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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
    Power Poster 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
    Power Poster 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:

  11. #11
    Junior Member eeraemore's Avatar
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    Thank you everyone for all of your input!! If it helps, this is what I am trying to make as the final block: http://www.quiltingboard.com/t-141590-1.htm. With this question (post), I'm focusing on the middle "diamond." I took a 4 7/8 square of the light green, made a half square triangle, and did the same for the dark green...then sewed them together to make the middle diamond! I've been thinking about this for over an hour. My head hurts. :)

  12. #12
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    Call me crazy, but this is one of reasons I loved piecing quilts! Color, creativity, and the challenge of figuring it all out! And yes, I am a believer that math is beautiful! (If you doubt it, check out tessellations, fractals, et al.)

    PaperPrincess is correct in reducing the starting lengths of sides A and B of the triangle by 1/2" to 4 3/8". (I was wrong in reducing them by only 1/4".) Her calculation of 6.18" (~6") looks right to me as the finished size of the block. (While I enjoyed reading it, I don't agree with the next step she took, however, which reduced the block to 5".)

    I am thoroughly enjoying the other ways that people are approaching this question! Vive les differences! (One proof of math's beauty is that it works no matter which way one comes at it.)

    eeraemore, PLEASE let us know what you learn from experience!!!

    Thanks!

  13. #13
    Super Member LyndaOH's Avatar
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    I think you're having trouble because you are trying to find a math formula for using half square triangles to make a quarter square triangle square. The two don't easily mix.

    I think what you should do is use quarter square triangles. Take two squares that are 7 3/4" square - one of each fabric. Cut them diagonally twice. This will still give you the 4 7/8" triangles, but the bias edges will be in the right place. I used 7 3/4" because it's 6 1/2" plus the extra 1 1/4" you add as per the standard formula for quarter square triangles. Your center block will finish at 6 1/2" which is just a smidge larger than the 6.45" that your center should be, based on a 12" corn & beans block.

    Does that make sense?

  14. #14
    Super Member LyndaOH's Avatar
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    Quote Originally Posted by sushi
    PaperPrincess is correct in reducing the starting lengths of sides A and B of the triangle by 1/2" to 4 3/8". (I was wrong in reducing them by only 1/4".) Her calculation of 6.18" (~6") looks right to me as the finished size of the block. (While I enjoyed reading it, I don't agree with the next step she took, however, which reduced the block to 5".)
    Actually there's only 1/4" from each triangle, because there's only one seam on one leg of the triangle, so you were right that it's 4 5/8" and about 6.5" finished.

  15. #15
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    Now I love this discussion even more! :-)

  16. #16
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    I'm glad I took too Aleve because all this makes my head hurt. To begin with there does not appear to be complete information. However, in rereading your original post which says you cut a square of 4 7/8 inches and then cut the square into a triangle. My quiltmaking knowledge tells me that the intention is to sew two of the triangls together back into a square. With the 7/8" the assumption is that you are going to sew them together with an accurate 1/4" or scant 1/4" seam which will make your square 4.5"
    However your illustration does not show a half square triangle square. So something is missing. So I don't understand your original question related to what size you should square up the square, according to your description it should be 4.5". Your illustration does not match your description. To my knowledge no instructions assume trimming down unless they tell you to. 7/8" is the basic addition to sewing triangles into square. I've talked enough.

  17. #17
    Super Member LyndaOH's Avatar
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    Quote Originally Posted by Holice
    I'm glad I took too Aleve because all this makes my head hurt. To begin with there does not appear to be complete information. However, in rereading your original post which says you cut a square of 4 7/8 inches and then cut the square into a triangle. My quiltmaking knowledge tells me that the intention is to sew two of the triangls together back into a square. With the 7/8" the assumption is that you are going to sew them together with an accurate 1/4" or scant 1/4" seam which will make your square 4.5".
    Just to clarify, this will make a finished square of 4" - unfinished it will be 4.5".

    If you look at her illustration, what she's doing is taking half square triangles and sewing four of them into a square.

  18. #18
    Super Member Val in IN's Avatar
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    I'm with Holice, this gives me a headache :-|

  19. #19
    Super Member GrannieAnnie's Avatar
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    Actually there's only 1/4" from each triangle, because there's only one seam on one leg of the triangle, so you were right that it's 4 5/8" and about 6.5" finished.[/quote]


    I've loved math all my life HOWEVER--how do you take a square that is 4.835, cut it in half ---even with the other square attached and get a square that is almost 2 inches larger?

    I've still not been able to see her picture, so that may be part of my problem.

  20. #20
    Super Member LyndaOH's Avatar
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    She is taking two squares that are 4 7/8", cutting both in half and then making a larger square out of the four triangles from both squares.

  21. #21
    Super Member OmaForFour's Avatar
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    I agree! Your finished size should be 4 inches. Therefore I would square them up to 4.5 inches so that you have the 1/4 inch on each side of the square to piece them together.

    Quote Originally Posted by katier825
    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

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    I bought the Quiltmaker's Gift book also and had to find corrections to many patterns. That might help with your math issues. This site has some corrections listed for the book.

    http://www.quiltmaker.com/pattern_correction.html

    Hope it helps!

  23. #23
    Senior Member sarahrachel's Avatar
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    don't worry. I passed trig first semester but by second semester senior year, I forgot just about all of it. and I graduated in June!! but I agree with everyone else, if that's what the whole quilt is going to be, don't worry about the measurements, just maybe make more squares until it's the size you want it

  24. #24
    Sherome's Avatar
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    I never understood any of it. I most likely will never have the correct size, since I can't figure out even why the 7/8.

  25. #25
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    All these beautiful magic formulas and no one has the same out come. I definitely will not throw stones because 9th grade algebra ate my sack lunch.
    The link for the pic isn't working so I don't know if this would work but can you not find your smallest block and trim the others to match like someone else here suggested? Can you not just decide I want the quilt this size, divide by # of block on a horizontal row and on a vertical row?
    Robin in TX

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