Guest blogger series: half square triangles

by kath_red on March 17, 2011

in Guest Series 2011, Quilting, Whip Up Tutorials

Today I want to welcome Christina Lane to Whipup – an all around crafty gal, she says that she loves everything from quilting, screen printing, embroidery and more! She has recently had patterns published in Quilts and More magazine and Lunch Bags!, a Stash publication. When she’s not creating for herself, you’ll find her longarm quilting for others. Christina loves to share her crafting journey at her blog The Sometimes Crafter.

Hello Whip-Up readers! It’s so nice to be back in this space guest posting at Whipup. Today I wanted to share a really great way of making half square triangles (HST) that I recently found online and give you some measurements to help you with making them just the size you need for your own projects. Ahead of me I have a quilt full of half square triangles, and I was not looking forward to the prospect of making all of them in the way i’m use to. Then I remembered having seen this video by The Missouri Star Quilt Company.

Her ingenious method of assembling a HST (half square triangles) for a pinwheel block is perfect for most HST projects I can think of. You simply take your two squares of fabric (usually a print and a solid), place them right sides together and sew around all four edges with a 1/4″ seam allowance. Then you cut the square on both diagonals and you end up with four pieces.

When you open out and iron them you get perfect half square triangles! It’s amazing every time. I recently made a bunch of these in different sizes just to get a feel for the math. I have some of the sizes figured out to get you started, but the basic math is a HST that is roughly 64% of the size of your original square.

So your math would be this:
Beginning Square Size x 0.64 = HST size.

If you knew the size you needed your HST to be, but wanted to find out how big to cut your beginning squares, you would use this equation instead:
HST / 0.64 = Beginning Square Size.

Of course it’s always a good idea to make a test square before beginning a project, as all of our sewing and cutting varies. Not every size is exactly 64%, but it’s a good place to start without getting too involved in the math. Below is a table that gives you the starting square size and the final HST size for sizes that I’ve actually tested. You may find as you make these that you can get 1/8″ to 1/4″ more from each size, but I like to play it safe. So many times when I’ve made HST in the past I find that I don’t have enough to trim away and get the size stated in the pattern, so i’d rather play it safe. Good luck and have fun creating!

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{ 57 comments… read them below or add one }

1 Grace March 17, 2011 at 7:26 am

wow thanks for the tip and for doing the math. I hate calculating so this helps!
best wishes

2 Patty March 17, 2011 at 7:56 am

You wouldn’t believe this but I was just thinking last night that I need to search for this information! I just bought fabric for a table topper and I wanted to design the top but I couldn’t remember the formulas. Thanks for a very timely post!

3 Siobhan March 17, 2011 at 8:09 am

Nice Math!

4 craftytammie March 17, 2011 at 8:12 am

I’ve done this before, it’s fast and easy! The cut edges will be on the bias though, so be careful not to stretch them out!

5 Lisa March 17, 2011 at 9:52 am

I’ve done a couple quilts based on that video. So easy! Thanks much for the math. Math and I don’t get along, so I’m super excited to have it done for me!

6 Seanna Lea March 17, 2011 at 10:06 am

Awesome. I want to quilt, but cutting is not my friend. Fewer cuts means I’m more likely to start (yeah, still haven’t done that) and finish a project!

7 Caitlin March 17, 2011 at 10:23 am

This couldn’t have come at a better time,thanks! Got all my fabrics stacked up for a HST quilt and needed the math!

8 Simone March 17, 2011 at 10:26 am

It looks very easy! But isn’t your grain in the wrong direction this way???

9 kath_red March 27, 2011 at 12:26 am

Be careful with your bias edges – reinforcing with starch will help stabilise while sewing the patches together, otherwise make pins your friends and be careful not to pull the fabric while sewing the patches together.

10 Jennifer Can Quilt March 17, 2011 at 11:37 am

I don’t understand why we are dividing by .64. Can’t we just add 7/8″ to the finished size? That is how I do it!

If I want my HST piece to be 4″ finished (4.5″ unfinished) then I cut my squares 4 7/8″, draw a diagonal line, and sew 1/4″ from it on either side. When I cut in half and press, my pieces are 4.5″, just as planned.

11 kabby March 16, 2012 at 9:48 pm

This method is giving you ’4′ 1/2square triangles. With adding 7/8 to your finished size you are only getting ’2′ 1/2 square triangles

12 Lisa March 17, 2011 at 12:02 pm

Wow! That’s the best method I’ve seen yet. Thanks for sharing.

