Seam allowances can be the bane of the sewer’s life, from quilt blocks coming out too small or too big, to clothes gaping or bunching, to bag gussets coming up long or short, how exactly does one measure them and ensure that they come out just right?

Establishing what the seam allowance should be:

The first step is to check the pattern so that you know what the seam allowance ought to be (not that I’ve ever used the wrong one *ahem*).  For quilts, the majority of patterns use a 1/4″ seam allowance, but for other things it very much depends on the designer.  The big commercial patterns, such as Butterick and Vogue, often use a 5/8″ seam allowance, while smaller companies can vary greatly – I usually use a 1/2″ seam allowance for my bags so that there is enough seam allowance not to rip through, and also because it’s much easier to measure to add on to the finished size than 5/8″ is!

Measurement options:

Once you’ve worked out what it should be, the next challenge is how to measure it on your machine.  There are three options, the first is to measure using the foot on your machine, the second is to use the markings on the needle plate of your machine (if you have them) and the third is to use a seam guide.  There’s no particular right or wrong way but it may take a bit of experimentation to work out which is best for you, and then how to get the perfect seam allowance using that method.

Using your foot to measure:


These are the 3 feet that came with Big Brother than I can use to measure.  The one on the left is the Standard foot – note the notches marked at the bottom of the opening for the needle, the centre one marks the centre needle point, and the right hand one is for 1/4″ from the starting needle position on the left.  The one in the centre has multiple markings in 1/8″ increments starting from the centre up to 1 1/8″.  The one on the left is a 1/4″ foot and has a bladed edge on the right that is meant to give a 1/4″ seam allowance when the needle is in the centre – I say meant to, but the reality is it’s around 3/8″ I suspect partly because the bladed bit got a bit bent at some time before I got it.

Using your needle plate to measure:


As you can see above, I actually have 2 needle plates for Big Brother.  One of the most important things on a zig zag machine (ie one where the needle can move) is to understand where the measurements start from, so let’s look at these needle plates a bit more closely (if you have a straight stitch only machine, you can skip the first one)


This is the standard plate that comes fitted to the machine.  Measurements on this plate are based on starting with the needle on the left, although if you choose a zigzag stitch it will zig zag on either side of the centre point.  The needle can be moved up to 7 mm in 0.5 mm increments using this plate on Big Brother, however your machine may go a little wider or narrower.


This is a straight stitch plate where the needle must be in the centre and can’t be moved at all.  Measurements on this plate, therefore, start in the centre – you can see the difference from the first one by looking at the lines around the right hand screw hole.

Using a seam guide:


Here are 3 of the seam guides that I have in my possession, although other guides are available.  The top two screw onto the bed of the machine (the screw isn’t pictured for the left hand one) and the screw allows the guide to slide backwards and forwards to the required distance.  The lower one is magnetic – do not use one of these on a computerised machine!  This one is the easiest of all, just plop him on the bed of the machine at the required distance and away you go.  You can also use washi tape and the like, stuck in place, but if you use many different seam allowances on a regular basis, you can end up with a ridiculous number of layers (Little Brother suffered terribly from this!)

Other options:


There are other options for more specialised sewing, this one in particular is my favourite for using when sewing half square triangles, the Clearly Perfect Angles cling by New Leaf Stitches.  Have a look around the notions section in your favourite shop, you may find all sorts of things like this!

Checking that the measurement is correct:

Once you’ve chosen how you want to measure your seam allowance, you need to check for accuracy.  To do this, take two pieces of fabric that you know the size of, and that you know what the finished size should be when sewn together.  For example, if you had 2 squares of fabric, each 3″, after they were sewn together you would have:

  • A piece 5 1/2″ x 3″ with a 1/4″ seam allowance
  • A piece 5″ x 3″ with a 1/2″ seam allowance
  • A piece 4 3/4″ x 3″ with a 5/8″ seam allowance

If the piece does not come out correctly, then you may need to adjust either the needle position or the seam guide until it is correct.  Bear in mind that when you open the fabric out away from the seam, there is always a thread or two of the fabric that is in the actual fold, and the thicker the fabric, the more may be consumed by the fold.  This is why we talk about scant seam allowances, where the distance from the needle to the measuring point is actually just under the seam allowance value.

If you’re using a seam guide and want to remember where the sweet spot was if you have to move it, using an old needle and an index card, put the edge of the card up against the guide underneath the needle, then lower the needle into the card.  When you come to use that seam allowance again, you can place the card with the needle through it and move the guide until it’s up against the edge of the card.

Happy measuring!