Why Commercial Patterns Don't Fit
Commercial patterns are drafted to a B-cup assumption — which means any bust larger than a B requires a Full Bust Adjustment to produce a garment that fits across the chest without pulling, gaping at the neckline, or lifting at the hem. Similarly, patterns drafted for one hip-to-waist ratio will not fit a different one without adjustment.
The Full Bust Adjustment (FBA)
An FBA adds width and length to the bodice front at the bust, without changing the shoulder, armhole, or side seam above the bust. The basic process: slash the bodice front pattern piece from the bust point to the hem; slash from the bust point to (but not through) the armhole; open the slash at the hem to add the required width; true the new seamlines; re-draw the dart to accommodate the added ease. This sounds technical but is learnable in a single sewing lesson.
Grading Between Sizes
When your measurements fall in different size ranges at different points (e.g., a size 18 hip but a size 14 waist), grade between sizes: use the larger size at the hip, blend smoothly through the waist to the smaller size above. Commercial patterns designed for broader size ranges provide grading lines; otherwise, draw your own blend lines between the size cutting lines.