | 104 | 40 and up |
| Pattern bust (cm)The body bust listed for that size in the chart, not the finished measurement. | 92 | 40 and up |
Step by step
Quick reference table
Results when only Full bust (cm) changes and everything else stays put.
| Full bust (cm) | Spread per piece (cm) | Total difference (cm) |
|---|---|---|
| 52 | -10 | -40 |
| 78 | -3.5 | -14 |
| 104 | 3 | 12 |
| 156 | 16 | 64 |
| 208 | 29 | 116 |
What each result means
| Result | At default values |
|---|---|
| Spread per piece (cm)What you actually spread on the pattern piece — a quarter for a front cut on the fold. | 3 |
| Total difference (cm)The circumference gap between body and pattern, before it is divided up. | 12 |
Common mistakes
The pattern bust has to be the body measurement from the size chart, not the finished bust — the finished figure already contains design ease, so using it either erases the adjustment or leaves the top swimming. A negative gap is not an FBA but its opposite, an SBA, folded out by the same amount. Under about 2.5 cm most sewists skip the adjustment entirely; pressing and wearing ease absorb that much. On a pattern whose front is two pieces rather than cut on the fold — a princess line or a centre-front seam — each piece is half the front, so 12 ÷ 2 = 6 cm is distributed across it.
Glossary
- Full bust
- The circumference measured round the fullest part of the bust.
- Pattern bust
- The body bust listed for that size in the chart, not the finished measurement.
- Spread per piece
- What you actually spread on the pattern piece — a quarter for a front cut on the fold.
- Total difference
- The circumference gap between body and pattern, before it is divided up.
Frequently asked questions
Q. How is Full Bust Adjustment (FBA) Calculator calculated?
Total difference = Full bust − Pattern bust, Spread per piece = Total difference ÷ 4 — Choosing a size by high bust leaves the pattern's bust smaller than yours. At a 104 cm full bust against a 92 cm pattern bust, the gap is 12 cm. But a front cut on the fold is a quarter of the body, so the spread on the piece is 12 ÷ 4 = 3 cm, and the dart intake grows by the same 3 cm. Spread the full 12 cm and the garment comes out four times too big.
Q. Can you walk through an example?
With Full bust 104cm, Pattern bust 92cm, the answer is Spread per piece 3cm.
Q. What do I need to enter?
Enter Full bust, Pattern bust. The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Full bust (cm) moves the answer to Full bust (cm) 52 → Spread per piece (cm) -10 and Full bust (cm) 208 → Spread per piece (cm) 29. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
The pattern bust has to be the body measurement from the size chart, not the finished bust — the finished figure already contains design ease, so using it either erases the adjustment or leaves the top swimming. A negative gap is not an FBA but its opposite, an SBA, folded out by the same amount. Under about 2.5 cm most sewists skip the adjustment entirely; pressing and wearing ease absorb that much. On a pattern whose front is two pieces rather than cut on the fold — a princess line or a centre-front seam — each piece is half the front, so 12 ÷ 2 = 6 cm is distributed across it.
Related calculators
Materials differ in density and absorption. If your label states a figure, use it — the defaults here are only common values.