What is a scant 1/4″ seam allowance? Why do I need it? How do I get it? This post will answer all your questions about this quilting term!

What is it?

A scant 1/4″ seam allowance is a seam allowance that is ever so slightly narrower than one 1/4″ inch. You can notice here that where the seam folds in, the top of the assembly dips a little. A few of the fibres of the fabric that get swallowed up in that dip.  A scant 1/4″ compensates for the loss of some fabric in the folds of a seam, so that the dimensions of your block remain true.

Why should I care?

See this block here? It’s from my newest pattern, Striped Scallops. There are eleven vertical seams in this 12 1/2″ (unfinished) block — ELEVEN! If you use a “true quarter inch” seam, you lose a little bit in each seam and by the end of it all, it’s noticeably smaller than that bottom rectangle, which has been accurately cut at 12 1/2″. See the teal block below.

In my experience, ignoring a scant 1/4″ is most noticeable and puzzling when a pattern has seams meeting a perpendicular (such as Banner Year and even a traditional log cabin). Multiple seams meeting at a single perpendicular, like in this Striped Scallops, exacerbate the issue. The tiny bites those seams take out your fabric start to add up!

How do I get it?

It’s a matter of testing! Cut six 2 1/2″ squares. It’s best to use contrasting fabrics and alternate them. Don’t follow my example of this six-squared gradient. Although it’s pretty, it’s harder to see what you’re looking for!

Sew the squares together into a row, using a reference that you think is a good 1/4″ seam allowance. This might be a tape line on your sewing machine, a 1/4″ foot, the edge of your normal foot. Press all the seams to one side or press them all open — whichever is your general preference. Get your ruler out and see what you’ve got.

The six squares above should theoretically add up to a row that measures 12 1/2″, right? It is actually only 12 1/8″! Yikes. If you look at the 8 1/2″ mark and follow the squares going to the left, you can see that each is a bit smaller than 2″. This means that the seam allowance is too big and I need to make it narrower.

Adjust your reference – move your tape line, change your sewing machine foot, or adjust your seam guide. I use a Janome Seam Guide (affiliate link) on my Juki. Repeat the exercise with new squares till you get accurate 2″ widths and the right overall length.

This one is five squares; each colour is 2″ and the overall length actually measures 10 1/2″. This is it! We’ve found it!

Keep a reference

I “recorded” this accurate scant 1/4″ on a postcard so that I can set up my seam guide in a jiffy. Once I found the scant 1/4″, I sewed a line on a postcard and labelled it. I keep it next to my sewing machine so if I have to set up my scant 1/4″ with my seam guide, it’s easy to find.

Some considerations

Pressing seams open vs. pressing seams to one side: This here is not a debate of which is better – open or to one side. Rather, I am telling you if you prefer pressing your seams to one side, make your scant test that way. If you are a press-open type, do your test with your seams open. Each method will yield different results because there is a differing amount of fabric folded under at the seam. No use testing in conditions that are not true to reality!

Different substrates: Different fabric manufacturers and different weights of fabric will have a different scant 1/4″ because of the amount of bulk at the seam. I usually use one manufacturer for a whole quilt and mostly in quilting cotton, so one test for that manufacturer usually suffices. If you like mixing manufacturers (and you should, it’s fun!), it gets a bit dicier. However, unless you are working with vastly different substrates — like cotton lawn and quilting cotton — you should be fine. Art Gallery has a noticeably different “hand” (different tactile feel to the fabric) because of its thread count so this may warrant some experimentation if you’re mixing it in. A linen/cotton blend is also noticeably heavier and will need another test.

This fabric is Spoonflower’s New Petal Signature Cotton. It felt a bit heavier than the quilting cottons I was used to using so I thought I should test it out with 5 x 2 1/2″ squares. And my hunch was right… I needed a narrower 1/4″ to get it to be accurate. This test doesn’t take too long, so if you’re in doubt, do a test with your chosen fabrics before you start a project. Especially if the project at hand is a higher-stakes one — high time or fabric investment!

There you have it. A scant 1/4″ seam allowance. It’ll help you with your accuracy!