How to Test for an Accurate 1/4" Seam Allowance

Sewing an accurate and consistent seam allowance is important for every quilt top, but it becomes especially important if you are piecing a quilt top that has a lot of seams. This is because an inaccurate seam allowance can add up quickly and will show in your pieced block and ultimately your final quilt top. In this blog post, I am going to show you how to test your seam allowance so you’re getting an accurate 1/4″ seam allowance every time. I will also explain the mysterious scant 1/4″ and why you should use it.

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A really important step, before you begin piecing your quilt, is to do a simple test to check your seam allowance. This is a quick and easy test that you can do with a scrap of fabric. It will give you confidence knowing you are piecing your quilt blocks accurately and allow you to get to know your sewing machine a little better.

What is Seam Allowance

Seam allowance refers to the distance between the stitches sewn and the edge of the fabric. Most quilt pattern directions specify a 1/4″ seam allowance, while others specify a scant 1/4″ seam allowance (more on that later). Most quilting patterns will not call for a larger seam allowance. However, you may find a 5/8″ or 1/2″ seam line required in sewing patterns for bags and clothes.

How to Test Seam Allowance

To test your seam width, cut at least 4 strips of fabric. The more seams you use in your test block, the more noticeable an inaccurate seam allowance will become. I suggest 2″ or 2.5″ wide strips (I used 2.5″). Make sure your strips of fabric are cut accurately. If your strips are not cut precisely, then this test will not be accurate.

Place two strips of fabric together, right sides together, taking care to line up the raw edges as close as possible. Stitch the long side of the pieces of fabric together using what you believe to be an accurate 1/4″ seam allowance. Repeat this process with the remaining fabric strips. Press the seams open and measure your sewn unit. Because I used four 2.5″ strips of fabric, the finished test block should now measure 8.5″ wide.

To figure out how wide your strip set should be, multiply the width of your strips times the number of strips used and then subtract the seam allowance (3 seams with 1/2″ lost to each seam). (2.5″ x 4) – (0.5″ x 3) = 8.5″.

If your strip unit is shorter than 8.5″, then you know your seam allowance is too wide. If your strip unit is longer than 8.5″ then your seam allowance is too narrow (or too “scant”).

Make any necessary adjustments and then repeat the test. I found that even a millimeter or 2 made a difference in the outcome.

What’s a Scant 1/4″ Seam?

A scant seam allowance can be very confusing for many quilters. I’d be lying if I said I didn’t cringe a little when I read “scant 1/4” in a pattern. Nonetheless, knowing how and when to use a scant seam allowance is very important for your overall piecing accuracy and is a handy tool to have in your quilting toolbox.

A scant 1/4″ seam is just slightly smaller than a standard 1/4″ seam allowance. Technically, it’s about 1 thread width smaller. The reason for this is that after a seam is pressed, a little bit of fabric has to wrap around the thread, causing your finished seam to be too short. This is most noticeable if you use a heavier-weight thread. Stitching with a scant 1/4″ helps to compensate for this small loss of fabric within the seam and keeps the dimensions of your quilt block accurate. Using a scant 1/4″ is more important for some quilt patterns than others, so it’s important to follow the advice of the quilt pattern writer when it comes to pressing and seam allowance directions.

Accurate Piecing Tips

Start your test by using the 1/4″ marks provided on your sewing machine (if there are any). If your sewing machine doesn’t have any 1/4″ marks (or they’re inaccurate) you can put some washi tape, diagonal seam tape, or even masking tape, on your machine. Just measure 1/4″ from the center needle position and lay the tape flat across the throat plate, making sure to avoid the feed dogs or any other moving parts. I have also seen quilters stretch a large rubber band around the throat plate. Test a few different methods to find what works best for you.

diagonal seam tape for accurate quilt piecing

When I am piecing, I prefer to use my patchwork foot (Bernina #37) which is designed specifically for 1/4″ seams. A quarter-inch foot like this allows you to line up the edge of your fabric with the edge of the presser foot and produce a pretty accurate and consistent 1/4″ seam.

If you don’t have a patchwork foot and are using a different foot, like a standard presser foot, I suggest using the guide line on your sewing machine. Some older machines don’t have any guide lines, if this is the case for your sewing machine, then I would suggest using the tape line described above.

Pressing Tips

A quick note on pressing… technically, pressing your seams open is going to lead to a more accurate seam allowance. However, the direction in which you press your seams can depend on the quilt patterns you are using and the piecing technique you choose. Pressing your seams toward the darker fabric, rather than open can make a small difference and is just something to keep in mind when you are considering your seam allowance.

Recently, I have become a huge fan of starching and pressing my pieced units. I have found that this is a super simple way to improve my accuracy when it comes to piecing. I would use a product like Best Press or Magic Quilting & Crafting Spray (my personal favorite).