What blending is
A multi-size pattern prints every size in the range as a set of nested cutting lines. Blending means starting on one line and moving to another, gradually, along a seam — so the piece is a 16 at the bust, a 12 at the waist and a 14 at the hip, with a smooth curve between each.
It is the same operation a pattern grader performs, done by hand for one body. There is nothing advanced about it. The skill is in deciding where the transitions belong and knowing which parts of the pattern must not be dragged along with them.
Pick the starting size from the upper body
The single most useful decision comes before any drawing. Choose the size for the bodice from your high bust measurement rather than your full bust, then treat any remaining bust width as a full bust adjustment.
The reason is that the shoulder, the armhole and the neckline are graded from the same size number as the bust. Choose the size by full bust and you get an upper body drafted for someone broader than you, which shows up as a gaping neckline and a shoulder seam sitting halfway down the arm. Those cannot be blended away; they have to not happen.
Once the upper body size is settled, the waist and hip are blends.
The mechanics
- Trace, do not cut. Work on tracing paper over the printed pattern so the original stays intact and the second version is easy.
- Mark the anchor points on the seam you are blending — the bust line, the waist and the full hip — each on the size you want at that point.
- Join them with a smooth line. A french curve helps; a steady hand and a couple of attempts in pencil will do. The transition should run over several inches, not turn a corner.
- Do the same on the matching piece. A bodice front blended at the side seam needs the bodice back blended identically.
- Check the seam lengths agree. Walk the front side seam against the back side seam with a tape measure or by pivoting the pieces. If they no longer match, the blend curve is not the same on both.
- True everything that crosses the blend — notches, the seam allowance width, and the hem edge, which must still meet the side seam at a right angle.
A worked example. Your measurements put you at a 16 bust, a 12 waist and a 14 hip on a chart, and your high bust puts the bodice at a 14.
Cut the bodice as a 14 through the shoulder, neckline and armhole. Add a full bust adjustment to bring the front bust up to what you need. From the bust line down the side seam, blend from the 14 line to the 12 at the waist. On the skirt, start at the 12 at the waist and blend out to the 14 by the full hip.
Three sizes appear on one garment and no single line is a compromise. That is the entire point of a multi-size pattern.
Where blending goes, and where it does not
Blend at the side seams. That is where the grading between sizes is mostly expressed, and it is the least structural place to change width.
Do not blend across the shoulder, armhole or neckline. Those are shaped curves whose relationship to each other matters. Move between sizes there and the armhole no longer matches its sleeve, and the neckline no longer matches its facing or collar.
Take care around darts. Blending past a dart moves the side seam relative to the dart legs. After a blend, check that the dart legs are still equal in length and that the dart still points where it did — closing the dart on paper and re-truing the edge is the quick way to confirm it.
Watch pockets, plackets and waistbands. Anything drafted to fit a particular seam has to be adjusted to the blended length rather than cut at its printed size. A waistband cut at a 12 will not fit a skirt blended out to a 14 at the hip if the waist opening changed.
When the gap is more than two sizes
Blending across three or four sizes is possible and needs a longer transition — spread it over as much vertical distance as the seam allows, and check the resulting curve looks like a body rather than a step. Where the difference is very large, it is often the sign of something the side seam cannot solve on its own: a full bust adjustment, a full or flat seat adjustment, or a waist that sits at a different height than the pattern assumes.
Sew a muslin at that point. Blending four sizes on paper is quick; discovering the result in good fabric is not.
Brands where the usual rules change
Patterns with printed cup sizes. Cashmerette prints a separate bust cutting line for each of three cup options, so there is nothing to slash and spread — you choose the printed line for your cup and then blend to the waist and hip sizes as normal.
Brands with two separate blocks. Closet Core drafts one block for sizes 0–20 and another for 14–32, and they are not a continuous grade: you cannot blend from one to the other, and a 14 in one is not a 14 in the other. Sew House Seven's Standard and Curvy ranges overlap at sizes 16–20 with genuinely different numbers, so check which range your pattern belongs to before deciding what to blend between.
The calculator reads your measurements against each of those charts separately for exactly this reason, and draws the blend for the chart your pattern actually uses.
General guidance following standard home-sewing pattern alteration practice. Brand specifics are taken from each company's own published size charts, reproduced in this site's calculator and linked to their source. Where a pattern's own instructions differ, follow theirs. Not professional fitting advice.