Quarter Inch Seam Accuracy: Why Pressing Direction Can Change Your Quilt Block Size
If you have ever measured a quilt block and found it was about 1/8 inch smaller than the pattern, no worries! It usually comes from something more nuanced than “you did it wrong.” The accuracy of one’s quarter inch seam can be dependent on a combination of many things, including: cutting, sewing, and pressing. If one variable is slightly off, the finished measurement can shift, even when you feel like you did everything right.
In this post, I’m sharing a simple strip test I ran, and what happened when I changed just one variable by pressing seams open and then pressing seams to the side. At the end, I’ll share what options you could pursue if your blocks are running slightly small.
What “quarter inch seam” really means in quilting
A quarter inch seam allowance sounds straightforward, but there can be more variables in play that can affect your finished size:
- How accurately you cut
- How consistently you sew
- How your machine forms stitches
- Thread thickness
- Fabric weave and how much it compresses
- How you press seams, including direction and pressure
The strip test experiment: 1.5 inch strips into a 1 inch finished result
To isolate the variables, I used a classic test laid out below. In theory, 1.5 inch pieces sewn with a quarter inch seam should finish at 1 inch each. This is a clean way to check whether your sewing and pressing habits are landing where you think they are.
- Cut multiple strips at 1.5 inches wide.
- Sew them together using a quarter inch seam allowance.
- Press the seams.
- Measure the thickness of the inner strips with a ruler, with the goal of 1 inch finished.
Results: pressing seams open vs. pressing seams to the side
Here’s what surprised me…
When I pressed seams open, the measurements stayed accurate down the strip set (see photo above), and the finished width continued to match the ruler in a satisfying way. But when I pressed seams to one side on another strip set, the first sections looked accurate enough. But as I looked down the strip set, the measurement slowly drifted away from the ruler (see photo below). It was just a tiny shift that became more noticeable the more seams were stacked in the same direction.
And this is probably what happened…
Pressing seams to the side creates a fold, which creates added thickness that asks fabric to travel a slightly longer path around the thread. And that effectively “uses up” the slightest bit more fabric. One seam pressed to the side might not show much difference but if a block has many seams, the smallest change can compound. And so this is why I now understand why some quilters prefer pressing open for accuracy and flatter results.
The part that makes this complicated, at least in my eyes, is that I do love pressing seams to the side. When seams are pressed open, the fabric is folded away from itself and the seam is held together mostly by thread. Pressing to the side keeps the fabric layers overlapping, which can add strength once the quilt is quilted. I also like pressing to the dark side to help hide seam allowances and bulk behind darker fabrics, and I love when seams nest cleanly because they’ve been pressed in opposite directions.
If your quilt blocks are 1/8 inch small, here are your options
If your quilt blocks are coming out about 1/8 inch smaller than expected, the first thing I want you to do is take a breath. Quilts are remarkably forgiving, and consistency matters far more than hitting a perfect measurement every single time. A block that is slightly small but consistent across the quilt can still come together beautifully.
One option is to simply keep sewing the way you are. If your blocks are consistently 1/8 inch small, you may find that everything still fits together smoothly as long as the same habits carry through the entire quilt. In many cases, this has little to no impact on the finished look, especially once the quilt is quilted and used. If it is extremely important for you to accurately match the finished size of the quilt, you could add a border around each block and trim to size, or add a border around the edge of the quilt top.
Another option is to experiment with a scant quarter inch seam. This can be particularly helpful if you prefer pressing seams to the side. A scant seam, which is literally just a hair (or thread) smaller than a true quarter inch, can help account for the tiny amount of fabric that gets taken up when seams are folded and pressed in one direction. This adjustment can make a noticeable difference in blocks that contain a lot of seams.
You might also choose to press seams open more often in areas where many seams come together. I know I threw a little shade on pressing open earlier, but there are still benefits in doing so. Pressing open reduces bulk and can help measurements stay closer to their intended size, especially in complex blocks with lots of seam intersections.
Quilting as an art and the point is the process
This whole experiment brought me back to something I learned through music. In any art, there’s an ideal we practice toward. As we improve we notice more nuance, listen to more details, our technique improves, and our standards start to shift. And because of that, we’re essentially never going to reach perfection, we are just improving. And the journey is the main point.
Hopefully, you’ll see that quilting can be approached in the same way. First you learn about seam allowances and how to use the right guides (and feet) to help you stay consistent with that. And now we can see that our approach to pressing can have an effect on the finished size of our blocks and quilts. There are variables that interact with each other, and hopefully this awareness is added to the appreciation and a part of learning the craft.
So if your blocks are off by 1/8 inch, no worries! Hopefully this post has loaded you up with some understanding and options with which to move forward. And whatever you choose to do, I hope you see that the goal doesn’t have to be perfect measurements as much as it is about the time you spend in your sewing room making something beautiful with your own hands.
Try the experiment yourself
If you try this strip test, I would love to hear what you notice. Different machines, threads, fabrics, and pressing habits can all change results!
Share your results and tag me so we can compare notes: contact@sweetpotatoquilts.com
