Pattern Grading
What Is Grading? How Do We Do It?
Grading is the messy and confusing part for most so let's start with some definitions. Grading is the process of creating ever increasing/decreasing sizes of something that is primarily the same shape as the original. Typically we start with two sizes (smallest and largest) and then start the process of breaking up the space in between those two sizes into gaps. Look at the image below ... I have two pentagons; one representing my smallest pattern and one representing my largest.
Now in the far greater majority of cases we simply draw a line between each node and break it up into equal gaps then join the dots (see top left image below). That's it ... that's grading explained simply. The line is straight, or linear. The distance between each node on the line is the same, or equidistant. But keep in mind that it's not always like this. The spacing between the dots might increase with increased size (nonequidistant) and the line may also be curved instead of straight (nonlinear, typical of plus size inclusion)
So we can mix both of these to get four different methods for working out the gaps between the smallest and largest sizes ...
A straight line with equal distances is easy, but how do you know what the spacing is if you go the non equidistant route. This method is about increasing the gap between sizes as you increase in size (see top right image above) ... its got very little really to do with actual sizing and more to do with how many people will fit into each RTW size. Demographically speaking (and much to the denial of plus sizes) there are more people that fit into the smaller sizes than the larger sizes. So if we broke up the smaller sizes into smaller grading gaps the majority could get a better fit ... at least that's the theory. The contra argument is that, while there's less of them to account for, the larger sizes with more variation in shape get less fit options. It comes from a desire to better satisfy the majority while still providing something for the minority AND being able to better control how many of each size you manufacture ... it's about business reality.
To determine how much to increase the gap between each subsequent size is an arbitrary business decision and nothing to do with anything else. Generally I find people do things like gap 2 is 10% bigger than gap 1, gap 3 is 10 % bigger than gap 2, etc ... this increases the gap with each size even though the percentage is the same ... it's nonequidistant. Equidistant would be gap 2 being 10% bigger than gap 1, gap 3 being 20% bigger than gap 1, etc. They aren't the same and the distinction is very important (see image 2).
Now for the fun one; nonlinear (see bottom two images above). This is something I've started seeing only in more recent times where there are a much wider selection of sizes than the usual 5 or 6 standard RTW options. It's pretty much come from the desire to satisfy an ever increasing group of plus sizes ... don't get me wrong, two paragraphs ago I said that weren't that many and there aren't demographically speaking, but the number in that group is indeed growing, especially in places like the USA and Western Europe. Previously We had normal sizes and plus sizes and each was a linear grade (dual linear) ... the plus size grading line being shallower than the normal sized line because after a certain point we decided that people got wider faster than they got taller. The argument then became at what point do we change the angle of the line ... which size is arbitrarily the start of plus sizes? So instead it stopped being a line and became a curve ... or nonlinear (see images below  green represents the plus sizes). I have only ever had two clients who have gone this route because I believe others found it hard to work out .... two straight lines was easier (or 1 straight line for normal, and 1 for plus: two different grading jobs). I never liked it much until I recently started designing body forms with zero ease and found it essential to getting everything right ... at zero ease it must be a curve or your middle sizes don't fit!
All this is based on the pattern maker creating two sizes to begin with ... the smallest and the largest. This is without a doubt the most common way to grade as it removes all the complex formulas and mathematics that you need if you grade up and down from a single size. I've honestly never understood why people grade from a single size .... the argument is about time saving which is cool if you've got a fashion CAD program with all the complex math rules programmed into it. But if not then it's faster to draft two pattern sizes and grade between them than it is to work out a grading rule set.
If you've designed your patterns off a block then it's quite likely you've already bought a fully graded block set or made your own according to whatever drafting method you used ... if you can make one block to a method then you can make two, right? .... and you only ever need to make these two sizes once. So let's just assume for a second that you do have a set of graded blocks for whatever you're doing. I, for example, as a swimwear pattern maker have a fully graded set of 12% negative ease swimwear blocks (obviously I have many others but I typically use this one). I choose my largest and smallest from that set and open them in CAD. Then I draft my design twice, once onto each block, step by step ... so I end up with two different patterns ... one in the smallest size and one in the largest size. If I was doing plus sizes I'd probably add the largest plus size to the end and draft all three concurrently from scratch (much like the green section in the image above).
This is without a doubt the easiest way to grade on paper ... a simple straight line between two similar points on two pattern pieces, broken up into even gaps.
