The rules on seam allowances
Seam allowances are always subject to loud and cantankerous debate. Okay, maybe not but that doesn’t mean folks don’t have tightly held opinions they’ll defend to the death, using their seam allowance as some sort of body armor. The answer is, it depends. However, it depends less on your comfort level or preconceived ideas than it depends on equipment. There is a whole chapter in my book about it starting on page 129. So, in order to answer the question of proper seam allowances, you must determine what kind of equipment will be used to sew the seams.
Let’s do the easy stuff first -overlock machines or sergers. Is it a three or four thread overlock or is it five thread with a safety stitch? The latter requires 1/2″, the former is 3/8″. Yes I know some people will use 1/4″ on a three thread but we don’t talk to those people. I kid, I kid. Context is everything, my suggestions are optimized for a commercial environment. You can get away with 1/4″ on a 3 or 4 thread home machine sewing for yourself but I wouldn’t do it with a pattern that contractors will be sewing up for you. In summary, any seam that will be overlocked, regardless of where it lies (cuffs, shoulder or side seams, collar etc) is either 3/8″ or 1/2″.
Single needle seam allowances
Wars have been started under flimsier pretexts. That thirty years war you read about in high school? That began over the proper hem allowance for draperies hung in Lutheran vs Catholic churches, bankrupting most of western Europe in the process. With single needle (meaning your typical lock stitch machine), seam allowance is relatively arbitrary because you can set to needle where ever you like. That said, there are preferred guidelines such as:
bound shirt sleeve vents used in conjunction with cuffs
tight curves (scallops etc)
ALL outside edges (cuffs, collars etc). Yes, really. ALL.
General seaming, shoulders, side seams, waists, armholes, sleeves, inseams, joining cuffs, waistbands, princess lines, turning under patch pockets (manually) etc. Pretty much everything else.
I have only ever seen 1/2″ seam allowances -remember, we’re talking about lockstitch machines- used by DEs. 1/2″ is hands down, their favorite. On everything. Everywhere. Knit or woven. Serger or lockstitch. It’s there. Maybe they even have little shrines to it. They’d probably invite the 1/2″ seam allowance to the company picnic if they could. What’s scary these days is that we have so many smaller contractors who’ve come up through the ranks and have never worked in a larger production facility themselves, that they use it the same way. ~shudder~ makes me shiver all over, blind leading the blind and all. And I am just having a bit of fun. Probably 75% of my readership uses 1/2″ allowances just as I’ve described them and I certainly don’t intend to alienate anyone. See caveat at close.
Now is when things get more confusing because some seams are more complex than merely laying two layers together and joining them. Consider a hem, typically you make two turns. Each turn can be different depending on a variety of things (preference, equipment, material etc). Complex (and simple) seams are described in seam classes. We love seam class specifications, yes we do. We even have books about them. Some seams we debate long and loud about are french (used on silk) or even flat felled seams. In the case of flat felled seams, it depends on whether you’re using attachments (see below) or doing it manually. And if manually, how you learned to do it. Just to fan the flames, silk seams are usually 5/8″ total. 1/4″ for the first pass, 3/8″ for the second. In industrial sewing, two sides joined together don’t necessarily have the same seam allowance. In the case of flat felled seams, one side of the seam is 1/4″ to 3/8″ (depending on material weight) and the other side is 5/8″ to 3/4″ (ditto).
Hems depend on preferences such as finishing and equipment. Expensive long dresses will have a much deeper hem, 3″ is common. If it’s a shirt tail hem and turned by hand, the total allowance is 5/8″ (first turn is 1/4″, second is 3/8″). However, if you’re using a folder to sew the hem, that’s usually 1/2″ (below). But even folder hems vary. A handkerchief hem can be minute, maybe 1/4″ total. If there aren’t two turns, it’s not technically a hem. One turn is just an edge finish which is not to suggest it is inappropriate on things like visible facings on better goods.
Then, there’s other stuff you sew to products, like zippers. It’s impossible to give a set allowance because this depends on the width of the zipper tape and the desired finish. Heavier zips have wider tapes. If there’s a lap to cover the teeth in a centered application, the allowance will be greater than if teeth are intended to be left exposed -such as in sportswear. Similarly, if you have an overlap construction, one side of the zipper will have a greater seam allowance. Well, not really seam allowance per se but most people think of it that way. A lapped construction will mean the two sides will vary in their nett distance from the (presumably) zero point of CB/CF (or where ever it is the zipper is inserted). Also, zipper allowance can depend on material weight because this affects turn of cloth. For example, a zipper in an outer weight jacket in heavier goods will take a 3/4″ allowance. A lighter weight will only take 5/8″. A dress or blouse made in very light goods may only be 1/2″ (invisible zipper) etc. In sum, yes it varies but it’s not arbitrary such as “whatever works best for you”. It’s more specific than that. You can derive rules for specific circumstances and apply them uniformly. See my zipper sewing tutorials for applied examples.
Binders and folders
The matter of attachments rear their ugly heads. Attachments are doo hickeys you can attach to a machine to partially automate a process. As I said in my book:
…binders and folders are more obstinate and opinionated than I am. Binders and folders don’t care what your favorite couture expert says, they don’t care about “opinion” or “whatever works best for you”. Worse, binders and folders are disrespectful and undermine your authority because they don’t care who owns the company. These attachments will only sew own way and one way only which is fortuitous. Being precisely engineered tools, they perform operations better than human hands.
And even then, you may have to alter the width of the start point of certain seams (like flat fells on jeans) so they will feed into the folder. Often, a contractor won’t say anything and just sew these up and you’ll be none the wiser unless you take really good spec measures of finished products and idly wonder where 1/4″ of the spec went. Ask me how I know.
Does it really matter?
And if you wonder, seam widths really do matter a great deal and for several reasons. As I bang on about non-stop, it’s easiest to sew curved seams (necklines etc) with a smaller allowance more accurately. The wider the allowance, the harder it is to hit the sewing line, so no, we don’t use smaller allowances because we’re cheap. Which is not the same as denying we’re cheap because we often are. We use smaller allowances because they’re more accurate to sew. The other reason is that professionals will automatically sew your goods to optimal widths regardless of what you had in mind. It really is a good idea to set your allowances to the preferences and habits of those doing the work. Meaning, that if your contractor uses 1/2″ on a lockstitch as a matter of course, that is fine. Seriously. If they’re not used to using 3/8″ in places the rest of us would but you decide to conform because of the little joke I made about inviting the 1/2″ seam allowance to the company picnic, you could be inviting a whole host of problems. Put it this way. If I were using a contractor who was otherwise stellar but habitually used 1/2″ allowances, I would change my patterns to match their working preferences.
Seams have names and official designations indicated by codes. If you buy industrial equipment (usually new), the machines are often rated or described by seam class codes. If you’ve ever seen a “service line” put out by machine manufacturers, each operation of a garment is usually listed by code. You can find out more than you’d ever care to know in my book (which explains anatomy of a seam class etc) and in the forum.