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From the pages of Threads magazine Making Sense of Pattern Grading If you want to use a too-small (or too-big) pattern, you need to know the way garments differ in proportion from one size to another by Terry Horlamus
I"ll explain the thought process behind grading and touch upon the three different methods used in the industry today. Then I'll take you step-by-step through a simple cut-and-spread grading method and provide the measurements and formulas you'll need to get started. So whether you want to take a beloved pattern up (or down) a size or two or you want to create a small line of clothing, pattern grading is an important tool. Once you understand it, you'll be able to take your sewing to the next level.
Historically, the science of grading went hand-in-hand with the advent of commercial patterns and the mass-production of pattern-built clothing some 150 years ago. To properly fit a pattern to a range of sizes, each pattern piece needed to be graded, or systematically increased or decreased. Today, pattern companies and apparel manufacturers take a middle-sized pattern (typically a size 12) and grade it up for larger sizes and grade it down for smaller sizes (see One pattern, three sizes). Methods of grading—There are three basic methods of grading: cut and spread, pattern shifting, and computer grading. No one method is technically superior and all are equally capable of producing a correct grade.  Cut-and-spread method: The easiest method (photo above), which is the basis of the other two methods, is to cut the pattern and spread the pieces by a specific amount to grade up, or overlap them to grade down. No special training or tools are required—just scissors, a pencil, tape, and a ruler that breaks 1 in. down to 1/64.  Pattern shifting: Pattern shifting (photo above) is the process of increasing the overall dimensions of a pattern by moving it a measured dis-tance up and down and left and right, (using a specially designed ruler) and redrawing the outline, to produce the same results as the cut-and-spread method. The most recent development, computer grading, is the fastest method, but tends to be an investment only larger manufacturers can afford. However, sophisticated home computer software is becoming affordable.
Pattern vs. body measurements The first step in any grading exercise is to accurately measure your base pattern. Sometimes you can find the key measurements you need (bust, waist, and hip) on the pattern envelope. Or you can measure the pattern itself. To determine the actual measurements of the sewn garment rather than the size of the paper pattern, keep two things in mind when measuring a pattern. First, account for all design details. If a garment has a bust dart that extends to the waist, then omit the dart area when you measure the waist; similarly, omit the volume given to pleats and gathers. Second, measure the pattern from seamline to seamline, not cutting line to cutting line.
Establish the grade Once you have the bust, waist, and hip measurements from both the pattern and your body, you can address the basic grading dilemma: "How much do I grade up if the pattern is too small? Or, if it's too big, then how much do I grade down?" To establish the overall grade, or the total amount needed to make the pattern larger or smaller, simply calculate the difference between the pattern and the body measurements. There are two types of overall grades: an even grade and an uneven grade. An even grade means that the bust, waist, and hip measurements change the same amount from one size to another. For example, if a pattern measures 35-27-37 and the body measures 37-29-39, then the difference between each measurement is 2 in. and the overall grade is an even 2 in. Sizes of commercial patterns and apparel always follow an even grade.
If the differences between these measurements aren't equal, then it's an uneven grade. This changes the shape of a garment, as well as its size, so strictly speaking, uneven grading isn't really grading. "Uneven grade" is synonymous with "pattern alteration." An even grade is the type of grade I will discuss because it's straightforward and easy to understand. Once you master the even grade, applying the methods to the uneven grade is the next step. See Four ways to address an uneven grade for more information. Putting it all together Once you've determined that you have an even grade, the actual mechanics of cut-and-spread grading are easy (see The cut-and-spread method for detailed instructions). Basically, you will use your measurements to do some easy calculations, create the cut lines on your pattern, then spread/overlap at each cut line the calculated amount. I've listed the necessary formulas in the grading chart, as well as pre-calculated values (no math needed) for three different overall grades (1 in., 1-1/2 in., and 2 in.). The cut-and-spread method can even be used when grading large amounts up or down, for example a size 4 to a size 18. The large gaps at the cut lines do tend to be a little awkward, so I recommend grading incrementally instead (first grade the size 4 to a size 12 and then the size 12 to a size 18).
Take it to the next level When you first try grading, start with a basic bodice front and back, skirt front and back, and sleeve, then graduate to more complicated designs. Remember that any pattern, no matter how "designed," is based on one or more of the five basic pieces. Even a strapless top began life as a basic bodice, so place the strapless bodice pattern over the basic bodice pattern and transfer the cut lines. A dress is simply a bodice joined to a skirt, so the cut lines and spread/overlap amounts remain the same. Just match up the vertical lines on the bodice with those on the skirt. Grading can be as complicated or as simple as you want it to be. So when you first approach any pattern, always start with these basics. Figure out your needs as a sewer, dressmaker, or designer; then map out your course, and grade, one step at a time.
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