How to Make Half Square Triangles: Three Methods and a Size Chart

There is no formula for a half square triangle.

That is the thing nobody tells you, and it is why the numbers never seem to agree. There are three common ways to make the unit, they cut different numbers of triangles out of the same two squares, and each one has its own sum. Look up "what size do I cut for a half square triangle" and you will find three different answers, all of them correct, none of them labelled. The math is different for each method, and almost nobody says which method they are doing.

So this page has all three. Yvonne made them on three consecutive Technique Tuesdays, two at a time, then four at a time, then eight, and her videos are in the sections below. Underneath each one is the math, written down so you can come back to it at the cutting table. There is a half square triangle chart you can print, a cheat sheet for all three methods, and it costs nothing.

The two numbers, and why they trip people up

A half square triangle is a square made of two triangles, cut on the diagonal. Quilters shorten it to HST, and the HST unit is the one most of us make more of than any other. Whole quilts are built from nothing else, and it turns up inside most of the quilt blocks you already know.

You will also meet it under older names. Pieced triangle squares, two-triangle squares, split squares, bias squares. Same unit every time.

Every HST has two sizes, and half the confusion in this subject comes from not saying which one you mean.

Finished is what the unit measures once it is sewn into the quilt, with its seam allowances hidden inside. Unfinished is what it measures on the cutting mat before it joins anything, and it is always the finished size plus 1/2 inch, a quarter for each side.

So a 4 inch finished HST is trimmed to 4 1/2 inches. A pattern that says "trim to 4 1/2" and a pattern that says "4 inch finished" are asking for exactly the same unit.

That half inch is not special to this unit. Every patchwork piece carries a 1/4 inch seam allowance on each of its four sides, so unfinished is always finished plus 1/2 inch, whether the piece is a triangle or a plain square.

What is special to the HST is the 7/8 inch in the two at a time formula. Half of it is the ordinary seam allowance. The rest covers the diagonal seam running through the middle of the unit, which a plain square does not have, and leaves a whisker to trim.

The half square triangle size chart

This is the half square triangle chart. Every number in the three method columns is the square size to cut for your two starting squares. Trim the finished units to the size in the second column, whichever method you used.

All measurements in inches. Two at a time gives 2 units from 2 squares, four at a time gives 4, the Magic Eight gives 8.

All three are speed piecing, which is the old name for the same idea: sew the larger pieces first, then cut them into units, rather than cutting small pieces and sewing them one at a time.

For any size that is not in the chart, the three formulas are these. Two at a time, cut both squares at the finished size plus 7/8 inch. The unit next door is the quarter square triangle, which is the same square split twice instead of once and is cut at the finished size plus 1 1/4 inches. It has a size chart of its own. Four at a time, take the unfinished size, multiply by 1.6, and round up to the next 1/4 inch. Magic Eight, double the finished size and add 1 3/4 inches.

Download the size chart as a one page PDF. It is the whole cheat sheet on one page: the table, the three formulas, the precut sizes and the bias note. Tape it inside the lid of your thread box and you will never look this up again in the middle of a quilt.

Two at a time

This is the method everyone learns first, written 2 at a time in most patterns and called sandwich piecing in the older books, and the one to reach for when you need one or two HSTs of a pairing, or when you are working scrappy and every unit is a different pair of fabrics.

Put two squares right sides together and draw a diagonal line on the wrong side of the lighter one. That drawn line is the cutting line, not the sewing line. Sew 1/4 inch either side of it, cut on the line, and press open towards the dark. Two HSTs.

Yvonne has one detail here that is worth more than the method itself. When you sew, aim one thread to the inside of your quarter inch rather than dead on it. Pressing the unit open folds the fabric over the seam line and eats a hair of width, and that one thread is what gives it back. It is the difference between units that trim easily and units that trim short.

If your quarter inch is not yet automatic, draw the two sewing lines as well as the cutting line. Three pencil lines and no thinking beats two lines and a guess.

Chain piece it. Sew down one side of every pair without cutting the thread, clip them apart, then go back up the other side. Eight units take about as long as two.