13 Dani March 17, 2011 at 1:06 pm

This. Is genius. I am about to start a pinwheel quilt and this will save some serious time!

14 Lori March 17, 2011 at 3:09 pm

My jaw just hit my desk. I would never have thought of this. It’s so genius but so simple! Thanks for doing the math…

15 julie March 17, 2011 at 4:29 pm

Thank you soooo much for this post. I have wanted to try this, but just couldn’t get my head around the math!!!

16 Natalie March 17, 2011 at 5:38 pm

I wonder what the math is for turning these into Pinwheels? I’ve never thought to break it down like this. Thanks!

17 Staci March 17, 2011 at 5:40 pm

I tried this method just this week, and it is now my FAVORITE way to do HST. Thank you for doing the math! The blog where I first saw this had math, too. But it was WRONG! Ooops!

18 Katie March 17, 2011 at 6:21 pm

I use this method all the time & love it, but I’m not sure it’s a good one for those new to sewing. It’s far too easy to stretch those bias edges.

19 beth March 17, 2011 at 8:19 pm

Great little chart to have handy!

20 aunt spicy March 17, 2011 at 8:59 pm


21 Susie March 17, 2011 at 10:13 pm

Yipes! There is a better quick way to do half square triangles where you don’t end up with all that bias all around the outside. If you do it this way, be really really careful with the iron so you don’t stretch the shape. This is a good way to use charm squares like in the video, but if you are cutting your own fabric, I predict less frustration if you do quick-pieced half-square triangles the traditional way.

22 Jennifer March 18, 2011 at 8:35 am

Thanks so much for the calculation! I also saw that video a month ago and was amazed. Just last night I was wondering how to figure out that math, as i am about to make a block with 5 small pinwheels and thought it would be a perfect time to try the new technique. Thanks!

23 Cherri March 18, 2011 at 12:27 pm

Yea Christina! I love the inner math geekiness! Thanks for figuring this out…I took a pic of the chart so it will always be available on my phone as a reference! I am sewing my last Neptune charm pack using this technique……thank you for the numbers!

24 vireya March 18, 2011 at 6:51 pm

These are actually quarter-square triangles, not half-square triangles. And they have all the outside edges on the bias, so are really tricky for beginners to handle. I wouldn’t recommend this method to a beginner patchworker, as it is a recipe for a really messed-up, discouraging result!

25 baylady March 7, 2013 at 6:09 pm

Hi…I can see why you’d think these are quarter-square triangles because you end up with 4 units. But if you look at each unit, you’ll see that fabric A and fabric B each occupy half of the resulting square. [a quick way to identify quarter-square triangles is to note if two 'hourglass' designs are formed inside the square.]

26 ChristyJ May 23, 2013 at 9:04 pm

Actually this tutorial is for quarter-square triangles.

HST & QST are defined by where the straight of grain is when you cut them. A HST has 1 bias edge on the long side of the triangle. On a QST both of the short sides are bias edges and the long side of the triangle is the straight of grain. Vireya is right, you will always get the best results when you keep the outside edge of your block on the straight of grain.

27 Mimi March 25, 2011 at 3:10 pm

Thanks for working out the math and the chart – I copied and saved it to have as a reference.

28 Cussot March 25, 2011 at 5:03 pm

I’m not sure it would be a good use of fabric, but here’s a way to use Christina’s math AND avoid ending up with the bias edges on the small squares: cut the big squares on the bias. Proceed as directed, being careful not to stretch the edges as you sew those outside seams.

29 Cindy Carter March 25, 2011 at 8:16 pm

Wow! What a fabulous idea!
Thanks for sharing.

30 Angie March 26, 2011 at 1:10 am

Thanks so much! This is just perfect…I have to save and print it out so I have it when I start my HST quilt!

31 Adeela March 27, 2011 at 9:18 pm

I made a wallhanging using this tutorial and it turned GREAT.

