The rules on seam allowances pt.2

I decided to write a follow up to my first entry because comments from two people who have every reason to expect they’d understand what I was talking about, didn’t. Perhaps my entry was too brief (1,500 words on seaming is brief) but I didn’t want to get too technical lest legions of readers expire from abject boredom, their hands affixed to the armrests of their chairs in the final stages of rigor mortis.

I have a silly question on overlock seam allowances. Are 3/8″ and 1/2″ the seam allowance as marked on the pattern or the finished seam allowance after sewing? I think most of my RTW knit garments have 1/4″ finished overlock seam allowances, and I’m pretty sure 1/8″ is trimmed during sewing. That would mean a starting seam allowance of 3/8″.

When we talk about allowances, we mean the total from nett (the sewing line) needed to complete the seam successfully. In the course of doing an overlock (serged) seam, it is presumed that successfully completing the seam means it is necessary to trim off a scant 1/8″ portion of the edge (or edges). In other words, the seam allowance (3/8″ for 3-4 thread, or 1/2″ for five thread safety stitch) will mean that after sewing and required trimming of the edge, the remainder will be 1/8″ less the finished viewable seam. So yes, a knit garment with a 3 or 4 thread overlock seam will finish at 1/4″ width from nett (the seam line) although 3/8″ was used to make it.

I understand where the confusion comes in. In the typical course of sewing a seam with a regular machine, the seam allowance is not trimmed away. The seam you see is the seam allowance you started with. Another way to explain it is if you were to dissasemble a knit garment (neatly undoing all the seam threads) to make a pattern from it, you would need to add another 1/8″ (that had been trimmed away) to all overlocked seams on the pattern to permit successful seam formation of the new garment.

Is 3/8″ cut off (and then the stitching takes up more on overlock) or is the actual seam at 3/8″ (so you cut off less).

I understand how this can be confusing. It is a matter of orientation, your point of reference, what you consider to be the zero point. With basic seaming, the point of orientation is (nearly always) the needle path, aka the sewing line. In the case of the single needle machine, it’s the position of needle (middle of the throat plate). This is the zero point. With usual handling, everything to the left of the needle is garment or product. Everything to the right of the needle is seam allowance. If it’s a two or more needle machine, the farthest left needle is the zero point of orientation (the sewing line). The space between the left and right needle and the fabric to the right of the right needle is all seam allowance. This is confusing because many people are oriented to lining things up on the right because there are (often) little lines on the throat plate and so, align the cut edge as their zero point or frame of reference on those markings. I just checked the machines I use most often. None of the industrials have these markings but all of the home machines do.

If you align the edges to cut off 3/8″, the needles to the left are seaming 1/4″ into the area that belongs to the body of the garment or product. To prevent this confusion, the guidelines on the throat plate of overlocks (only on my home machine) are measured and aligned in accordance with the needle path (the farthest left needle). If you do not have markings on the little guide that holds the knife of the overlock, you can get one of those pattern rulers and measure the distance between the left most needle and the knife. On my five thread overlock, this distance is 3/8″ -but that’s because it is a safety stitch. In order to to have a bit to trim away, I need to make the allowance 1/2″. [My problem is I forget to. My previous machine was a 3/4 thread that finished at 1/4″ (total 3/8″) so for the past year, I’ve consistently made my seam allowances too small for the new machine.]

Again, the above refers to most (but not all) basic seam formations aka “joining”. If you’re top stitching a patch pocket, the edges of the pocket are the point of orientation (the zero point). You’d stitch inside the area. This is similar to top stitching, say a collar edge. The edge of the collar becomes the zero point. This may not seam logical but think of it this way: you don’t need to add seam allowance to a pattern for top stitching. Top stitching is not a joining seam, it’s “ornamental”. Ornamental is the name of a separate and specific seam class. Not needing separate seam allowance is but one reason ornamental stitches are an entirely different seam class.

In the section on industrial sewing in my book (specifically 131-132), I explain the anatomy of seam class pictographs to analyze the manner used to illustrate them. You can also find out more about seam classes here or read related topics on sewing machine attachments. I was sure I’d written more about seam classes before but I’m not finding anything. Should I do something like that? I suppose it’s the only way you’ll see what I mean when I say that seam allowances vary according to the seam class and the equipment used to form them. If I ever write a production pattern making book, this will be a big part of it mostly because I don’t like having to figure it out over and over, asking around or measuring or whatever. I don’t know why somebody hasn’t cooked up a chart and posted it around long before now. Sure it’s minutia and a tedious task; where’s an autie when you need one?

