Lesson 1: Allowing for Seam Allowances

In quilting, the standard seam allowance is 1/4”. So, when figuring out what size to cut pieces for quilt blocks, you need to add ¼” for each seam allowance, or ½” total to each piece. If the finished size of the block below is 6”, that means the finished size of each square will be 3”. Therefore, the size of squares to cut for this block will be 3 ½”.

In the next example, if the finished size of the block is 10”, that means the finished size of each rectangle will be 2 ½” x 5”. Therefore, the size of rectangles to cut for this block will be 3” x 5 ½”.

Practice: If the finished size of the blocks in this example is 12”, what size would you cut each piece?

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Taking it to the Next Level: Half Square Triangles

Many quilt blocks use half-square triangles as elements. The math for diagonal seam allowances is different than the math for square or rectangular blocks. The simple formula for figuring out how big to cut squares for half-square triangles is to add 7/8” to the finished size of your half-square triangle unit, then cut the squares in half once diagonally to make half-square triangles that you will then sew together with a ¼” seam.

Take Note: Many quilt designers add MORE than 7/8” to the finished size of their half-square triangles. That leaves plenty of room to square the half-square triangle up after it is pieced together. ALWAYS REMEMBER that you add at least 7/8” to the finished size for cutting, and when you square up the half square triangle unit, add ½” to the finished size of the unit, and square it up to that measurement.

In the following example, the finished size of the half square triangle is 3”. Therefore, you will cut squares that are 3 7/8”, cut them in half once diagonally, piece together the resulting half-square triangles, and square them up to 3 ½”. Then, when you sew the half-square triangle into a quilt block, the finished size of the unit will be 3”.

Practice: Determine what sizes to cut for the half-square triangles in the following examples, then determine what size to square up the resulting half-square triangle units to.

A. B. C.

Finished Block Size: 2 ½” Finished Block Size: 15” Finished Block Size: 8”

Cut Size: Cut Size: Cut Size:

Square to: Square to: Square to:

Assignment:

Figure out the measurements to make a 6” block from this month’s block, Diamond Panes, on p. 81.

Definition

Finished Size: The size of a block element AFTER it has been sewn into the block.

Perfect Piecing Tip: Strive for the Perfect Quarter Inch

Here are a few ways to achieve the perfect quarter-inch seam:

1. Use a ¼” foot. Many newer machines come with a ¼” foot. If yours doesn’t have one, you may be able to purchase it separately from a sewing machine dealer. Please note that all ¼” feet are not created equal! You will still want to check your seam allowance (See Below) and make adjustments as needed. 2. Visually determine where the ¼” is on your existing presser foot by using an acrylic ruler. Lay the ruler under your presser foot. Carefully lower the needle to it touches the ¼” mark on your ruler. Notice where the outer edge of the ruler is…is it right on the edge of your presser foot, or slightly inside or outside the edge? That is where you will want your fabric to line up as you sew. 3. Use a piece of painter’s tape, moleskin, or weather-stripping to serve as your guide. Line up your acrylic ruler as described in #2, then put the tape on your machine, along the edge of the ruler leading up to the presser foot.

Checking Your Seam Allowance:

Cut three 1 ½” squares. With right sides together, stitch them together in a row, using a ¼” seam. After stitching, press your seams to one side. Measure the center square of the newly stitched row. The width should be exactly 1”.

1”

If your center square is too wide or too narrow, it may indicate an issue with cutting or pressing or it may be caused by the bulk of the thread. When seams are pressed to the side, one layer of fabric is pressed over the seam and seam allowance. The bulk caused by the seam and seam allowance can make the block slightly smaller.

If you find that your blocks are consistently a thread or two shy, you may want to adjust your quarter inch seam to a scant quarter-inch seam — a seam allowance that is just a thread or two shy of being a full quarter inch. You’ll find that this will resolve any issues created by the seam and seam allowance.