This is a question most very new sewers don't ask until they've experienced the frustration of making a few garments that don't fit them. In my Screaming Mimi's Get Started Sewing course, I go over this very issue. The problem is, there is really no hard & fast rule. What I can say for sure is that most commercial patterns are not sized the same as off the rack clothing. So where does one start?
Because commercial pattern envelopes contain various size groups, it's best to start by comparing your body measurements to those on the pattern envelope so that you can determine which size group to purchase. Then, before cutting your pattern tissue, and fabric, consider the many other factors involved. Click on "Read more" to continue...
All patterns are not created equal. Most experienced sewers have gained a sense of how the various pattern companies differ in their sizing, and which are best suited for their body type. Of course you'll want to start with "easy" patterns, but be sure to try various brands to get a sense of their sizing. And don't be afraid to try the indie patterns.
Comparing the pattern tissue to a like style garment that fits you can be helpful in determining the correct size. For example, compare the pattern tissue for a pair of pajama bottoms to a pair that fit you...don't forget to consider seam allowances.
Tweak your assembly a little bit so that it will be easier to make adjustments while sewing. The tunic top pictured above is a Very Easy Vogue pattern (V8731). Using my measurements, and holding the pattern tissue up to my dress form, I thought I would need to cut an X-Large. Before hemming the sleeves, neck, and bottom, I tried the top on, and it was too big -- this is where tweaking my assembly came in handy. Rather than following the pattern directions to set the sleeve in, I attached the sleeve, and then sewed one long seam from the wrist to the bottom hem. This way I was able to take it in by sewing a new seam 5/8" from the old seam, and trimming off the excess.
Note to beginners: A set in sleeve is done by sewing the side seam of the garment, sewing the sleeve seam, and then sewing the sleeve "tube" to the bodice at the arm hole.
One way to determine if you're about to cut the proper size is to make a muslin first. Purchase some inexpensive muslin fabric, and cut the main pieces of the garment. You can quickly assemble it, and try it on to determine fit. Admittedly, I don't do this often, but if you're making a very fitted garment, a lined jacket, or using an expensive fabric, this will end up saving a good deal of time, and frustration in the long run.
Adjust sleeve, and leg lengths on the pattern tissue before cutting. These adjustments need to be made in the correct place, not by cutting off the bottom, otherwise the sleeve or leg lose their proper shape.
What I've learned over the years is that Vogue, and Simplicity (especially their A B, C, & D cup size, and their Amazing Fit patterns) are best suited for my body type, so if I'm going to use a pattern from another company I need to be prepared to make some adjustments. If I'm sewing for someone else, I need to make a muslin. And no matter what your experience level, you will always learn something new by taking a class.