Quilter's Math ; Change triangle block sizes

[theothermarion]

When I make HST's, I add 1 inch to the finished size, then trim to the right cut size.
For QST's, I add 1.5 inch to the finished size. I don't trim the HST on the interim step.
For flying geese, Most tutorials ask you to add 7/8 inch to the smaller dimension for the 4 squares. Then they ask to add 1 1/4 to the larger dimension for the large square, I add an extra 1/8 to each dimension, and trim. That gets complicated: I trim the long side with the point first, making sure the point is 1/4 inch from the edge. Fortunately, I have a pineapple ruler that helps. 99 percent of the time, that 1/8 inch is sufficient.

Anyway, now I have perfect HST's, QST's, and flying geese. A lot of the time, that is the last time I'm perfect in a quilt!

[MEMA1]

I am not a beginner quilter. However, I have started teaching a friend how to quilt and found a huge gap in my knowledge. I need to know if there are "math rules" for changing the size of a quilt block, especially those with triangles (whether they be HST, QST, or any others). We (my student and I) decided we wanted the block 1 inch bigger all around. So, we just added 1" to all the measurements. The HST are coming our WAY off for the flying geese that we are trying to make. Could someone tell me what we are doing wrong or point me toward a book, a video, or any other medium that could give us a tip or tips on how to correct what we have obviously done wrong? Any help is appreciated.
Thank you.

[CarrieG]

I have a hard time with that, especially triangles! I find it helpful to use paper. If I want to make a 9" finished, I draw a 9" square, Then divide it up. Just remember to add your seam allowance to each piece. Not real helpful, I know!

[bbquiltmaker]

I have a tutorial for making 4 geese at once, oversized, then trim to perfection:

http://bbquiltmaker.blogspot.com/2014/08/four-flying-geese-tutorial.html

Yes, there is Quilter's math. HST: cut square 7/8" larger than finished size, cut in half for 2 triangles. QST: cut square 1.25" larger than finished size, cut in half on BOTH diagonals, for 4 QST with straight of grain on the long side. Squares and Rectangles are cut .5" larger than finished size, in both directions.

[rehak]

What you add to each element of a block when increasing the size of the block depends on the structure of the block itself. Basically, you have to distribute that added inch across the pieces of the block. For example, if your block is a 4-patch, you would add 1/2" to each square and that would increase the block by 1". However, if your block is a 9-patch, you would add 1/3" to each square to get the 1" increase. As you get into half-square triangles and more complicated blocks, the math becomes more complicated. Like Carols said, it's usually easiest to draw out the new block on a piece of paper and measure the pieces. Then also note that the measurements could be difficult depending on size and characteristics of the original block and how much you want to increase it. In the examples I gave, increasing the size of the 4-patch by 1" is easy to do. But increasing the 9-patch by 1" gives you dimensions that are harder to measure with our rulers. If odd measurements like that bother you, you will have to be careful about how you choose the blocks to increase.

[twiglet]

When I increase/ decrease a block size I take the finished size and look at how big each element will be. Then add seam allowances to each one using the amounts Barbara quoted. It's easier than trying to compare it with the original measurements.

If it's an odd shape that doesn't meet with those formulas I draw it out and add 1/4" seam allowance then measure the overall size.

[JudithA]

I use Electric Quilt to change block sizes. You just have to decide what finished size you want your block. EQ7 does the math. It tells you exactly what size to cut the pieces in the block. Works beautifully.

[grandedesign]

Check out the book "Patchwork Math" from Lynn Wilder. She gives you a formulas for all kinds of blocks as well as different approaches to construction. She is a great teacher and even a guest on TQS; her book is in the TQS Store. Please note - this is not intended to be a "commercial"! I teach quilting and found this to be a great staple for my classes. Cheers Wendy

[Lorchen]

You TEACH?!!!

Come over and work your magic so that I produce a decent miniature quilt for my lovely partner!

[grandedesign]

Hi Lorchen - I believe your partner is going to get a fabulous quilt Unless you are interested in Ribbon Applique..... you don't need a class from me!

[MEMA1]

I have EQ! I haven't used it for ages, but love it. My daughter and I actually designed her wedding quilt on EQ together. Thanks so much! Best solution to our problem.

[MEMA1]

Granddesign: Thanks for the tip. The book has been very helpful to both of us. We refer to it quite often. Very helpful tip.