32 S Thoreson May 11, 2011 at 3:31 pm

This works well and if HST are small the bias does not seem to be a problem. There is another way that gives straight grain edges and yields 8 HST. I searched for instructions and the math but could not find a site.
One needs 2 larger beginning squares. Just as in making a single HST mark a diagonal line from one corner to corner and also from the opposite corner to corner. Now stitch 1/4 inch on both sides of each diagonal line.
Then cut vertically and horizontally in the MIDDLE of the square giving four squares that are sewed on the diagonal. Cut between the diagonal stitched lines and there are 8 HST. The only math I could find was for 3 1/2 inch HST start with the beginning two blocks at 8 3/4 inches. It may be helpful to do a bit larger and trim the finished HST.

33 kath_red May 14, 2011 at 8:08 pm

Thanks everyone for all the super great advice – so great to have so many talented and knowledgeable people able to add to the knowledge base


34 Jenny October 2, 2011 at 11:32 pm

Oh! I want to remember this! How in the heck am I going to bookmark this comment?

35 kath_red May 20, 2012 at 1:52 am
36 Barbara July 18, 2012 at 6:37 pm

So, if you want to calculate the size square to begin with for the method that yields 8 HST’s think of it this way. You are really joining 4 sets of the traditional method used to create 2 HST’s from a 2 squares. (take the size you want in the end and add 7/8″)… So S Thoreson’s example of an ending 3 1/2 inch HST with a beginning blocks of 8 3/4″ can be proved as follows:
One set of HST requires a square of 4 3/8″ (3 1/2″ + 7/8″)
Multiply this by 2 and you get 8 6/8 or 8 3/4″ – so you cut a square 8 3/4 x 8 3/4 and you will end up with 4 sets of the traditional method joined as one block.
Generically you can apply the formula of 2 x a + 1 3/4″ = length of one side of your square. a=size of ending HST
Therefore a ending HST of 4 1/2″ would require a 10 3/4″ beginning square.
( 2 x 4 1/2 + 1 3/4 = 9 + 1 3/4 = 10 3/4)
finished beginning
1 3 3/4
2 5 3/4
3 7 3/4
3 1/2 8 3/4
4 9 3/4
4 1/2 10 3/4
5 11 3/4
5 1/2 12 3/4 …etc
hope this helps

37 Jennifer June 28, 2011 at 8:37 pm

This is awesome! Thanks so much. Can’t wait to try this out. So appreciate your hard work!

38 andreatrib August 2, 2011 at 3:23 am

Here is one that worked for me… Start with 9 7/8 squares, sew each edge with ¼ seam allowance. You’ll get 4 – 6” finished half square triangle.

My sweet husband worked out the formula for me… Square root of {2*[(Finished square with seam allowance + 0.5) squared]} = the size of square you start with.

Then I put it into excel to get the following measurements….

Basically the numbers in order show the size finished block (1), the block with seam allowance (1 1/2), the starting block size as a decimal (2.82843), and the approx yardage square calculation (2 7/8).

I generally rounded up… and I still have to test them… but here they are… :)

Finished HST / HST w/1/2” seam allowance / Starting Square actual / Starting Square approx. yardage calc
1 / 1.5 / 2.82843 / 2 7/8
2 / 2.5 / 4.24264 / 4 ¼
3 / 3.5 / 5.65685 / 5 2/3
4 / 4.5 / 7.07107 / 7 1/8
5 / 5.5 / 8.48528 / 8 ½
6 / 6.5 / 9.89949 / 9 7/8
7 / 7.5 / 11.31371 / 11 5/16
8 / 8.5 / 12.72792 / 12 ¾
9 / 9.5 / 14.14214 / 14 1/8
10 / 10.5 / 15.55635 / 15 9/16
11 / 11.5 / 16.97056 / 17
12 / 12.5 / 18.38478 / 18 1/2

39 ingrid@morestylethancash August 25, 2011 at 9:24 am

I could cry…no really. Thank you so much for posting this. I am going to be making a quilt from all the scraps I have left over from sewing for my daughter when she was little. I was dreading doing the hst because they always turn out wonky for me. Thank you so (sew) much!

P.S. I found you on Pinterest

40 Manda September 8, 2011 at 10:25 am

I’ve been making HSTs this way since being introduced to the video on Missouri Quilt Company. As a newish quilter, I don’t know alot about bias, but I haven’t had any frustration in making blocks this way. Someone contacted me via flicker wanting to know how to make the block. I posted the link to the video, then remembered I’d seen this on Pinterest, and shared the link. I’m working on a diamond quilt block using this method now:

41 teresa g January 10, 2012 at 6:11 pm

THANK YOU for posting this. So glad you figured the math out for me.

