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Thread: Quilt math...ugh!

  1. #1
    Junior Member eeraemore's Avatar
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    Boy, I wish I remembered trigonometry...I hated that class. :)

    Anyhow, I cut four half square triangles from square blocks measuring 4 7/8 by 4 7/8. And then I sewed them together to make a square. I was following a pattern in a book (Quiltmaker's Gift). I'd like to square up my squares, but it doesn't say what the finished size of the block is...

    I've added a diagram here if it helps any. Each arrow is pointing to a side of the triangle which was 4 7/8 inches (before the were sewn together).

    Can anyone tell me what the size of this block SHOULD be? It's somewhere between 6 and 6 1/2 inches.
    Attached Files Attached Files

  2. #2
    Super Member GrannieAnnie's Avatar
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    Quote Originally Posted by eeraemore
    Boy, I wish I remembered trigonometry...I hated that class. :)

    Anyhow, I cut four half square triangles from square blocks measuring 4 7/8 by 4 7/8. And then I sewed them together to make a square. I was following a pattern in a book (Quiltmaker's Gift). I'd like to square up my squares, but it doesn't say what the finished size of the block is...

    I've added a diagram here if it helps any. Each arrow is pointing to a side of the triangle which was 4 7/8 inches (before the were sewn together).

    Can anyone tell me what the size of this block SHOULD be? It's somewhere between 6 and 6 1/2 inches.
    Don't see what we're supposed to be seeing.

    If the whole quilt is half square triangles, don't worry what you're supposed to have. Check about 5 or 6 pieced squares. Find the largest square you can get that is common to all. Remember to use a ruler that is giving you a diagonal mark from at least one corner. (didn't realize until the last couple weeks that some folks didn't realize a diagonal was necessary to trim a square.)

    Trim your squares up to a common number, maybe 4 1/2". Then proceed with your pattern. Or simply trim down to the size you're told to use.

    I flunked trig-----------and quilt math is simple for me.

  3. #3
    Super Member SueSew's Avatar
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    There is a tute on this with some helpful links - from Crafty Bear and others also - just try a search. Don't feel bad, it is confusing even with geometric formlua! Try graph paper and scale the sides at 4 sq to inch or whatever.
    Good luck!.

  4. #4
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    Without knowing what your seam allowances were, all I can give you is the straight-up math. Your outside edges are all the hypotenuses of the right triangles. Remember the Pythagorean Theorem: A-squared + B-squared = C-squared (where C is the hypotenuse and A and B are the other two sides).

    Side A is 4 7/8" (or 4.875"), and Side B is the same. You want to know what side C is.

    4.875 squared + 4.875 squared = C squared
    4.875 x 4.875 = 23.765625 so ....
    23.765625 + 23.765625 = 47.53125 = C squared
    The square root of 47.53125 = 6.8942911 so...
    C (the hypotenuse and your outside edges - without having taken into account seam allowances) = 6.8942911" (or roughly 6 7/8")

  5. #5
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    As I look at the diagram again, I'm guessing you lost only 1/4" on each of the A and B legs of your triangles if you used a 1/4" seam allowance. Doing the math again with that in mind:

    Legs A and B are now 4.625" long. 4.625" x4.625" = 21.390625 sq. "
    21.390625 sq. " + 21.390625 sq. " = 42.78125 sq. "
    Square root of 42.78125 sq. " = 6.5407377" which is within spitting (and quilting) distance of 6 1/2 "

    If I were you, I'd square them to 6 1/2" (unless some are too small - in which case I'd square them all to match the smallest one :-)

  6. #6
    Super Member katier825's Avatar
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    according to this calculator, your finished size would be 4". it says you add 7/8" to the finished size to get the cutting size. personally, i would have rounded up to 5" and trimmed back...i hate working with 1/8 inches! :)

    http://www.waukinvnetcorp.com/myquiltgenie/mgq_hst.php

  7. #7
    Super Member PaperPrincess's Avatar
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    Except that there were 4 triangles cut from the 4 7/8" square. So, if you assume you loose two 1/4" seams that makes it 4 3/8 on each side.

    4.375 X 4.375 = 19.141
    19.141 + 19.141 = 38.282
    Square Root is 6.18 which is the diagonal and is bisected, so each HST would be trimmed down a bit to 3"
    If you sew four 3" squares together, your block will wind up 5" finished.

    So, are you thoroughly confused??????? ;-)

  8. #8
    Super Member katier825's Avatar
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    Quote Originally Posted by PaperPrincess
    Except that there were 4 triangles cut from the 4 7/8" square. So, if you assume you loose two 1/4" seams that makes it 4 3/8 on each side.

    4.375 X 4.375 = 19.141
    19.141 + 19.141 = 38.282
    Square Root is 6.18 which is the diagonal and is bisected, so each HST would be trimmed down a bit to 3"
    If you sew four 3" squares together, your block will wind up 5.5"

    So, are you thoroughly confused??????? ;-)
    Yes! LOL I saw half square and that's what I focused on. I did find a link for quarter square triangles. It says the magic number is to add 1-1/4" to the finished size. Here's the link I found.

    http://www.patchwork-and-quilting.co...triangles.html

  9. #9
    Super Member Jan in VA's Avatar
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    Let's back up.
    Let's scrap most of the math.

    You cut 2 sqs. that were 4&7/8", right?
    You cut these on both diagonals to make 8 total triangles, right?
    Then you sewed the triangles cut from one sq. to the triangles cut from the other square, right?

    And now you're asking what size these sewn together triangles are supposed to be so you can square them up, right?
    And you want to know what size the finished block using these HSTs should be, right?

    I'd like to know how many HSTs are in the whole block.
    What is the name of the block?

    If there are a total of 4 HSTs in the block, then they probably should be trimmed to 3.5".....which will give you a block of 6.5".

    (The problem with this method is that all your outside edges of the HSTs are on the bias.....a pain in the butt, IMHO.)

    I'll be happy to PM with you if you wish.

    Jan in VA

  10. #10
    Super Member PaperPrincess's Avatar
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    OK, This was driving me crazy (very short drive, BTW), so I went & cut two 4 7/8" squares. Sewed a 1/4 seam on all 4 sides. Made 2 cuts from corner to corner, as I thought I saw in the attachment. The 4 HST came out at 3", so I stand by my calc ;-)
    Except now that I re-read her post and look at the pic, I realize that I misunderstood what she was asking.
    :oops: :oops: :oops:

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