But it's even easier in CAD. In Corel draw I select my smallest size and my largest size (see image directly below) and hit the blend button (next image). It asks for how many items between the smallest and largest and I hit apply ... Corel Draw spits out all the in between sizes in one click ... linear equidistant grading at its fastest without expensive fashion CAD software ... Adobe Illustrator does a similar thing. It's instant and perfect and you don't need complex grading rules or fashion specific software.
Both Corel Draw and Adobe Illustrator blend evenly along a straight line by default, but they will also blend along a curved line (called Blend to Path in Corel) ... you just select the two sizes, the curved line representing the path (you need to select all the curved lines between nodes), tell it how many between smallest and largest and it'll spit out the new graded sizes.
But wait there's more! While Corel Draw comes standard with equidistant blending, you can edit the button's script to change from equidistant to percentage or logarithmic or any complex equation should you wish even though you'll rarely if ever use it ... it is still possible ... I created my own scripted button for this to make the process faster ... I doubt most will ever require anything like this.
But what are the gaps if I want to do it from one pattern? I used to give people an idea of how much to grade between sizes. In nonstretch people tend to use fixed arbitrary increments that show very little mathematical relationship with reality. In stretch patterns we use percentages... it only works, however, when the amount of ease is the same between sizes. If you're mathematically alert you'll immediately realize that a percentage negative ease is not an equal increment between each size. Instead it is proportional to each size. Let’s consider this for an Australian RTW size 10 bust measurements;
Size 8: 80cm x 12% reduction = 70.4cm or 4.8% smaller than size 10
Size 10: 84cm x 12% reduction = 73.9cm
Size 12: 88cm x 12% reduction = 77.4cm or 4.8% larger than size 10
Size 14: 92cm x 12% reduction = 81.0cm or 9.6% larger than size 10
So if you start with a size 10 you have a difference in bust measurement of 4.8% per size. These calculations will come out the same no matter what negative ease you use, as long as it's a percentage and not a fixed amount (like you would have learned for nonstretch). Now keep in mind that each size category is based on a standard metric bust measurement and that all the other measurements vary depending on your market demographics. True proportional grading means you'd grade by percentage across all horizontal measurements. In other words your waist and hips, for example, would be accurate for a size 10 but applying 4.8% per size would lead to minor variations in other sizes.
But what about the vertical? The length of our one piece blocks are nape to waist plus waist to crotch plus 1/2 Gusset. If we try those based on the size 10 again we get;
Size 8: 39 + 24 + 8.5 = 71.5 or 3.1% smaller than size 10
Size 10: 39.5 + 25 + 9.25 = 73.75
Size 12: 40 + 26 + 10 = 76 or 3.1% larger than size 10
Size 14: 40.5 + 27 + 10.8 = 78.3cm or 6.2% larger than size 10
So again, if we start with an Australian RTW size 10, we get a difference of 3.1% per size completely irrelevant of how much ease we apply. Of course, on paper, you still have to calculate every single point if you're grading by hand, but always by the same percentage. You never have to remember fixed incremental amounts ever again. The massive advantage, however, comes if you have a CAD package. For the incremental method your computer must know where your nodes are before you can grade and this requires specialist software which is expensive. For the proportional (percentage) method you can skip having to calculate the true increments, ignore that nodes even exist, and simply scale up your block or pattern a fixed percentage for horizontal and a fixed percentage for vertical. It's so easy it means any simple off the shelf CAD program can do it, cheaper, faster and just as accurate.
This is a good way to get your second size. You could use this way to get all your sizes but things don't always work out the best if you just apply flat percentages. For example ... your body length might increase by 3.1% per size, but your armhole height might increase only 2.6% ... hence why the fixed increments were used .... but really its a matter of knowing where things change with increased body size. So I start with my smallest size, apply a percentage to get my largest size, tweak the pattern to bring parts of the largest size back to reality, then blend between the two. In CAD this is remarkably easy, fast and incredibly accurate.
CAD will save you the time that you might fear you'll lose having to learn the software ... but it's even more important for a bigger reason ... taking less time to do a job means getting more jobs done in the same time ... or more profitability!
Lastly I have one comment that I really unfortunately need to make. NEVER GRADE A PATTERN WITH SEAM ALLOWANCE ADDED. Always add seam allowance after you've finished grading. If your pattern has seam allowance already added then remove it prior to grading. Why?

You need to verify the seam lengths of each newly graded piece match up correctly.

You don't want to grade the seam allowance itself (ie; seam allowance getting bigger with increasing size).

You want to create a habit of doing things in a specific order so you don't forget steps.

If it's unmarked and presumed by you, others who use your work later might think it hasn't been added and add it again so always clearly mark on your pattern how much seam allowance is added and where (if it's different on different parts of the pattern).