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Although quilting is one of the most creative and
fun things you can do, sometimes it is necessary to make mathematical
calculations, especially if you like to design your own patterns.
Listed on this page is some useful information that can be referred to
when you are feeling math challenged.
Yardage Measurements
Fabric is usually purchased "by the yard", 36 inches, or a fraction of
a yard. Fabric is traditionally manufactured in 44" widths; however,
recently some manufacturers have shortened that to 42". These
measurements explain what the length of the fabric cut is when you buy
a fraction of a yard.
Yardage 
1/8 
1/4 
3/8 
1/3 
1/2 
5/8 
2/3 
3/4 
7/8 
1 
Inches 
4.5 
9 
13.5 
12 
18 
22.5 
24 
27 
31.5 
36 
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Fat Quarters
Normally 1/4 of a yard of fabric would be a piece
that measured 9" x 44". Quilters use a special cut of fabric called a
"fat quarter". It results in the same area of fabric, but the yardage
has been cut differently so that the piece measures 18' x 22", enabling
quilters to cut larger pieces than you could from a regular quarter of
a yard of fabric.
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Quilt Dimensions
There are variations in what determines the
appropriate size for a bed quilt. When determining how big you want
your quilt to be, you must first know the dimensions of your mattress.
These are listed below.
 Standard Mattress Sizes
Crib Mattress: 28" x 52"
Twin/ Single Mattress: 39" x 75"
Full/ Double Mattress: 54" x 75"
Queen Size Mattress: 60" x 80"
King Mattress: 76" x 80"
California King: 72" x 84"
Next you must know how far you want the quilt to come down on the side
of the bed. This is called the "drop". A quilt needs at least a 12"
drop on each side and the bottom if you want it to cover the side of
the mattress and you need to add an additional 15" to the top edge if
you want to cover the pillow. If you want the quilt to serve as a
fullsize bedspread with a 21" drop on each side, the chart below lists
common sizes.
Twin /
Single 
Full /
Double 
Queen 
King 
81" x
110" 
96" x
110" 
102" x
115 
120" x
115

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On
Point Blocks
Blocks that are arranged "on point" are rotated so
that the square block is laid out in a diamond effect. While this is a
beautiful layout, it creates some mathematical complications when
designing a quilt. This is because the diagonal measurement of the
block is creating the width. To determine this measurement, you simply
multiply the finished block size by 1.41. For example, 6" square block
no longer has a 6" width on the quilt top, but has a width of 8 1/2".
The chart below gives diagonal measurements for a number of common
block sizes.
Finished Block Size 
1" 
2" 
3" 
4" 
5" 
6" 
8" 
10" 
Diagonal Measurement 
11/2" 
27/8" 
41/4 
55/8" 
71/8" 
81/2 " 
11 3/8" 
141/8" 
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Decimal Conversion
Sometimes quilt making requires that we cut pieces
with strange, fractional measurements. Knowing the fractional
equivalent in decimals can be a help. The following chart provides
decimal equivalents for a number of common fractions used in cutting
quilt pieces.
Fraction 
1/8 
1/4 
3/8 
1/2 
5/8 
3/4 
7/8 
1 
Decimal 
.125 
.25 
.375

.50 
.625 
.75 
.875

1.0 
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Corner Setting Triangles
Corner setting squares are made from half square
triangles. That means that a square block has been cut on the diagonal,
resulting in two triangles. The legs of the triangle form the outside
edge of the quilt and the hypotenuse is sewn to the block edge. There
are two formulas that can be used to make corner setting triangles.
Formula #1 has you divide the finished block size by 1.414; then add
.875 for the seams, and round up to the nearest 1/8". The square block
would be cut this size and then cut corner to corner with one diagonal
cut, resulting in two corner triangles.
Formula #2 has you multiply the finished block size by 1.41 and divide
that answer by 2. Then add .875 for seams and round up to the nearest
1/8". Cut a square block this size and then make one diagonal cut,
corner to corner, resulting in two corner triangles.
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Side Setting Triangles
Side setting triangles are used along the side a
quilt with onpoint block layout. They are made by cutting a square
twice diagonally, resulting in 4 triangles. The legs of the triangles
are on the bias and the base is on the straight of grain, thus it goes
on the outside of the quilt top.
The formula for figuring side setting triangles is to multiply the
finished block size by 1.414, add 1.25 to this measurement, and then
round up to the nearest 1/8". This is the size to cut the square block,
which will then by crosscut diagonally twice, resulting in 4 triangles.
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Common Setting
Triangle Sizes
This chart lists the sizes of squares that will be
cross cut to create setting triangles.
Finished
Block Size

Side
Triangle Squares

Corner
Triangle Squares

4"

7"

33/4"

5"

83/8"

41/2"

6"

93/4"

51/8"

7"

111/4"

57/8"

8"

125/8"

65/8"

9"

14"

71/4"

10"

151/2"

8"

12"

181/4"

93/8"

14"

21"

103/4"

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