Grading is the process of creating a size range of patterns for a particular garment. It’s the process of creating a size 4, 2, and 0 pattern from a size 6 pattern or a size 8, 10, and 12 pattern from a size 6 pattern. Typically, a middle size such as a size 6 or a size medium is drafted and then graded up and down because grading can only be done two or three sizes at a time before pattern shapes are distorted.
To make it more confusing, grade rules depends on the particular garment. An example is the body length grade of a t-shirt. On this type of garment, the grade for body length depends on the length of the garment. If the body length is longer than 25,” then body length grades ½”, if the body length is between than 25” and 18,” then body length grades 3/8”, and if the body length is less than 18,” then body length grades ¼”. So essentially, every garment has a different grade and grading is done on a case-by-case basis.
The first step in grading is defining a size range, such as XS-XL or 0-14, and making sure that your size range is consistent and proportional. This means that if a size 2 has a bust/waist/hip ratio of 33”/25”/35”, then a size 4 has a bust/waist/hip ratio of 34”/26”/36” – each one increases by 1.” A grade would not be proportional if a size 2 had a bust/waist/hip ratio of 33”/25”/35” and size 4 has a bust/waist/hip ratio of 35”/26”/36” – the bust increases by 2” while the waist and hips increase by 1.”
Usually grade rules stay fixed from size XS to sizes S and M, the total circumference from size to size at the waist and hip usually
increases by 2” (which means that the garment increases 1” flat and each pattern piece increases ½” at waist and hip). From sizes M to L and XL, the total circumference from size to size at the waist and hip increases by 3” (which means the garment increases by 1 ½” flat and each pattern piece increases ¾” at waist and hip)