42 Jeanne Crowell April 5, 2012 at 4:32 pm

I tried doing the math but it wasn’t the same size as yours. To do a 2 1/2″ my calculator says to cut a 4″ square, not a 4 1/4″. And I have to do it this way: 2.5 x 64%=4. If I do 2.5 x .64 I get 1.6. What am I doing wrong?

43 Joan April 30, 2012 at 12:44 pm

Thanks for this great time saver and easy to do!

44 linda August 22, 2012 at 10:53 am

CAN YOU PLEASE TELL ME . . . does this HST tutorial end up making the 4 hst’s in the size shown as 1) what a finished size hst would be in a quilt before the finished 1/4″ seam allowance? Or 2) and hst that is ready to use before sewn into a finished quilt top?

45 Rebecca Grace October 1, 2012 at 5:36 pm

Hi, Jeni! I know you posted this awhile back, but I am just now seeing it. I took a patchwork class with Kaye England a couple weeks ago and she showed us how to use the Cut for the Cure Specialty Rulers from Nifty Notions to easily cut HST, QST, half rectangles and more, all from the same size strips without having to do any math — I was in LOVE. Have you tried them?

46 lizellen December 29, 2012 at 12:14 pm

Thank you for doing the math. I saw this on Jenny Doan’s you tube but there was no directions for figuring out other sizes. This is great way to do HST and I hate points so this makes it a little easier.

47 Heather January 6, 2013 at 5:43 am

Wonderful tutorial! Thanks! I had a tiny bit of bias issue. I think I threw out two HST. But otherwise, it’s not really a problem.

48 Kathy Holm February 22, 2013 at 12:29 am

Thank you for saving my life on these HST..what a nightmare they can trusty notebook is at my side recording all this be used tomorrow , as t prepare to do battle with those pesky HST. This time..I’ll win the battle!

49 Kathy Holm February 22, 2013 at 12:32 am

Thanks for your great have saved my life on doing the explanation of HST.and the math.

50 Patti March 20, 2013 at 1:56 pm

Boy thanks you so much for this. I am so going to be using it for my month of a Bee.

51 Cindy April 16, 2013 at 4:07 pm

To eliminate the bias edges on HST’s-simply remember: Finished size of HST x 2 + 1.75 = the beginning size of the square.
Take 2 squares of fabric facing each other. Draw diagonal line corner to corner to create an ‘X’. Sew scant 1/4″ on each side of the lines. Do not cut that marked line yet. Now, cut those squares in half of both ways. There are 4 squares that will need the center line cut from corner to corner. Now cut those. You end up with 8 HST’s that do not have bias edges. No frustrations about starching and curling and stretching and fitting into your blocks. The result is the same as if making 2 HST’s at a time, but aking 8 at a time eliminates sewing those lines on tiny squares. Also, as everyone’s scant 1/4″ is different than the next person’s, it is much easier and less fretful for many to simply round out the measurement of the beginning squares, and then square them up after pressing the HST’s.

1 x 2 + 1.75 = 3.75 (1″ final HST size x 2 = 2″. Then 2″ + 1.75″ = 3.75″, the size of original 2 squares of fabric. You could round up to 4″ and square up the HST’s after final pressing.)
1.5 x 2 + 1.75 = 4.75
2 x 2 + 1.75 = 5.75
2.5 x 2 + 1.75 = 6.75
3 x 2 + 1.75 = 7.75
Etc. etc. See the pattern?

52 Judith Inge October 7, 2013 at 9:56 am

A friend just gave me a chart with that method. I used it to get the HST to fit 13″ blocks for a quilt set on point. Worked great and no bias edges to the outside.

53 Wendy Dillon October 7, 2013 at 11:54 am

thanks for the conversion table

54 Hedgehog October 7, 2013 at 12:17 pm

Much easier to make them right and not have bias edges. Look at this website: or at Cindy’s instructions above. The math for these is to add 1 3/4″ to the desired finished HST size. This method makes eight HSTs at the same time and they do not have bias edges.

55 Hedgehog October 7, 2013 at 12:20 pm

Oops! I meant to say add 1 3/4″ to TWICE the desired finished HST size. Cindy is correct.

56 Edie Moen October 7, 2013 at 6:30 pm

Does this method not leave you with all bias edges

57 andromeda October 9, 2013 at 5:42 pm

thanks so much for doing the math for us! math is not my friend!!!!

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