Make 8 Half-square Triangles Using 2 Fabric Squares

Photo of block made with half-square triangles

by Cathy Ellsaesser, posted on

One of my UFOs is a quilt with different scenes from Alaska. Each block has a different quilting block as feature, along with an appliqué. The block for the bear has sixteen 1¼” finished (1¾” unfinished) squares made from half-square triangles. Now, I am really new to piece quilting, so tricks and hacks that make sewing those little pieces of fabric together are a blessing to me. I am sure that all of our experienced quilters are rolling their eyes at my new-found wonder on how to make eight half-square triangles by sewing four seams across the diagonals of a square. Since I didn’t know about this technique, chances are some of our customers new to quilting don’t know about it either.  Thus, the topic of today’s blog!

Photo of cover of Northern Wilderness quilt pattern

What Size Square for Eight Half-Square Triangles

In several quilting books and instructions I usually see instructions to cut out squares of fabric then cut them into two triangles. These individual triangles are then sewn together on the diagonal to make a square the size needed.

Photo of instruction page for bear showing 16 half-square triangles

So the cutting directions do not tell you how to cut a square to make 8 appropriately-sized triangles. Don’t worry, though, the math on this one is easy!

  1. Determine what size finished triangles you need.
  2. Add 7/8″ to that measurement.
  3. Multiply that sum by 2.

This is the size you need to cut your square.

For example: To make 2″ finished half square triangles;

  1. Add 7/8 to the 2″ measurement. Your total is now 2-7/8″.
  2. Multiply 2-7/8″ by 2. That equals 5-3/4″.
  3. Cut two 5-3/4″ fabric squares to create eight 2″ finished half square triangles.

For my 1¼” finished squares, I cut  4¼” blocks from the brown fabric and the beige fabric to make the half-square triangles ((1¼ + 7/8) x 2 = 4¼”). Each square will make 8 half-square triangles, so I’ll need two squares of each fabric.

Fabric squares to be used to make 8 half-square triangles

Mark The Square for Sewing

Use any type of marker that won’t bleed into (or rub off onto) fabric. I like to mark the lighter fabric with a fine point pencil, but if you have dark fabrics, just keep a light marker handy.

Note: Fabric is usually easier to handle when you use sizing or another pressing aid like Mary Ellen’s Best Press to add stiffness.

Draw diagonal lines on the reverse side of one of the squares as shown. If you have a ½” wide ruler with a center line, mark the seam lines easily by placing the center line of the ruler along the diagonal of the square and then drawing the ¼” seam lines on either side. Otherwise, draw a diagonal line from corner to corner and then mark parallel seam lines ¼” from that diagonal line. If using a ¼” presser foot, you don’t need to mark the sewing lines.

Photo of marking seam lines on fabric block to make half-square triangles. Photo of pencil lines drawn to indicate seam lines for half-square triangles

Sew the Seams

  1. Align the two fabric squares, right sides together and all edges matched. Secure with pins to keep fabrics from shifting.
  2. Sew on the marked lines if using a ½” ruler as described above. If you only marked the center diagonals, sew a seam 1/4″ on each side of those.
    Photo of seam lines sewn for half-square triangles
  3. Press the sewn square to set the seams, a step that removes little puckers and makes it easier for fabrics to lie flat later.

Cut Out the Triangles

Note: It is really helpful if you have a rotating mat, or a small mat you can rotate as you cut. A non-slip ruler like the Quilter’s Select rulers are also very helpful in keeping your fabric from shifting as you cut.

  1. Cut each of the drawn cutting lines along the diagonal, from corner to corner.
    Photo of cutting sewn square to make half-square triangles  Photo of cutting sewn square to make half-square triangles
  2. Slice through both layers of the sewn square through its vertical midpoint. Use the center ‘box’ created by seams and the original marked lines as a guide,
    Photo of cutting sewn square at vertical midpoint to make half-square triangles
  3. Now cut along the horizontal midpoint of the square.
    Photo of cutting sewn square at horizontal midpoint to make half-square triangles
  4. Now you have 8 half-square triangles
    Eight half-square triangles created from two squares of fabric
  5. Place the eight unopened HST units on your ironing board with the fabric seams will be pressed towards facing up. Press one more time, unopened, to set the seams.
  6. Flip the top fabric of one unit back and use your iron to (carefully) press the unit open. Note: The eight HSTs still have their little ‘dog legs’ attached — tiny triangles that extend past the seam when patchwork is pressed. Clip them off before use.
  7. Repeat for all HST units.
  8. Sew together as pattern requires.

Here’s a diagram of all the sewing and cutting lines to make 8 half square triangles.

Diagram of sewing and cutting lines to make 8 half-square triangles

Here’s my finished quilt square.

Photo of block made with half-square triangles

Now I just have to add the rest of the fabric and applique to finish the block.

After adding the borders and the applique, I have a completed block.  (9 down, 7 to go!)

Photo of completed quilt block with half-square triangles

Cut to Size

If you’re new to all this and worried about accuracy, make your squares ½” larger than called for. You can then cut your half-square triangles down to size after sewing and cutting. This is a little more work, as you have to cut around each triangle, but it gives you a little cushion to make sure your final half-square triangles are exactly the size you need.

To do this:

  1. Increase size of fabric squares by ½”.
  2. Draw lines and cut as described above.
  3. Mark a line on your triangle ruler with a dry erase pen to indicate where the seam line of the half square triangle should be. (Draw the line on the shiny side of ruler if your ruler is coated on the bottom.)
  4. Place your triangle ruler on top of the half-square triangle, aligning the seam line with the line you drew on the triangle ruler.
    Photo of triangle ruler with line drawn at seam line for trimming down over-sized half square triangles
  5. Trim excess fabric along sides of triangle.

Best Press

I mentioned that it’s a good idea to starch or size your fabric before cutting out your pieces. I highly recommend Mary Ellen’s Best Press for this. Not only does it smell good (Linen is my favorite), but it really helps your fabric from stretching as you cut and sew it.

Our July/August newsletter has a coupon for 30% off the gallon size, but here’s one for 30% off the smaller spray bottle as well. Just click, print and take to any of our locations to purchase at this discount.

Happy Sewing!