Half Square Triangles (HST) Chart & Formula

Learn how to make half square triangles with this easy tutorial, which will show you three different methods. HSTs, or half-square triangles, are squares cut in half diagonally, creating two triangles. It is far easier to create triangles for quilting in this manner than to cut triangles separately and then sew them together.

Half Square Triangles Tutorial

Half square triangles are typically abbreviated to HST in most quilting patterns. They can be made in any size to suit the patchwork masterpiece you are creating.

Half square triangles can form a basis for many varied quilt blocks and are extremely versatile. They are one of the most popular units used in patchwork. They are also a great way to contrast two fabrics. Fabrics can be contrasted with patterns in a similar color, or be completely different colors.

If you are new to quilting, read my article on quilting for beginners for a great overview to this fantastic hobby.

Half Square Triangles Supplies

• FABRIC – Quilting fabric and tightly woven cotton fabrics are best for making your half square triangles. You can use some of your scraps to make a quilt or purchase pre-cut bundles of layer cakes or charm squares. Fat quarters are also a good way to purchase fabric and maximize the number of patterns in your quilt. For quilting, choose smaller patterns so you can not cutting into the designs.
• THREAD – Strong thread suitable for quilting.
• PRESSER FOOT – You can use your all-purpose sewing foot and mark the stitching lines or use a foot with a 1/4 inch guide. A walking foot is also useful for eliminating puckers.
• QUILTING RULER, CUTTING MAT & ROTARY CUTTER – Accurate cutting is essential for great results and a quilt that has matching seams. Quilting square rulers can help.
• MARKING TOOLS – Tailor’s chalk or pencil to mark the sewing lines.
• PRESSING TOOLS – Ironing board and iron

Half Square Triangle Video Tutorial

Ways to Make Half Square Triangles

Here are 3 methods of making half square triangles. They vary in the amount of squares you end up with.

Half Square Triangle Methods:

1. Method #1 – 2 patches
2. Method #2 – 4 patches
3. Method #3 – 8 patches

How to Make Half Square Triangles (2 at a Time)

This basic method is the traditional method and produces the best-quality half square triangles. It is, however, the slowest method, which may not be ideal if you are making a large quilt. You will make 2 half-square triangle units.

• PROS – Accurate and good quality
• CONS – Slow

Formula for Half Square Triangles

Now, of course, you want to work out what size squares to start with, to end up with the correct size pieced square for your final block. Here is the math:

So, if you want a 3-inch (7.6 cm) square in your final block, your original squares must be 3 7/8 inches (9.4cm). Occasionally, it may be easier to add a full inch (2.5 cm) and trim your blocks down when squaring them up rather than worrying about an exact size.

2 at a Time Half Square Triangles Chart

Here are some common half square triangles and the measurements for the square you need to start with.

HST FINISHED SIZE	CUT SQUARES SIZE
1″ (2.5cm)	1 7/8″
1 1/2″ (4cm)	2 3/8″
2″ (5cm)	2 7/8″
2 1/2″ (6.3cm)	3 3/8″
3″ (7.6cm)	3 7/8″
3 1/2″ (8.9cm)	4 3/8″
4″ (10cm)	4 7/8″
4 1/2″ (11.4cm)	5 3/8″
5″ (12.7cm)	5 7/8″
5 1/2″ (14cm)	6 3/8″
6″ (15.2cm)	6 7/8″

How to Sew Half Square Triangles

Here are the step by step instructions for how to sew half-square-triangle blocks.

Step 1 – Cut out Squares

First, measure out and cut out your squares. A rotary cutter is ideal for this.  Don’t forget to include your seam allowances!

Step 2 – Mark the Diagonal Seams

Draw a diagonal line on the wrong side of one of your squares. Use a marker that won’t bleed into your fabric and can easily be removed.  This will be a 45 degree line.

Place your two contrasting fabric squares right sides together. Be precise when matching up your squares.

Step 3 – Stitch

Pin your squares together, then sew a ¼ inch (6mm) seam on either side of your diagonal line. You can draw stitching lines for yourself or simply use your sewing machine foot as a guide.

Step 4 – Cut the Diagonal

Cut along your diagonal line, open out, and you will have two half square triangles.

Step 5 – Press

Press these new squares you have created, and square them off neatly to make sure your seams match up perfectly when you are putting your final block together.

PRESSING TIPS

• When pressing, don’t move your iron along the fabric, as this can distort your bias edges. Just place your hot iron on top of your seam lines.
• Don’t be tempted to skip the pressing and squaring off, you will be glad you took this extra time later!
• If you are using a light and a dark fabric, press your seam towards the darker fabric side so that it doesn’t show when you turn your block over to the right side. 

Step 6 – Trim HSTs

After pressing, your squares will need a trim.

When you open them up, you will find little dog ears from your seam allowances. Trim these neatly as well as checking that the final size is accurate.

Half Square Triangle Variations

Here are 2 variations of sewing half square triangles. These will quicken the process by allowing you to sew more HSTs at a time.

