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How to Calibrate Your Scant 1/4" Seam

Maybe you've heard about it before, or even know what it is, but have never actually sat down and calibrated your scant 1/4" seam allowance. This tutorial will take you through that process step-by-step so you're sewing perfectly accurate scant 1/4" quilting seams.

First things first: some ground rules

There are a couple of really important things that you want to do in order to do a useful 1/4" seam calibration.

  1. Make sure you sew exactly as you do when you piece quilts. That means sewing with the same thread, the same presser foot, at approximately the same speed, and with the same amount of care that you would normally use when you're piecing quilts. (if you sew fast while watching Netflix, sew the calibration fast while watching Netflix.) It also means pressing according to your usual preference. If you tend to press open sometimes and to one side at other times, you are going to want to do multiple calibration pieces.

  2. You have to calibrate off of a sewn piece. There are quite a few tutorials out there proclaiming that you can calibrate off a seam gauge or by just measuring the distance between your needle and the edge of your foot. This is not true. while those techniques might get you into the ballpark range, if you want true precision, you have to actually do some sewing. This is because fabric is lost when it folds (hence the need for a scant 1/4" to begin with) so if you're not sewing with fabric and folding fabric to do your calibration, you're not going to be accurate. You can read more about this here.

Step 1: Prepare your fabric

To perform a calibration, you will need 3 pieces of fabric. These can be small scrap pieces and don't have to be anything special (as long as they're quilting cotton, or whatever fabric you happen to be using). The actual calibration piece is the centerpiece, and only the size of it matters; the others can be any size.

Your calibration piece can be whatever size you like as long as you know what size the piece should be when it's sewn into a block. Practically speaking, I recommend cutting a strip at 4.5" x 1.5". This size will give you a good idea of how consistent you are sewing as well as reveal any inaccuracies easily so you can make adjustments.

When you cut this calibration piece. Make sure you are very accurate. Iron it beforehand (starch only if you normally starch your fabrics), cut carefully, and verify the measurement after you cut it to make sure it is very accurate otherwise you're whole calibration will be off.

For the other two pieces, they can be any width strips as long as they are at least as long as your calibration strip (4.5"). I've cut the same sized pieces (4.5" x 1.5") for demonstration just for simplicity.

NOTE: If you normally press seams to the side, you are going to want to prep at least 2 calibration sets because you are going to need to do this calibration twice. More on that later.

Step 2: Sew your seams

Piece one outer strip to one side of your calibration strip. Piece the other to the opposite side of the calibration strip. Now your calibration strip should be sandwiched in between two other pieces of fabric. As you piece, do everything the same as you normally would when you're quilting. You're just attaching fabric in this step.

Step 3: Press your seams

Press each of the seams you just sewed on either side of your calibration strip according to your preference. If you normally press open, press both seams open. If you normally press seams to the side, sew two sets of calibration sandwiches and press one set with both seams towards the center strip and one set with both seams away from the center strip. If you normally do a mixture of pressing open vs. to the side, sew 3 calibration sandwiches and press one open, one towards the center, and one away from the center. This is really important because the fabric folding during pressing is what controls how scant you need to sew your seam.

Step 4: Measure and assess

Once you have your calibration sets sewn and pressed, place your ruler over the center calibration strip and measure it's with seam-to-seam. Because we cut that strip at exactly 1.5", if our scant 1/4" seam is calibrated, its sewn width should be exactly 1" wide. Exactly 1"; not a thread over or under.

Here, you can see I'm just ever so slightly under 1".

If your strip is wider than 1", your seam is too scant. Move your needle away from the sewing edge of your 1/4" foot by just a couple of notches and repeat this calibration process starting from Step 1 changing nothing about how you are sewing. We're adjusting your machine to how you sew, not trying to change your sewing habits.

If your strip is narrower than 1", your seam is not scant enough. Move your needle closer to the sewing edge of your 1/4" foot by just a couple of notches and repeat this calibration process.

Rinse, wash, and repeat until your calibration strip is exactly 1". Then, and here's the important part, WRITE DOWN THE SETTINGS that have allowed you to achieve a perfect scant 1/4". Put it on a piece of masking tape and stick it to your sewing machine so you know exactly how you need to sew up your sewing machine each time you sit down to precision piece. For instance, I know on my machine with my flanged 1/4" foot, I need to set my stitch width (needle position) to 9.0 mm in order to get a scant 1/4" seam. That's where I set it EVERY time I sit down to piece. This also not what the manufacturer says will be my scant 1/4"; I needed to adjust it for the particular way I sew.

About that pressing to the side. It is entirely possible that if you press to the side and you sew two calibration pieces, one piece may end up narrower than 1" and the other wider than 1". If that happens to be the case for you, the issue is with your pressing technique. You are not as accurate in pressing one direction vs. the other (i.e. the seams aren't completely extended). This is something you will need to address before you can calibrate your scant 1/4" seam accurately and it's also one of the reasons why I like pressing open so much. More on that to come next month.

That's it! Once you've calibrated your scant 1/4", just set it and forget it. Every time you piece you can rest assured that your seam is accurate . . . for at least a little while anyways. Once a year (or so), or right before I start a really precise project, change the type of fabric I'm using, or change the type of thread, I will repeat this calibration process to make sure that I am still accurate. Why? because your technique can drift over time. Maybe you injured your arm and you don't really even notice how that's changed your sewing because it doesn't hurt anymore, but now your seams are listing to the left. Maybe your machine might start sewing differently after a service. Or maybe you're sewing with linen instead of cotton for a project. All of that can change how much fabric you lose in the opening of the seam. Most of the time you will find that things are still pretty close, but on a precise project, it really matters if your seams are accurate and this simple exercise can make a huge difference!


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