How to make half-square triangles 2 at a time

Half-square triangles, affectionately known as HSTs, are one of the main building blocks of quilt patterns. They show up everywhere from traditional to modern quilts, and there are even improv versions of them.

Because they're such a staple block, I thought I'd write a tutorial series on the most common methods of making them, so you can pick and choose whichever method suits your project the best.

I'm starting this tutorial series with the absolutely easiest method to make half-square triangles, which is two-at-a-time. This is the first method most people will learn when they start out with quilting, and is a staple in everyone's quilting repertoire.

This post contains some affiliate links. If you make a purchase through one of the links, I may get a small commission, at no extra cost to you. Find more info here.

Three easy ways to make HSTs

There are three main methods for making half-square triangles, and I will be showing you all three in this tutorial series. Just click the links to go to the other blog posts:

• Two at a time - My favourite, and the most beginner-friendly way, to make half-square triangles is the two-at-a-time method. This method is perfect for patterns that use HSTs with lots of different colour combinations, and for scrappy quilts. 
• Four at a time & Eight at a time - For patterns that use many of the same HSTs, there are ways to make them four or even eight at a time. These are time savers that help you cut fewer fabric pieces, and are a good way to make many HSTs with the same fabrics. These often require yardage or fat quarters to start with, since the starting piece of fabric is bigger, and therefore aren't always suitable for scraps.

A quick note on finished vs. unfinished measurements:

• Finished means the measurement of your unit or block as it is sewn into the quilt. This does not include any seam allowances, and is basically what you see once the quilt top is sewn up.
• Unfinished means the measurements of a block or unit including the seam allowances. So this is what you would see if you measured a unit on your cutting table, before sewing it into a block or quilt. Seam allowances in quilting are normally 1/4", so a quilt block or unit that is 6" x 6" finished, for example, would be 6 1/2" x 6 1/2" unfinished. 

Quilt math for 2-at-a-time half-square triangles

Making half-square triangles two at a time is the easiest and most beginner-friendly method of making HSTs, and also the easiest one to calculate.

Simply add 1" to the finished measurement of your half-square triangle unit, then cut two squares to that size, one of each fabric you need in your HST. This gives you enough room to trim the final HSTs even if your sewing was a tad wonky.

So, let's do an example. If you want to have an HST that measures 4" x 4" finished (this would mean 4.5" x 4.5" unfinished), add 1" to the finished measurement.

This means you need to cut two 5" x 5" squares of fabric to make two HSTs.

To download a cheat sheet with diagrams and the math chart that you can print out and keep near your sewing table, enter your email address below. You can opt in to my regular email updates as well, if you like, and hear about new tutorials, pattern launches, freebies etc., or get just the cheat sheet emailed to you.

How to make half-square triangles 2-at-a-time

Cut two squares of fabric according to the measurement on the chart. Mark a diagonal line on the wrong side of one of them.

Put the two squares right sides together, with the diagonal line visible on the top.

Sew two seams, one on either side of the line, 1/4" away from the line.

Cut along the drawn diagonal line between the seams and open up the two half-square triangles. 

Press the seams either to the dark side, or open, depending on what you prefer. I pressed these to the side.

Trim both HSTs to their unfinished size. 

How to trim half-square triangles

The easiest way to trim HSTs is by using a square ruler if you have one, but it can also be done with a rectangular one (scroll down a bit to see how to use a rectangular ruler for this). You can see which rulers I use on my Resources page. There are a few specialty rulers created specifically for trimming half-square triangles as well, but I'm showing you the basic method here.

Line up the 45 degree line on the ruler with the diagonal seam of the half-square triangle. Check that the fabric extends past the measurement you need (4.5" in our example) on all 4 sides (blue arrows). If not, slide the ruler up or down along the diagonal seam until everything fits.

Trim the top and side of the block. I am right-handed, so I am trimming the top and right-hand side of the block, but if you're left-handed, it works the same way on the left, just turn the block around. There are even left-handed rulers that have all the markings the other way around so you're not having to read things upside down.

Turn the block around so the two untrimmed sides are now at the top and right. Line up the 45 degree line again on the diagonal seam, then line up the two edges you just cut with the exact measurement you need (the 4 1/2" line in our example).

Trim the remaining two sides.

And there you go, you have two matching half-square triangles ready for your quilt project.

If you don't have a square ruler and are using your rectangular one, here's how that looks. It's lined up exactly like the square ruler, with the diagonal line on the seam and the edges on your chosen markings. I like that my Creative Grids ruler has markings all the way around, but if yours doesn't, just use some masking tape on the ruler to mark the line you need. That should help to keep your eye from slipping to the wrong line.

Quilt patterns that use 2 at a time HSTs

I have multiple HST patterns available, either as a freebie, in magazines or in the shop. You can get the Polar Peaks pattern for free when you sign up to my emails here.

The Facets pattern is a bold and simple gemstone pattern that uses two-at-a-time half-square triangles to make the scrappy gemstone blocks.

The Ripple & Swirl quilt pattern uses the same method to achieve the scrappy look of the ribbons. This quilt comes with both the Ripple (shown in pink) and Swirl (shown in blue) layouts. This beginner-friendly pattern is a great stash buster that works with colour-coordinated scraps just as well as it does with a rainbow of random scraps.

Both patterns use the 2-at-a-time method of making half-square triangles and they are beginner-friendly and fast to put together.

Both patterns are available in the pattern shop.