Adding Seam Allowances

One of you emailed me with a question that got me thinking: how do you add seam allowances?  I wonder if it’s one of those things so simple that everyone assumes they know how everybody else does it.  I hope some of this looks familiar!

Method 1 With a Sewing Gauge

Adjust slider to the desired SA and place along the stitching line.  Draw little lines against the edge of the gauge.  Repeat at regular intervals and join with a ruler.

For curved areas, draw smaller lines at shorter intervals.  To join up, I go freehand: a good method if you haven’t a fashion curve (or prefer not to use one).  Turn the pattern so the line curves in towards you (i.e. not away from you) and with your elbow pivoting on the table and your hand still, turn your forearm in a smooth arc with the pencil skimming along the little lines.

Method 2
Using a Fashion Curve

Align the desired seam allowance with the stitching line and draw along edge.

As before, on curves such as the armscye, keep realigning and mark lines a little and often.


So far so simple.  But there’s more…


3 How to Add Seam Allowances to Darts

Draw seam allowances up to the dart (on both sides).  Pin dart closed, folded in the direction you want it pressed on the garment.  The seam allowances and stitching lines should now meet. Cut along the seam allowances.

Remove pin and the pattern looks like this:

Try not to skip this step or you may have a shortage of fabric in your dart when you come to sew the seam.

4 Adding Seam Allowances to Angles

When two stitching lines meet at a right angle, no problem: the SAs are also right angles.  It gets trickier when the angles are sharp, large or curves, for example in the neck to shoulder corner, or waist to side.  If you serge your seams, what follows is perhaps less of a concern.  If you press seams open or don’t trim the allowances much (for example, if you might need to let out a garment later), you may want to add the following to your method:

Step 1 Draw the Seam Allowances up to the corners but don’t cut.

Step 2 Fold the pattern back along the stitching line then cut along the seam allowance on fold.

Do this on all curves and angles that are not L-shaped: i.e. fold back at stitch line, cut on fold along seam allowance edge.

The opened up piece will have pointy bits like this:

When the seam allowance is pressed open after stitching, there is enough fabric to align with the fabric of the cross seam.


So, any revelations?  Did I miss any tricks?! How do you add yours?



22 thoughts on “Adding Seam Allowances

  1. I use the sewing gauge with the slider and I fill in free hand on smallish curves, on bigger ones, I use a French curve. It’s a bit weird but I like adding seam allowances, you can disengage your brain!

    • I’ve only seen it on blogs, not in shops. And I haven’t seen many reviews, strangely. It’s certainly a very clever gadget and I can’t believe it’s only a couple of years old!

  2. Thanks for this. It’s always interesting to see how others do things and the folding method to create ‘pointy bits’ (I love a technical term) is new to me. I’m actually guilty of just eyeballing seam allowances, but I can see that this will no longer do!

  3. That last one is nice and I shall do it from now on. Thank you Marianna! I should do the darts more often but just wing them and ignore the fabric non-matchy when sewing them up. Terrible lol.

    • I’ve just had a look at your review and at their website. Great for quilting especially, I’d imagine. Again, as with the ‘magnetic barrel’, such a simple idea, you want to kick yourself for not having invented it first! Thanks for pointing it out!

  4. I haven’t had to yet but I’ve bookmarked your page so I can remind myself of the options! Incidentally, there’s a free pattern I’ve wondered about. It’s printable in a range of sizes but without seam allowances… would using the next size up work (instead of adding a seam allowance to my required size)?

    • Hi Clipped Curves,
      Good question! The answer is: it’s not a good idea but you might get away with it for a simple skirt, a loose garment or something with lots of right angles, but the results wouldn’t be “perfect”. Where you’d notice the imperfections the most is at the angles such as armholes and neckholes where you’d lose a bit of the corners. Let me know if you try it!

  5. You have no idea how much you’ve helped me. I really am not a natural “sewer” and pretty much always struggle with everything i try to do. When it came to seam allowances it confused me, sometimes. me. You’ve clarified many things. Again thank you and for your wonderful little tricks 😉

  6. I made a template for a seam allowance that is a circle made of a stiff material like card or plastic with a hole in the middle with the radius being the size of the seam allowance. I put the tip of the marker through the hole in the middle, the edge of the circle goes along the edge of the pattern.

  7. I just use a sewing machine with no thread and stitch around with whatever seam allowance I need and get a perforated edge. Super quick and accurate!

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