What's a 1/4" Seam Allowance?

That pesky 1/4" seam allowance.  It's the standard in quilting.  It's the key to making things fit together.  It's everywhere, but what does it actually mean in practice and how do you measure it?

Here's the most important thing I want to say about the 1/4" seam allowance.

It isn't the distance from the edge of the fabric to the line of stitching.  It's whatever gives the correct result.

Then why is it called a 1/4" allowance?  Because 1/4"  is the measurement we use for the quilt math we do to figure out how make parts fit together the way we want them to.  

Quilt math works with a perfectly 2-dimensional design, but fabric and thread exist in a 3-dimensional world. When you press a seam, the thickness of the thread and the thickness of the fabric itself result in the fold taking up a bit of the fabric.

In the 3-D world, what's left in the block is what you originally cut, minus 1/4" and that little bit lost in the fold.  You'll need to take that little bit into account for things to work out as planned in a pattern.

Read on for more questions and do's and don'ts of seam allowances.

How do I measure the seam allowance?

If you sew two 2" squares together, you should expect  a 2" x 3 1/2" unit (mathematically, each square loses 1/4" into the seam, so 2" - 1/4" + 2" - 1/4" = 3 1/2").

So, sew two  2" squares together with what looks like 1/4" seam allowance, press the seam to one side, and measure the unit.  

• If it's shorter than 3 1/2", your seam ate too much fabric.  Try again with a narrower seam allowance.

Seam allowance is too wide

• If it's longer than 3 1/2", your seam didn't eat enough fabric.  Try again with a slightly wider seam allowance

Seam allowance is too narrow

• If it measures 3 1/2", you've found the correct seam allowance.  You can mark the bed of your machine with tape or a stack of sticky notes as a guide to where to position the edge of the fabric to repeat that seam allowance, or take note of the needle position if you adjusted that to get the right seam allowance. 

Seam allowance is correct

You might be tempted to skip testing the seam allowance.  I get it.  Let's start building blocks already! Nevertheless, fudging might slow you down or mess you up later, and lead to more fudging to fix issues caused by earlier fudging, and so on, and so on.  I highly recommend taking the time now to save time later.

Can I just sew a "close enough" seam, as long as I'm consistent in all my seams?

Unfortunately, most of the time, this won't work out.  Sorry, it's a math and geometry thing.

For example, suppose a pattern includes this unit.

The pattern gives cut sizes that will make the two squares sewn together add up to a unit that is the same length as the rectangle it's being joined to, so they will fit together.  If the two squares add up to a shorter, or longer measurement, they just don't fit together, even if you use the same seam allowance on the horizontal seam.  The length will still be mismatched.  

Seam allowance too narrow

Seam allowance too wide

Can I just trim off any extra off the sides if parts don't quite match?

There are at least two possible problems with that approach.

Changing the dimension of a unit 

For example, in the unit I showed with a seam allowance that was too wide, trimming off the excess on the lower rectangle will make the unit measure 3 1/2" x 3 3/8" instead of 3 1/2" square.  It doesn't look like much of a change, but it's going to change how well it fits with other units in your pattern.  Over several units, it can really affect the total size in one or more directions, and possibly how square your project is.

Changing where seams fall in the unit

For example, in the unit I showed with the seam allowance that was too wide, shaving off some of the white is changing the proportion of the white piece.  When the green square and the white square were the same size, the seam was exactly in the center.  The white is now smaller than the green, and the seam is no longer in the exact center of the unit.

Depending on the design this might not be a big deal.  However, if the design needs that seam to line up in a particular way with some element in another unit, shifting that seam line away from the center might mess up the alignment.

Any other questions?

I think I've covered the basics. Do you have any other questions about seam allowance?  Add them in the comments or send me an email.  Now I'm off to sew the units for Week 2 of the Two-Colour Mystery.  After testing my seam allowance, of course.

Happy quilting,

Joanne