One thing about chain piecing triangles rather than squares. The point of the next pair does not always feed in cleanly on the tail of the last one, so lift the presser foot a fraction and position it yourself rather than letting the feed dogs find it.

Four at a time

Two identical squares, right sides together, and a 1/4 inch seam all the way around the outside. No drawn lines at all. Then cut both diagonals, corner to corner, and four units fall apart in your hands.

The cut goes from stitching corner to stitching corner, not fabric corner to fabric corner. Those are not the same point, and the difference is what decides whether the four units come out even.

The sum is the awkward one of the three. Take the unfinished size you want, multiply by 1.6, and round up to the next quarter inch. For 4 1/2 inch unfinished units that is 7.2, so cut at 7 1/4 and trim down.

There is a catch with this one, and it is the reason the section below exists. Read that before you decide where these HST units go in a quilt block.

The Magic Eight

Eight matching units in one pass, in under ten minutes. When a block or a quilt wants six or more of the same pairing, nothing else comes close.

Two squares right sides together. On the wrong side of the lighter one draw both diagonal lines, so the square carries a large X, then a horizontal and a vertical line through the same center point. Sew 1/4 inch either side of both diagonals, which is four seams. Then cut on the horizontal and the vertical, and cut on the diagonals.

The straight lines are cutting lines only. Sewing on them is the mistake people make the first time, and it costs the whole square.

The sum is the friendliest of the three: double the finished size and add 1 3/4 inches. For 4 inch finished units that is 9 3/4 inches, which is why a 10 inch layer cake square is such a comfortable starting point. It gives you the wiggle room to trim.

One note about the sewing machine. Use a quarter inch foot without the guide for this method. You are sewing across the middle of the fabric rather than along its edge, so the guide has nothing to run against and only pushes the fabric around.

We have longer walkthroughs of the last two on their own, if you want to sit with one at a time: four at a time and the Magic Eight.

Where the bias ends up, and why it picks the method

This is the part that turns three techniques into one decision, and it is the reason to know all three rather than just your favourite.

Fabric does not stretch along the weave. It stretches on the diagonal, and a cut edge on that diagonal is a bias edge. Every half square triangle has bias in it somewhere. The methods differ in where they put it.

Two at a time and the Magic Eight put the bias inside the seam. The outer edges of the finished unit come from the original edges of your squares, on the straight of grain, and they do not move.

Four at a time puts the bias on all four outer edges. Pull one and you can feel it give.

That single fact tells you where each unit belongs. A stretchy outer edge is no trouble in the middle of a block, where the units around it hold it steady on every side. It is trouble on the outside edge of a block, where it gets pinned, pressed and handled with nothing to stabilise it, and where it ends up next to the seam that joins one block to the next in the quilt top.

So: four at a time for units that will sit inside, two at a time or the Magic Eight for units that touch the edge. Size matters too. A little bias on a 3 or 4 inch unit is manageable. The same proportion of stretch on a 6 inch unit is a bowed block.

Trimming, and the eighth of an inch that adds up

Every unit gets trimmed, whichever method made it. This is the step that decides whether the rows line up later, and it is the one people skip because the units look close enough.

Lay the 45 degree line of a square ruler along the seam. Set the two edges of the ruler on the size you want. Cut two sides, turn the unit 180 degrees, line the diagonal up again, and cut the other two. You can tell which side you have not done yet, because it still has its dog ears.

How you hold the ruler decides more than people expect. Splay your fingers across it, well away from the cutting edge, and keep one finger or thumb off the ruler and down on the mat. That contact is what stops the ruler twisting as you cut.

The number to carry around is the eighth of an inch. It looks like nothing on the mat. Eight units later it is a whole inch, and a row that will not meet the row above it.

Two things help here more than trying harder. A square ruler with a 45 degree line through the middle, in the size you trim to most often, and a sharp rotary cutter. Our square rulers carry that 45 degree line, and a fresh rotary blade before you start a stack beats one changed halfway through. A blunt blade stops slicing and starts dragging, and the unit shifts under the ruler without you feeling it.

Pressing without stretching the seam

Press towards the dark, and set the seam first, which is just running the iron over the closed unit to settle the stitches.

