Friday, July 8, 2011

Quilt Math and Drafting Part 1 - squares, rectangles and half square triangles

There is no new block this week, but we are going to talk about "quilt math" and designing your own blocks.  (Sad about no new block?  Don't worry, I have a special block challenge at the end of this post, keep on reading.  :) )

First, some definitions.  A finished block, square or HST is one that is sewn onto other fabric on all four sides.  It is the size you want the piece to be when your quilt is finished.  Unfinished measurements include enough extra fabric for seam allowances.  So our blocks are 12.5 inches square unfinished and 12 inches finished.

Why did I choose for our blocks to finish at 12 inches?  First of all, the bigger the block, the easier it is to sew.  Since we are working with new techniques I figured we could give ourselves a break.  :)  But almost more importantly a 12 inch block is very versatile.

 It can be divided in 2, 3, 4, 6 and 8 evenly.  This allows for a number of different blocks without working with weird fractions.

Let's say I wanted to design my own block and I want to have the block divided into 4 columns and rows.  12 divided by 4 is 3, so each square in my block has a finished size of 3x3 inches.  I start sketching and divide the middle 4 squares on the diagonal and make a pinwheel design.  

But it needs something more so I color in some squares radiating off from the pinwheel.  Looks good!  But I notice that the two white blocks on the sides could be combined.  No point sewing two pieces of fabric together when you can just cut one, so I sketch the block again leaving out the lines that divided the two squares.

Here are the different pieces that I will need.

Now I need to figure out how large to cut my pieces.  I know that each finished square is 3x3, the finished rectangles are 3x6 and the finished HST measure 3x3.

I will need to have a 1/4 inch on each side of the square and rectangle for the seam allowance.  The finished height of the square + 1/4 inch on top + 1/4 inch on bottom =  finished height + 1/2 inch.  

The same holds for the width of the square and rectangle.  Finished width +1/4 on the right +1/4 on the left = finished width +1/2.

The rule is to add 1/2 inch to the finished length and width of squares and rectangles in order to find the unfinished size that you need to cut out.  So I would need to cut my square 3 1/2 x 3 1/2 inches and my rectangle 3 1/2 x 6 1/2 inches

Squares and Rectangles - add 1/2 inch to length and width

A HST unit is made out of two right triangles.  Each triangle needs it's own seam allowance.

They need a 1/4 inch on the side with the 90 degree angle and a 1/4 inch seam at the point, but the point extends farther as it tappers down to a point from the necessary 1/4 inch.  That extra length is 3/8 inch.  Please, let's not go deeper into the geometry of it and just trust that it is so.  :)  I add 1/4 + 1/4 + 3/8 = 7/8.  

I will need to cut squares 7/8 inch larger than the finished HSTs.  In this case the squares I would use to make my HSTs would be 3 7/8 x3 7/8.  If I planned to trim my HSTs to size I would cut my squares slightly larger, 4x4 inches.

Half Square Triangles - add 7/8 inch to the length and width of the squares you use to make them 
(add 1 inch if you plan to trim to size) 

Clear as mud? 

To finish my block I would count up how many different shapes I needed.

4 light 3 1/2 x 6 1/2 inch rectangles
4 medium 3 1/2 x 3 1/2 inch squares
2 dark 3 7/8 x 3 7/8 inch squares
2 light 3 7/8 x 3 7/8 inch squares (or 4x4)

I would make my HSTs, attach them to the medium squares, sew the rectangles onto the sides and then join the four subsections into a whole block.  And voila!  My block would be done.

I know there is way too much math and way too few pretty pictures in this post.  :)  Please, if you have any questions let me know.

I have an optional challenge for you this week:  design and make your own 12 inch finished block!

Sketch a large square and divide it into 4 sections length and width wise, just like I did.  Now start sketching.  You can use squares, triangles and rectangles.  Here is a series of 4x4 blocks that I did to come up with ideas for a bee block.  It doesn't have to be pretty!  :)

Really, the possibilities are endless.

Feeling up to the challenge?  You Can Do It!  I would love to see what you make!  (Flickr group here.)
Be sure to leave a comment on the next post to be entered in the "I've Made All 3 Blocks!" giveaway.  


  1. Ooh! I may have to try this. I already have one I did for a quilt design but I could try it as a single block. Just have to figure out the math to make it a 12 inch block!

  2. Thank you for explaining the math on the HST. I knew you were supposed to add 7/8 to the finished size of the square, but for the life of my I didn't know WHY!?! Thanks!

  3. Ohhh, I am sooo doing this- hope it turns out decent :) hehe! Getting paper out now :)

  4. Thanks for this post. Although I haven't been participating in the challenge -- we're moving, and I can't take on anything new! -- I have learned a lot from what you've posted here and in others about these blocks. :)

  5. i just finished up the 3rd block! im almost caught up. :) now its time to sit and plan. I love using graph paper. Do you use graph paper?

  6. Woohoo! I did it. I'll post to Flickr!

  7. I have done my first three blocks; I will see if I can find time this week to design my own! We move on Friday, so no guarantees :)

  8. you crack me up--I"m following the skill builder but not participating. I scrolled down to the triangle image and thought "how the heck did she get 3/8 out of that?!" Then I read the commentary--you're a hoot!!

  9. Clear as mud :D
    But seriously, that's so helpful, thank you. I'm not participating (yet?), but following anyway.

  10. This block will be used to finish the quilt sampler? Or I can wait for the next, I'm busy these days

  11. Well, after saying that I'm not creative, I thought 'what a wimp', and got out some fabrics to mess around with triangles. I tried the other technique on the video link you posted, but I had to cut so much off to even the squares up, I decided to stick with your methods. I didn't make a 12" block, but I'm happy with what I ended up with. I moved everything around many times to see which pattern I liked best, not looking at any books or magazines, so it is my own work, but I'm sure it isn't the first time it has been done, and I don't know what it is called! I've posted some pics on your Flickr site.

  12. Finally done the challenge block and all three original blocks...Definitely a challenge and definitely not the same size as the rest of the will add a border or something...Keep the challenges coming!

  13. I really enjoyed this assignment! It kinda felt like a test of everything that we've just worked on. So, if I were to give myself a grade it definitely wouldn't be an A, lol. My block came out to be a tad bit small but I'm so proud of it!

  14. Did the challenge Block and Blogged it here... can you take a look and see if you can guide me in the right direction?

  15. Thank you so much for your post. I'm new to quilting and the measurements were really not making sense to me, but you've helped me clear away the fog! Your post was the only one I could find on the internet that wasn't totally confusing :)

    Thank you again. :)

  16. Hello Leila! Thank you for your help. I was looking for some explanation about sizes of squares and triangles. I am brasilian and appreciate the blog's ideas.
    I will return to enjoy your knowledge.
    Thank you again!

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