I am lucky -- my measurements are perfect stock sizes! 39-31-41. And I never got the hang of refitting. I can go one size up, but more than one size up and I make a mess! LOL. Going down a size I could never do right, but beginning to learn (just ONE size down, though!) Then there is the issue of EASE. I candidly put my measurements out there: 39-31-41. Through trial and error, I've found that these sizes for these eras and brands fit me perfectly:
These just so happen to be my preferred eras to make, so this is why I have it all figured out. I don't have any original 1920s patterns. I have original and modern repro patterns spanning 1945-1965, but I just don't like that era of fashion very much so I have no clue what size I would wear in the vintage originals. Modern repro is always a size 14 so I usually make those if I get a hankering for a 1950s circle dress ;)
Vintage pattern from the 30s and early 40s are almost ALWAYS the size they say they are on the envelope. Hollywood patterns are pretty exact to my measurements whereas Simplicity has a bit of room to play -- not excessively like modern patterns, but just enough for a snug, nice fit (like I like!) I don't know when 4 inches of ease came in, but modern patterns have just that -- 4 inches of ease is usually the standard. My bust fluctuates between 39 and 40 inches depending on hormones and water weight and all that fun girl stuff.
This is a size chart on a brand new 2014 pattern. Going by this, I should be wearing a size 18 because I have a 40 inch bust. Nope. I have to cut and make a size 14. Here's why:
There will be three areas on the patterns with this little circle and a cross symbol. These are the finished garment measurements for the bust, waist, and hip. As you can see, even though it says size 18 is a 40" bust on the envelope, when the garment is all made up you end up with a 43.5 inch bust line. Look at size 14 -- 39.5 finished garment bust line. Perfect for me! Even though the envelope says size 14 is a 36" bust, it's really a 39.5" bust. Three and a half inches of ease is HUGE. This is why a lot of new sewers get disappointed that their finished garments are so big. I've been there! I had no idea there was a "Finished Garment" measurement on the actual pattern until recently! Most often, size 14 is a 39.5" bust, 31" waist, and 41" hips. Could be an inch bigger sometimes, but when I have shaping garments underneath a size 14 ALWAYS fits perfectly.
So, with this, I hope I've saved someone out there the agony of having your first sewing project turn out TOO BIG. And it will if you go by the measurements on the back of the envelope.