I have always been told to measure myself and use the back of the envelope to choose the size to make. Well, this seems to give me a very large size. If I use the size suggested, I have to then cut my finished garment down several sizes until I get a good fit.
This method of pattern sizing obviously is not working for me. So there are two other methods that I have found useful. The first is to use the Finished Garment Measurements and the second is to Measure The Pattern Pieces.
~ 1 ~ Thankfully there are some patterns that give the Finished Garment Measurements. If I am lucky enough to be using one of these, that is the sizing that I go by. For example, here is an image, showing the finished garment measurements printed right on the pattern paper itself:
This is the finished garment measurements for New Look 6433 skirt pattern. As you can see the hip measure for a size 8 is 37 1/2" once finished. The hip measure for a size 10 is 38 1/2", the measure for a size 12 is 40" and so on. If you look at the bottom of the image, you will see where it states that there is a 4" ease added to the pattern.
My hip measure is 39". Since I don't have a lot of fluff, or soft tissue, this measure does not change when I sit, bend or stoop. I can use this as the basis for my largest and smallest needed finished measurement.
According to the back envelope, I should round up to 40" and make up a size 16. A finished size 16 will give me 43 1/2" in the hips, according to the above information. For me, 4 1/2" is a lot of unwanted room. I want a skirt that fits well in the hips so I will cut a size 12 and have a finished 40" hip measure.
~ 2 ~ The second way that I choose a pattern size is to Measure The Pattern Pieces. There are patterns that, for some reason, don't have the finished garment measurements with them. They could be old, used, or partial patterns or just a new pattern that doesn't include that sort of information.
To find my true size, I measure the actual pattern pieces instead. I have a very low hip to waist ratio so I generally focus on the waist measure for each piece. As I go, I subtract the amount of the seam allowance and any darts and then add up what I have.
Do keep in mind that changing the seam allowances will also increase or decrease the finished size of your garment.