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  1. Bryan Lund says:


    As I said today on the phone I know nothing about your industry. With that said, thank you! And to answer your question about writing more on this…judging from what I experienced today, I don’t think you can overwrite on this stuff…based on what we talked about today: a simply seam on a strap. In my world of mechanical engineering, we have Mark’s book of engineering formula’s and the Machinery’s Handbook – loaded with tables, fits, tolerances, cross references, constants, formulas, conversions and the like.

    I felt really stupid asking about a seam, but when I learned the hundreds (thousands?) of variations on seams, I was quite overwhelmed. I have a new appreciation for the fashion industry based on a silly lil’ SSb-2!

    Thanks again!

    Bryan Lund

  2. Kathleen says:

    SSb-2? I thought it was SSa-2 but should have been the EFa-2. Btw, let me know how that worked out for you later.

    Btw, there’s a third seam class standard beyond the 751a and ISO designations -colloquial. We’d call the EFa-2, a simple or single turn double stitched. Welcome to my world. :)

  3. LisaB says:

    Kathleen, thank you for clarifying.

    My industrial overlock is a 3-4 mock safety stitch. I just double-checked to make sure I wasn’t crazy… The left needle finishes at 5/16″, and the right needle finishes at 7/16″. For the last year since getting the machine, I’ve been adding 1/8″ more than that as seam allowance on my knit patterns. It’s frustrating to mark off 7/16″ or 9/16″. Anyway, DH and I adjusted the knives to the right a while ago trying to get the more standard 1/4″ & 3/8″, but I couldn’t get the stitches to form correctly. Evidently, something else needed to be adjusted, too, but I didn’t know what. I ended up moving the knives back to their original position and just deal with the odd seam width. Maybe I AM crazy…

  4. Lisa Brazus says:

    Anything written on seam allowances and different types of seams is beneficial. My students look at me like I am an alien when I tell them that the seam allowance depends upon the machine being used, the type of seam being sewn, and the type of fabric being used. They want a more definitive answer.
    If you ever write that pattern book I will purchase it yesterday!!!
    Btw I was lol about your reference to rigor mortis.

  5. Betsy Johnson says:

    “DH and I adjusted the knives to the right a while ago trying to get the more standard 1/4″ & 3/8″, but I couldn’t get the stitches to form correctly. Evidently, something else needed to be adjusted, too”

    Did you loosen the tension on the looper threads? With the additional distance between the needles and the edge, each stitch needs more thread length.

  6. Kathleen says:

    Lisa -good point, I should have mentioned this. I don’t remember what model you have but some overlocks (and single needles) are not configured to imperial measures but metric. Can you try that again and measure in centimeters? I imagine it will come out in even measures. The only modification you’ll need to make is to add your seam allowances in metric rather than imperial measures. And maybe Betsy’s suggestions will work too, I wouldn’t know not having had that experience.

  7. Elizabeth says:

    Both my industrial sergers/ overlocks do the same 3/16″ and 7/16″. I have a Juki MO3316 5 thread serger and a Merrow 3 thread overlock. Both have been adjusted and serviced, the threads are not pulling, even if you remove the thread and let the needle mark the fabric/ or paper, the markings show the same measurements.
    My question is when I do patterns for clients, do I make them according to the seam allowence on my equipment since I am the one sewing the samples or with the 3/8″ and 1/2″ standard seam allowences in case they have a sewing contractor who has standard machines, especially if the client doesn’t have a contractor yet to ask?

  8. Dawn says:

    Ohhhh! She exclaims! No rigor mortis here. You just clarified everything for me! I provided a pattern that specified 1/4″ seams, and in trying to adhere to that they are sewing too close to the edge, maybe because they’re not used to it. It’s hard when the factory is trying to do “whatever you want” and you don’t really know what you’re doing! I did not understand to draft the pattern to 3/8″ (which the factory said is their standard allowance) – I did 1/4″ because the seams are being sewn using a 4-thread overlock. Need to add 1/8″ to the patterns asap. Thank you!!

  9. LisaB says:

    I’ve got a Juki MO-2512. In metric it looks to be exactly 5 mm and 8 mm. Interesting! Juki overlocks must be configured in metric, which would make sense.

    Betsy, I know I fiddled with tensions, but it’s too long ago to remember which ones and which direction. Sigh. Maybe we’ll give this another try following your advice.

  10. Million says:

    Kathleen, you must have just lost it when you saw the seam allowances I used for my older projects (since I hadn’t perfected the patterns I added extra material to the SA in order to compensate). Thanks for clarifying.

  11. Are there any hard & fast rules to serging seam allowances? I work with some silk (and other thin fabrics) & if I serge the edges and then press it, you can see the serging. I’ve been pinking some edges, but was wondering about other options. Any help is appreciated. Thanks!

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