The Elusive Quarter-Inch Seam Allowance

18 01 2012

Have you ever carefully cut the pieces for a block or quilt, then just as carefully sewed the 1/4″ seam allowance and yet the pieces didn’t fit together correctly?  Most likely it wasn’t a mistake in cutting, but rather that 1/4″ seam allowance wasn’t really a 1/4″.  I can hear more than one response, “but I used the quarter-inch foot,” or “I set the quarter-inch setting on my machine.”  Sadly there is no guarantee either of those things will result in an accurate 1/4″ seam allowance.  Actual settings for a quarter-inch seam vary from machine to machine.  So to ensure an accurate seam allowance you need to test your machine.  Here’s how:

1.  Cut three strips of fabric that are 1-1/2″ wide, my samples just happen to be 5″ long.

2.  Stitch the long sides together using either a quarter-inch foot or the quarter-inch setting on your sewing machine.  I use a longer stitch so it is easier to remove the stitches from the test piece if the seam allowance isn’t exactly 1/4″

3. Finger press the seams, and measure the center strip. If the seam allowance is accurate that measurement should be exactly 1″.  In the picture below the arrow points to the edge of the center strip.  You will notice the strip is slightly less than 1″.  Which means the seam allowance is more than 1/4″.

4.  Since the seam allowance isn’t accurate, remove the stitches . . .

5. and try again, after changing the needle position.  This is how my screen looked when the needle was in the center position.


Because I determined my seam allowance is too wide, I moved my needle one position to the right.  (If the seam allowance was too narrow and your center strip measured more than an inch you would need to move the needle to the left.)

6.  After stitching the seams, I again finger press and measure the center strip.  This time my strip is exactly 1″.  If your’s isn’t repeat the above steps until it is.

7.  Just to be absolutely sure, I press the strips with an iron.

8.  Then measure one more time.

9.  Now that you have that perfect quarter-inch setting, write it down!  If you change your thread to a different weight you will need to test again.  Yes, thread weight can affect the seam allowance width.

On a side noet, but related — if you have trouble with the sewing machine pulling the beginning of your pieces into the feed dogs, you might want to consider a straight stitch plate for your machine.  Below is a picture of the two throat  plates for one of my machines.  The one on the left is the stand zig zag plate that came with the machine and the one on the right is the straight stitch plate.

If you do use a straight stitch plate, be careful when adjusting the needle position to get an accurate 1/4″ seam.  The plate is intended to be used with the needle in the center position, however the hole was large enough on mine that moving the needle one position did not result in broken needles.  I think two positions from center is probably the most I could squeeze out.

My seam allowance was just slightly off on my first try.  Some might think, what’s the big deal.  Let’s say you were working with 2-inch squares.  If you were off just 1/16″ of an inch, in 16 squares you would have lost a whole inch.  So what should have been 32″ is now 31″ and if you were making those two-inch squares with half-square triangles you have lost more like two inches.  It adds up faster than one might expect and can cause loads of frustration!




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