Then open it, give the fold a small finger press to start it, and hook the point of the iron into the corner and press straight across the unit rather than along the diagonal. The seam of an HST runs on the bias. Push the iron along that line and a perfectly cut unit turns into a slightly leaning one.

Press, do not iron. Lift and place.

Straight from a precut

Half the appeal of this unit is that precut squares are already the right shape, and two of the common sizes land on friendly numbers.

• 5 inch charm square, two at a time. 4 inch finished units, trimmed to 4 1/2.
• 5 inch charm square, four at a time. 2 1/2 inch finished units, trimmed to 3.
• 10 inch layer cake square, two at a time. 9 inch finished units, trimmed to 9 1/2.
• 10 inch layer cake square, Magic Eight. 4 inch finished units, trimmed to 4 1/2.

A charm pack and a layer cake between them cover most of what a scrappy HST quilt needs. For the background, which repeats in every block and is what the triangles are read against, fabric by the yard keeps the whole quilt on one bolt, and solids keep the points crisp.

What to make with them

A pile of half square triangles trimmed to one size is never wasted, because the layout is what changes and the math never does. Most half square triangle quilt patterns are the same unit in a different order.

Four of them in a two by two grid, each turned a quarter turn from the last, make a pinwheel. The same four turned differently make broken dishes. Four on the sides of a nine-patch, with plain squares in the corners, make a friendship star. Three quilt blocks, one HST unit, no new math.

Set enough of them together and you have a half square triangle quilt with no other unit in it, which is why so many modern quilt patterns are built this way. Two of our block guides go further into it. Hovering Hawks is a 12 inch block that deliberately uses two of these methods in one block, so you can see the bias rule decide something real. And the Ohio Star is what happens when you take two half square triangles and cut them a second time, which is the next unit along.

Half square triangle questions we are asked most

What size squares do I cut for half square triangles?

It depends on the method, which is the part that catches people out. Two at a time, the finished size plus 7/8 inch. Four at a time, the unfinished size multiplied by 1.6, rounded up. Magic Eight, the finished size doubled plus 1 3/4 inches. The chart above has all three worked out.

What is the formula for a half square triangle?

There is not one. Each method cuts a different number of units out of the same two squares, so each has its own sum. That is the single most common reason quilters end up with units that are the wrong size.

What is the Magic Eight method?

Two squares right sides together. Draw both diagonals on the back of the lighter one, then a horizontal and a vertical through the center. Sew 1/4 inch either side of both diagonals, cut on the horizontal and vertical, then cut on the diagonals. Eight matching units in one pass.

Which half square triangle method is best?

The one that matches how many you need and where they are going. One or two, or scrappy, use two at a time. Exactly four, going inside a block, use four at a time. Six or more the same, use the Magic Eight.

Do I have to trim them?

Yes, and it is the step worth defending when you are in a hurry. Units that were never square do not become square when the block is squared up. They lose their points instead.

Why do my half square triangles come out too small?

Usually the fold at the press rather than the seam itself. Pressing the unit open takes up a hair of fabric across the seam, which is why sewing one thread inside your quarter inch line, rather than on it, makes the difference between trimming and re-cutting.

Where to start

Before any of it, test your quarter inch. Cut a 2 1/2 inch strip, cut three 1 1/2 inch segments off it, sew two of them together down the long side and press, then sew the third across the top. It should fit exactly. Too long and your seam is too wide, too short and it is too narrow.

Then print the chart, cut two 5 inch squares out of whatever is nearest, sew them two at a time, press and trim to 4 1/2 inches.

If it trims to 4 1/2 with a whisker to spare, your quarter inch is honest and every number on this page will behave. If it trims short, move your needle one thread and do it again. That is a five minute test that saves a quilt.

Marbelia Fernandes
The Fabric Hut Co-Founder

P.S. When you make a Magic Eight and only need six, do not put the two spares in the scrap bin. Put them in a jar with the others. A jar of odd half square triangles all trimmed to the same size becomes a quilt of its own about two years later, and it is usually the one people ask about.