To grade between sizes, choose the size for each part of the body from the measurement that governs it (a size 40 bust, 42 waist and 44 hip, for example), then draw a new cutting line that leaves one size's line and meets the next one in a smooth curve over at least 10 to 15 cm (4 to 6 in), usually between the bust and the waist and between the waist and the hip. The neckline, shoulders and armholes stay on the size chosen for the bust, the sleeve follows that same size, and length is adjusted separately. Do it on every piece that shares the seam, front and back, so the seams still match.
What does grading between sizes mean?
Most PDF and paper patterns are printed with nested sizes: all the cutting lines, one inside the other, on the same sheet. Blending between them means following one line at the top of the piece and another lower down, with your own line drawn between. People also call it "blending" or "grading out"; it is not the same as pattern grading, which is how a designer creates the whole size range from one block.
It works because the sizes differ by a fixed step at each seam, generally 1 to 1.5 cm (3/8 to 5/8 in) per size at the side seam. Moving from one line to the next adds or removes that step on each side of the body, so one size up at the hip adds about 4 to 6 cm (1 5/8 to 2 3/8 in) in total around the hip, since two side seams each have two cut edges.
Which measurement chooses which size?
Compare your measurements with the pattern's body chart, not the finished garment chart, and pick a size for each area. The size guide explains how to read the chart; if your numbers fall between sizes, take the larger for a woven fabric and the smaller for a knit.
| Body measurement | Decides the size of | Pieces affected |
|---|---|---|
| High bust, or bust if the difference is under 5 cm (2 in) | Neckline, shoulders, armholes, upper chest | Bodice front and back above the bust, facings, collar |
| Bust | Bodice at bust level, bust dart position | Bodice front and back, side seam at the underarm |
| Waist | Bodice at the waist seam, skirt and trousers at the top | Bodice lower edge, skirt and trousers waist, waistband length |
| Hip | Skirt and trousers at hip level, lower bodice of a long top or dress | Skirt and trouser front and back, side seams, pockets |
| Upper arm | Sleeve width | Sleeve, cuff |
The sleeve is cut in the size of the armhole it goes into, which is the bust or high bust size. If your upper arm needs a bigger sleeve than that, a full bicep adjustment adds width without changing the sleeve cap.
How do you draw the transition between two sizes?
- Trace or print the sizes you need. On a layered PDF, turn on only those two or three sizes so the sheet is readable. Print at 100 % and check the test square.
- Mark the governing points. Put a dot on the larger-size line at hip level, a dot on the smaller-size line at bust level, and so on, for each measurement.
- Draw the join. Starting on the upper size's cutting line, draw a line that curves gently to reach the lower size's line, spread over at least 10 to 15 cm. A short, abrupt step makes a visible jog in the seam and a kink in the finished garment. A French curve or hip curve helps, but a careful freehand line is fine.
- Keep the ends. Above the first dot and below the last dot, follow the chosen size's line exactly, including notches and the hem.
- Repeat on every piece that meets at the seam. A front side seam blended from 40 to 44 needs the back side seam blended the same way between the same levels, or the two seams will not be the same length.
- Check the seam lengths. Walk the front seam along the back seam with the paper on edge, or measure both with a tape. They should match within 2 to 3 mm (1/8 in).
- Trace facings, waistbands and pockets from the new lines. A waistband is cut to the new waist length; a hip pocket piece is blended like the side seam it sits in.
Tip: Mark the new line in a bright color and write the sizes it joins on the piece. In six months you will want to know what you did.
How do you grade a bodice between the bust and the waist?
The bust size sets everything above the underarm. From the underarm point on the bust size's line, draw the side seam so it reaches the waist size's line at the waist. On a bodice with a side bust dart, keep the dart of the bust size and let the new side seam pass through the dart legs; when you fold the dart closed the seam should be straight, so redraw the dart legs' cut edge through the fold.
Waist darts are a second place the sizes differ: a larger waist size has narrower waist darts. Use the dart of the waist size, and keep its point at the height of the bust size's dart. If your waist is two or more sizes larger than your bust, take some of the difference out of the darts and some at the side seam, rather than all at one place.
If a full bust adjustment is needed as well, do it first on the bust size, then blend.
How do you grade a skirt or trousers between the waist and the hip?
Mark the waist size at the top edge and the hip size at the hip line, which is usually 18 to 22 cm (7 to 8 3/4 in) below the waist and often marked on the pattern. Draw the side seam from one to the other, then follow the hip size straight down to the hem. On trousers do the same on the inseam side of the back piece if the sizes differ there, and check that the front and back crotch curves still meet at the same inseam length. The crotch depth belongs to the hip size, so if the waist is the smaller size, the crotch length is taken from the hip size and the waist is shaped with deeper darts or a slightly curved side seam.
The waistband is cut to the waist size plus its overlap, and the facing or yoke is traced from the blended skirt. A pencil skirt tends to show a short transition more than a flared one; give it the full 15 cm.
What mistakes do people make when blending sizes?
- Blending only one piece. The front and back of a seam must be blended between the same levels; otherwise the seam puckers or one piece has to be stretched.
- Changing the armhole. If the bust size is right, leave the armhole alone. A bigger armhole needs a bigger sleeve, and the sleeve cap ease goes wrong.
- A transition that is too short. Spread it over 10 to 15 cm. The jog from a 5 cm blend shows as a bump on the hip.
- Forgetting the length. Larger sizes are often drafted longer. If you blend to a larger size at the hip, check the back waist length and the skirt length against yours and use the "lengthen or shorten" lines rather than letting the size do it.
- Blending the seam allowance line and the cutting line differently. If the pattern shows both, draw the new seam line first and add the allowance parallel to it.
When is blending between sizes not enough?
Blending copes with a difference of one or two sizes between adjacent areas. Three sizes or more, say a 38 bust with a 46 hip, makes a side seam that swings out sharply and a waist dart that no longer suits the body. At that point it is better to treat the areas separately: use a full bust adjustment instead of a bigger bust size, or draft the bodice and skirt as two garments and join them.
A pattern drafted to your own measurements removes the problem. Sewly draws each piece from your bust, waist and hip, so there is no size to blend; if you start from a standard size, replacing one measurement with your own redraws the pattern, and the custom-fit page explains what is drafted and what still needs a toile. If you are checking a bought pattern, the size converter lines up EU, US and UK sizes so you compare the right columns.