The B cup assumption
When a pattern company drafts a size 16, it does not draft it for a person. It drafts it for a set of proportions: a bust, waist and hip in a fixed relationship, on a body of a stated height, with a bust volume corresponding to roughly a B cup. Every size in the range is that same proportional figure scaled up and down — which is what grading is.
The assumption is not laziness. A printed pattern can only carry one set of lines, and the company has to choose a body to draft them for. What it means in practice, though, is that the bust measurement printed on the envelope is doing two jobs at once: describing how wide your body is across the chest, and describing how much bust tissue you have. On a B cup those two things move together. On anyone else they come apart, and the chart cannot tell the difference.
The "cup" here is not your bra cup letter, and this trips people up constantly. Bra cups are the difference between your under-bust and your bust, measured on a band that changes with the band size — a 32D and a 40D are not the same volume of bust. The pattern cup is the difference between your high bust and your full bust. Someone who buys a DD bra can easily need no adjustment at all, and someone who buys a C can need a large one. Use the measurements, not the label in the bra.
What goes wrong without one
Suppose your high bust is 36 and your full bust is 40. Choose your size from the 40 and you get a pattern drafted for someone whose entire frame is four inches wider than yours. The bust will fit. Almost nothing else will.
- The shoulder seam falls off the edge of your shoulder, so the sleeve head sits on your upper arm and the whole garment hangs from the wrong point.
- The neckline gapes, because it was drafted for a wider chest.
- The armhole is too deep and too wide, which is what makes a bodice feel like it is wearing you.
- The back is loose and blousy while the front is still pulling.
- Everything rides up at the centre front, because the fabric that should be going over the bust is being borrowed from the length.
Choose your size from the high bust instead and the shoulders, neck, armhole and back all land where they should — and now the only problem left is that the front is too small over the bust. That is a solvable problem, and solving it is the full bust adjustment.
The arithmetic
Take full bust minus high bust. That difference is the whole diagnosis.
- Under 2 inches. You are close enough to the drafted cup. Cut your size from the full bust and get on with it — no adjustment needed.
- 2 to 3 inches. Borderline. Many people are fine here, particularly in a loose or semi-fitted style, but on a close-fitted bodice or a princess-seamed dress a small adjustment of half to one inch will improve it. This is the band where a muslin earns its keep.
- Over 3 inches. An adjustment is genuinely needed on any fitted garment. Choose your size by the high bust, then add roughly (difference − 2) inches to the front.
The reason for subtracting 2 is simply that the pattern already contains about two inches of bust difference — that is the B cup it was drafted with. You are only adding the amount by which you exceed what is already there.
Worked example. High bust 36. Full bust 40. Difference: 4 inches.
Four is over three, so an adjustment is needed. The amount is 4 − 2 = 2 inches total across the full bust, which is 1 inch added to each front piece — patterns are cut on the half, so a two-inch full-bust increase is a one-inch adjustment on the piece in your hand.
Choose the size whose chart bust is closest to your high bust of 36. On the Big 4 misses chart that is size 14 (bust 36). Cut the 14 at the shoulders, neck and armhole, apply a one-inch FBA to the front, and blend out to your own waist and hip sizes below the bust. The finished front now measures what a 40 would have, but on a frame that is a 14.
What the cut actually does
An FBA is not "make the front bigger". If you simply widened the piece you would add width at the shoulder and the armhole, which is the problem you were escaping. The adjustment adds fabric only where the bust is, and it does it by pivoting the pattern around fixed points.
On a dart-fitted bodice front the classic method runs three cuts. One goes from the hem up through the bust point and stops just short of the armhole, leaving a paper hinge. A second goes from the bust point out into the side bust dart, again hinged. A third crosses the piece horizontally below the bust. Spreading the first cut opens the piece by the amount you calculated; because it hinges at the armhole, the armhole itself does not change size. The spread automatically enlarges the bust dart, which is correct — a larger bust needs a larger dart to shape around. The horizontal cut drops the lower front slightly, which restores the length that a fuller bust consumes going over the curve, and it is why an FBA usually leaves the centre front longer than the side.
The two things to notice are that the armhole and shoulder do not grow, and that the centre front does not move. Those are the fixed points. Everything else pivots around them.
On a princess seam the method is different in execution and identical in intent: the seam allowance on the side front is widened over the bust curve and blended back to nothing above and below, adding volume exactly where the curve is. On a knit top with no darts you can often skip the whole thing, because the fabric supplies the shaping — which is one reason knits feel so much more forgiving than wovens.
Brands that draft more than one cup
Some pattern companies solve this at the drafting stage instead of leaving it to you. They publish separate bust measurements for two or three cup options within each size, and print separate cutting lines for the bust area on the pattern itself. Cashmerette drafts C/D, E/F and G/H options; Closet Core publishes a C/D-cup block alongside its standard range.
The workflow with those brands is different, and simpler. You choose your size from the waist and hip — not from the high bust — and then pick the cup whose published full bust is nearest your own. There is nothing to slash and spread, because the adjustment has already been drafted for you. A large high-bust-to-full-bust difference on a multi-cup brand is not a warning; it is just the instruction to pick a bigger cup letter.
This is worth knowing before you buy. If your difference is four or five inches, a multi-cup pattern removes an entire class of work from every project, and the calculator flags which brands offer it as soon as your measurements say you would benefit.
The small bust adjustment
The same logic runs backwards. If your full bust is less than about an inch and a half above your high bust, the pattern's drafted B cup is more bust than you have, and a fitted bodice will bag or wrinkle over the front. The fix is the same three cuts run in reverse — overlap rather than spread — removing (2 − difference) inches. It is a rarer conversation online, but it is exactly as real, and gaping necklines on otherwise well-fitting bodices are very often this rather than a neckline problem.
Where to start
Take both measurements properly first — a high bust taken too low is the most common way to arrive at a wrong answer here, and the measuring guide covers where it sits. Then run them through the calculator, which gives you the difference, the verdict, the size to cut from the high bust and the blend to your waist and hip in one pass.
Then cut a muslin. The FBA is a starting position, not a guarantee: bust apex height, how much of the difference sits at the side versus the front, and the specific style all move the answer. But it moves you from a garment that is wrong everywhere to one that is close everywhere, and that is the difference between a fitting problem you can see and one you cannot.
General guidance following standard home-sewing fitting practice. The 2-inch B-cup baseline and the (difference − 2) rule are the conventional starting points taught for commercial patterns; individual patterns and bodies vary, and a muslin decides. Not professional fitting advice.