Quilt Math – Oh My

Some people really hate math, but love quilting. They like the artistic flair of quilting, but not the technical part. I must admit, I am a geek and trained engineer. So, I kind of like math. Scary – I know. Let me share a few math secrets to help make quilt math a little less puzzling.

Quilt math is based upon 1/4″ seams and the finished size of the block. These blocks are

Basic Squares and Rectangles

For basic blocks, add 1/2″ to finished length and width. The Nine Patch (purple, yellow and white block above) finishes at 6″, so unfinished it is 6 1/2″. Each of the smaller pieces are 2″ finished, so they will need to be 2 1/2″ unfinished. That extra 1/2″ allows for 1/4″ seam on each side.

Square or Rectangle = Finished Length + 1/2″ and Finished Width + 1/2″

Basic Triangles

Basic triangles, like the Half Square Triangle or HST (orange and white) also need the 1/4″ seam. To get that 1/4″ for the points, you need a more than 1/2″.  An extra 3/8″ is needed for each point. Since the HST has one point on a side, you need to add 1/2″ + 3/8″ = 7/8″. For the 6″ finished HST, cut a 6 7/8″ square. Then, subcut that square once on the diagonal to make the triangle. This provides the extra 3/8″ for the point.

HST= Finished Length + 7/8″ and Finished Width + 1/2″

Multiple Triangles

Blocks with more triangles, like the green and white Quarter Square Triangle (QST), have two points on a side. This means you will need 3/8″ for the first point, 3/8″ for the second point, and 1/2″ for the seam. That is a total of 1 1/4″. So, our 6″ QST will need 1 1/4″ square, which will be cut twice on the diagonal.

QST = Finished Length + 1 1/4″ and Finished Width + 1 1/4″

At this point, you may wonder why some triangles are cut once on the diagonal, while others are cut twice on the diagonal. The diagonal cuts are bias cuts, which mean they stretch more. You don’t want the outside of your block to stretch, so you don’t want the bias cut on the outside of the block. The straight cut on the outside minimizes stretching.

Flying Geese

Flying Geese are special, like our red and white ones above. For the Geese or large triangle, the width is equal to two times the height AND the width has twice as many points as the height. Ok, it would if it were rotated 90-degrees. Please bear with me. So, the 6″ finished Flying Geese will be 3 1/2″ x 6 1/2″ unfinished. Since the height (3 1/2″) has one point, you need to add 1 1/2″ + 3/8″ or 7/8″ total. The width (6 1/2″) has two points, which means adding 1/1/2″ + 3/8″ + 3/8″ for a total of 1 1/4″. The initial rectangle will be 3 7/8″ high and 7 1/4″ wide.

Flying geese = Finished Height + 7/8″ and Finished Width + 1 1/4″

Meanwhile the air or smaller triangle is the same as for an HST.

On Point or Diamond in a Square

Now that you understand adding 1/2″ for seam allowances and 3/8″ for points, it is time to twist things with the Golden Number of 1.414. This is also known as the square root of 2. You will need this number for our blue and which Diamond in a Square.  We know from our previous examples that the finished block is 6″ square.  The white square is smaller than that. In fact, it is 6″ / 1.414 or 4 1/4″ finished. Therefore, the unfinished square is 4 3/4″.

 On-point Square = Finished Height/1.414 + 1/2″

Then, how big are the blue triangles? The finished blue triangle is half the size of the block, right? It is 3″ finished. It has one point, which means adding 7/8″. The blue square needs to be 3 7/8″, which will then be cut once on the diagonal to make the triangle.

Setting Triangle = Finished Height/2 + 7/8″

Those are the basic measurements for quilt blocks. You can plug any numbers into these equations

Binding

Straight cut bindings are generally cut 2 1/2″ wide and mitered together to create longer strips. How long do you need?

Binding = (Finished Quilt Length x 2 + Finished Quilt Width x 2) + 10″

Backing and Batting

Let’s start with batting. Most quilters add 8-10″ to the finished length and width for batting. The provides wiggle room when quilting.

Batting = Finished Quilt Length +10″ and Finished Quilt Width +10″

Now add the backing. This should be at least as big as the batting. There are three ways to calculate backing. The first is to assume one piece, in which case the batting and backing will be the same size! The second way is to assume you will piece the backing so the seam will run the length of the quilt back. Round up to the nearest 1/4 yd

Lengthwise Backing (yd) = Batting Length/36″ + Batting Width/40″

The third way is to assume you will piece the backing so the seam will run across the back or crosswise. Round up to the nearest 1/4 yd

Crosswise Backing (yd) – Batting Length/40″ + Batting Width/36″

In a nutshell (at least for math nuts), these are the basic equations you will need for quilting. For more complicated quilts, there is more complicated math. But let’s keep it simple and enjoy the holidays!

Happy Quilting!

Laureen

 

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