How to measure and cut triangles

A few years ago, I was surprised to realise that many people in my local quilt group did not know the trick for calculating what to cut for triangles. They were lost without a pattern. I cannot remember where I learned this little gem of information, but it means that I can work out the size to cut my fabric for almost any pieced quilt block, and it allows me to sew whatever block catches my fancy, with or without a pattern.

With Yvonne (Quilting Jetgirl) and Cheryl (Meadow Mist Designs) hosting a festival for sewing tips and tutorials, I thought I would share some simple quilt maths, which I first shared in my local guild newsletter.

When you look at traditionally pieced quilt blocks, you notice that most are made up of squares and triangles.

Measuring and cutting squares and rectangles is easy; to determine your cut size, you measure the finished size of the sides and add ½ inch to each measurement for the seam allowances.

Understanding HSTs and QSTs

This first step is understanding the difference between the cut size and the finished size of a block.

With just two numbers, calculating the cut size for triangles is just as easy. The two numbers to remember are ⅞ and 1¼.

Cutting half-square triangles – ⅞ inch

A half-square triangle is, as the name suggests, a triangle that is formed from drawing a diagonal line through the centre of a square to form two halves. The grain direction of a half-square triangle is as pictured.

Understanding HSTs and QSTs

A half-square triangle has the grain running parallel to the short edges of the triangle.

To calculate the square of fabric you need to cut to make two half-square triangles, measure the outside of the finished square and add ⅞ inch.

It does not matter what your finished size is, just add ⅞ inch to it to determine the minimum cut size.

For example, if I want to sew a 2-inch square (finished size) made up of two half-square triangles, I know I need to cut 2⅞-inch squares in each fabric and then cut them in half on the diagonal.

If I want to make a 6-inch finished pinwheel block, I need four sections, each made up of 3-inch half-square triangle blocks. To determine the fabric cutting size, I add ⅞ inch to 3 inches and cut two 3⅞-inch squares in each fabric, which I then cut in half on the diagonal. When sewn together with a scant ¼-inch seam allowance, these form neat 3½-inch squares. With four of these, I have my pinwheel.

Understanding HSTs and QSTs

A pinwheel block is a simple combination of half-square triangles.

The same measurements work whether you prefer to cut your triangles and then sew them together in pairs or you like to place two squares right sides together, mark a diagonal line through the middle and sew a scant ¼ inch on either side of the line before cutting along your marked line.

If you want to be super neat or you do not trust the accuracy of your seam allowances, you can add a little extra and trim after sewing. I generally prefer this method. When cutting, I add a full inch instead of ⅞ inch to the finished size, and I then trim it square after sewing to be sure everything is perfectly tidy.

When making the pieces for the 6-inch pinwheel block, I added 1 inch to the finished size of 3 inches and cut two 4-inch squares in each fabric. I then cut these all on the diagonal, sewed them back together in pairs, pressed and trimmed to 3½ inches before assembling the pinwheel.

The trimming wastes very little fabric and gives a precise result, which was especially important when I was working on my Dear Jane quilt.

Understanding HSTs and QSTs

To make half-square triangles that finish at 3 inches, I added 1 inch and cut 4-inch squares. This allows a little margin for trimming.

Cutting quarter-square triangles – 1¼ inch

A quarter-square triangle is a triangle that is formed from drawing two diagonal lines through the centre of a square.

The important difference between half-square triangles and quarter-square triangles is the grain direction. The grain of a half-square triangle runs parallel to the short sides of the triangle; the grain of a quarter-square triangle runs parallel to the long side of the triangle.

Understanding HSTs and QSTs

A quarter-square triangle has the grain running parallel to the long edge of the triangle.

To calculate the square of fabric you need to cut to make four quarter-square triangles, measure the outside of the finished square and add 1¼ inch.

For example, if I want to make the 12-inch finished Ohio star block pictured, I need four yellow quarter-square triangles. To determine the fabric cutting size for the yellow areas, I add 1¼ inches to 4 inches and cut one 5¼-inch square, which I then cut in half twice on the diagonal. When sewn together with one white and two blue quarter-square triangles, also cut to the same measurements with a scant ¼-inch seam allowance, these form neat 4½-inch squares.

Understanding HSTs and QSTs

This block is made up of squares and quarter-square triangles.

Again, if you want to be super neat, you can add a little extra to the measurements and trim after sewing. I usually add ⅛–¼ inch to the magic measurement so there is a little extra to trim each section neatly once it is sewn; in other words, I add 1⅜–1½ inch to the finished size to determine my cut size. If you do this, however, be sure to trim evenly from the centre.

Understanding HSTs and QSTs

For my demonstration I cut two 5½-inch squares, allowing lots of trimming room.

Understanding HSTs and QSTs

When trimming quarter-square triangles, be sure to trim evenly from the centre.

Now you know the trick, you too will be able to calculate the size to cut for whichever block takes your fancy too.

And please check out the tips and tutorials festival. You are sure to learn something new!

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3 thoughts on “How to measure and cut triangles

  1. This is going to come in very handy for an F2F block I have planned… Thank you for taking the trouble to produce this clear and easy to understand tutorial 🙂

  2. This is a very handy tutorial. I have been caught out a few times when I tried cutting HST’s in a hurry. I actually forgot to add the seam allowances for the diagonal bit!

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