Half Square Triangle Calculator and Chart

Finished, unfinished and cut size

Three different sizes can cause confusion. The finished size is what the unit measures once it is sewn into the quilt. The unfinished size is the finished size plus 1/2 in, because it still has a 1/4 in seam allowance on every side. The cut size is the square you start with. A 3 in finished HST is 3 1/2 in unfinished, and it starts from a square of 3 7/8 in if you make two at a time.

Patterns may use any of these, so always check which one you are looking at. If a pattern says to make HSTs that measure 3 1/2 in, it usually means unfinished. To use this tool, subtract the 1/2 in and enter 3 as the finished size. If your HSTs keep coming out too small, check this before you change anything else, as it is by far the most common mix-up.

The 2-at-a-time method

This is the classic method. Place two squares right sides together, draw a line along one diagonal, sew 1/4 in on each side of the line, and cut on the line. You get two identical units. The cut size is the finished size plus 7/8 in, and this is the most fabric-efficient method if you need only a few units from each print pair.

That 7/8 in figure comes from the geometry. Each seam runs along the long side of a triangle, and the 1/4 in seam takes more out of a diagonal than a straight edge. Many quilters add a full 1 in rather than 7/8 in and then trim, since 1/8 in lines can be harder to read on a ruler. This tool can add a 1/2 in for you, which gives you plenty of room to trim each one square.

The 4-at-a-time and 8-at-a-time methods

To make four at a time, place two squares right sides together, sew 1/4 in around all four edges, and cut along both diagonals. This is fast, but the outer edges of each unit are on the bias, so they stretch easily. Handle them gently, and starch the fabric first if you can. The cut size is the finished size times 1.414, plus 1 1/4 in. It rarely lands on a neat ruler line, so we round up to the next 1/8 in.

To make eight at a time, sew 1/4 in on each side of both diagonals, then cut along both diagonals and through the middle in both directions. The cut size is twice the finished size plus 1 3/4 in. This method is efficient for a large number of matching units, such as in a big star quilt. The outer edges fall on the straight grain, so these units are stable. It does require accurate sewing, because small errors are multiplied by the number of cuts.

Why trimming gives you better points

Exact cut sizes leave no room for error. A little drift in your seam, a slightly dull rotary blade or some fabric stretch will all change the final size. When hundreds of units are sewn together, those tiny errors add up to blocks that do not match. That is why so many experienced quilters make HSTs slightly oversized and trim them all to the same size after pressing.

To trim, place a square ruler on the unit with its 45-degree line on the seam. Trim two sides, then turn the unit, line up the cut edges with your unfinished size, and trim the other two. A ruler with a clearly marked diagonal helps a lot. It takes a little longer, but the blocks will go together far more easily. Tick the oversize option in the tool, and it adds 1/2 in to the square for you.

How to work out your fabric

Each pair of squares makes 2, 4 or 8 units, and you need the same number of squares from each of the two fabrics. Divide your total by the number per pair, and round up. Then see how many squares fit across the fabric width, and work out how many strips you need. For 40 HSTs with a 3 in finish, made two at a time, you need 20 squares of 3 7/8 in per fabric. That is 10 squares per strip, so 2 strips, which is 7 3/4 in of fabric, or 1/4 yard.

You can often use precuts for this. A 5 in charm square makes two HSTs that finish at about 4 in, once you trim them. A 10 in layer cake square gives eight HSTs finishing at about 4 in, using the 8-at-a-time method. If you are using fat quarters, the fat quarter calculator tells you how many squares fit on each piece. Also consider how the units combine: a simple change in the way they turn gives a completely different quilt.

Common half square triangle mistakes

The most common mistake is a seam that is slightly too wide. Over two seams, this makes a noticeable difference to the size of each unit, and the error multiplies across a whole quilt. Sew a test unit, press it, and measure it before you cut everything. If it comes out small, narrow your seam a little or add to your square size. Another common problem is pressing with a hard back-and-forth motion. Because the seam runs on the bias, this can stretch the unit out of shape. Lift the iron and press straight down instead.

Placement causes trouble as well. When two fabrics have a directional print, half of your units will have it turned the other way, so check before you commit to a large batch. Keep an eye on the value too, because a light and a dark fabric show the triangle pattern most clearly, while two similar fabrics blur it. Finally, chain piecing is a great way to save time, but do check your seam guide every so often. A small drift at the start of a long chain can spoil every unit that follows.