Quick 4 Patch Triangle Blocks

This method will give you 4 patches of half square triangles at once. You will need to use slightly larger squares to start off with so your final patches don’t end up too small.

• PROS – Faster for making large quantities
• CONS – Once you cut through the stitching there is a chance that the stitching will unravel. This can be partly solved by using small stitch lengths. Also due to rounding in the formula, you will need to cut down the final squares slightly.

4 Patch Sizing

Here are some common half-square triangles resulting in 4 patches and the measurements for the squares you need to start with. There is a small amount of rounding in these calculations so check your finished sizes and trim accurately at the end.

4 at a Time Half Square Triangles Chart

Here are the measurements to cut 4 half square triangles:

HST FINISHED SIZE	CUT SQUARE SIZE
1″ (2.5cm)	1 5/8″
1 1/2″ (4cm)	2 3/8″
2″ (5cm)	3 1/8″
2 1/2″ (6.3cm)	4″
3″ (7.6cm)	4 3/4″
3 1/2″ (8.9cm)	5 1/2″
4″ (10cm)	6 1/4″
4 1/2″ (11.4cm)	7″
5″ (12.7cm)	7 7/8″
5 1/2″ (14cm)	8 5/8″
6″ (15.2cm)	9 3/8″

Sewing 4 Half Square Triangles

1. Cut 2 squares using the formula or chart above.
2. Draw diagonal cross lines on the back of one of your squares. Use a marker that won’t bleed into your fabric and can easily be removed. 
3. Place your two contrasting fabric squares right sides together. Be precise when matching up the edges of your squares.
4. Pin your squares together, then sew a ¼ inch (6mm) seam all the way around the outside of the square. Use a small stitch length since you will need to cut through the stitching and you don’t want it to unravel.

5. Cut along your diagonal lines, open out, and you will have four half square triangles.

6. Press these new squares you have created, and square them off neatly to make sure your seams match up perfectly when you are putting your final block together.

Quick 8 Patch Half Square Triangles

If you are piecing together a large quilt, there is an even faster way to make these half square triangles. The problem is, what you gain in speed you lose in bias stability and more complex size calculations! With this method, you can create eight half-square triangle blocks at a time.  For those who want to give it a go, here is the method:

• PROS – By far the fastest method if you need to sew a lot of HST.
• CONS – Lots of cutting and measuring is needed. You will need to trim the final squares due to rounding in the formula.

8 Patch Sizing Formula

When sizing your squares for this method use the formula or follow the table below.

Formula: Cutting Size =  (finished half square triangle + 7/8 inch) x 2

8 at a Time HST Chart

HST FINISHED BLOCK	CUTTING SIZE
1”	3.75”
2”	5.75”
3”	7.75”
4”	9.75”
5”	11.75”
6”	13.75”
7”	15.75”
8”	17.75”
9”	19,75”
10”	21.75”

8 Half Square Triangle Quilt Blocks

1. Place two equal size squares of fabric right sides together. Remember that you will be need large squares here, as each one will be cut into eight pieces.
2. Mark your square on the diagonals and through the center, as shown in the picture below.
3. Sew your ¼ inch (6mmm) seam on both sides of the diagonal lines, all the way across the square.

4. Cut the square into eighths along each of the marked lines.

5. Continue as before for trimming and pressing when making the single square units.

Half Square Triangle FAQs

What is The Best Half Square Triangle Method?

The best method will depend on how many half square triangles you will need and whether it will be a problem to have bias cut edges. Using the basic method where you get 2 HSTs at a time will give the best results but is much slower than making 4 or 8 at a time.

What Completed Blocks Can I Make With my HST?

You can see from all these examples what a very versatile little block this is. If you add in a few solid squares, you can create even more blocks.

Here are just some of the most popular hst blocks.

• Diamond shapes
• Negative image diamonds
• Zigzag
• Diagonal stripes
• Trapezoid shapes
• Flying geese
• Broken dishes
• Pinwheels

What About Half Rectangle Triangles

Half rectangle triangles are another common quilting block shape. Because they are not symmetrical, they need a slightly different method than the HSTs. Read more about how to sew a HRT.

Half Square Triangles – In Conclusion

Once you have completed all your quilt blocks, you can decide what your finished object will be. Will you make a teeny little mug rug? Or something in between like cushion covers, tote bags or a table runner? Or will you go all out and create a full-blown quilt?

You can coordinate your fabrics to match your décor or create a wonderful gift with a friend or family member’s favorite colors. The choice is yours! There are endless possibilities with these half-square triangles! Enjoy experimenting with this handy quilt block.

More Quilting Articles

You can use the how to make a baby quilt tutorial to make a quilt with half-square triangles instead of square patches. This easy tutorial can be used to make a quilt of any size.

• Quilting for Beginners
• Quilting Tools
• Quilt Sizes
• How to Make a Baby Quilt
• Quilting Terms
• How to Bind a Quilt