Digging in to the Quilty Math for the Half and Half Quilt

 Digging in to the quilty math for the Half and Half Quilt.

The Half and Half Quilt Pattern + eBook is in the works. It's going to include the pattern (fabric requirements and assembly instructions for the quilt) plus a gallery of 130 blocks you can make that all use the exact same amounts of fabric. The only thing you change is how you rotate your half square triangle units. It's so easy but gives you so, so many options.

I'm scrapping everything I wrote for the first draft so far. But not really. I'm sharing it with you right here. This is the bridge between the Quilty Math Workbook and the Half and Half Quilt. This is how I figured out exactly what the fabric requirements are for all of the options I'll be offering in the Half and Half Quilt Pattern + eBook.

I use Quilter’s Dream Cotton Batting which comes in these sizes:

  • Craft 46" x 36"
  • Crib 46" x 60"
  • Throw 60" x 60"
  • Twin 93" x 72"
  • Double 96" x 93"
  • Queen 108" x 93"
  • Super Queen 93" x 121"
  • King 122" x 122”

I want the batting to hang over 4 inches on each edge of my quilt top which means my quilt top can be a maximum of:

  • Craft 38” x 28”
  • Crib 38” x 52”
  • Throw 52” x 52”
  • Twin 64” x 85”
  • Double 85” x 88”
  • Queen 85” x 100”
  • Super Queen 85” x 113”
  • King 114” x 114”

Do the math yourself:

  • If you know the size of your finished quilt top, add 8 inches total to the width and 8 inches total to the length.
  • If you know the size of your batting, subtract 8 inches total to the width and 8 inches total to the length.

I used 12 inch (finished size) blocks for my sample quilt.

  • Craft 2 blocks x 3 blocks = 6 blocks total
  • Crib  3 blocks x 4 blocks = 12 blocks total
  • Throw 4 blocks x 4 blocks = 16 blocks total
  • Twin  5 blocks x 7 blocks = 35 blocks total
  • Double 7 blocks x 7 blocks = 49 blocks total
  • Queen 7 blocks x 8 blocks = 56 blocks total
  • Super Queen 7 blocks x 9 blocks = 63 blocks total
  • King 9 blocks x 9 blocks = 81 blocks total

Do the math yourself:

  • Divide the desired width and length of your quilt top by 12.
  • You’ll need to round up or down so you can work with a whole number of blocks. Rounding up will give you a larger quilt top and less overage on your batting to work with. Rounding down will give you a smaller quilt top, you can add a border to your quilt top if you need it to be a certain size.
  • Multiply your blocks across by your blocks down for the total number of blocks you’ll need to make.

All of the blocks in the gallery are 16 patch blocks that are 4 half square triangle units across by 4 half square triangle blocks down.

I wanted my finished blocks to be 12 inch blocks so each of my finished half square triangle units is a 3 inch square unit.

Why did I pick 16 patch blocks versus 4, 9, or 36 patch blocks?

I wanted to be able to cut my half square triangle pieces as efficiently as possible from fat quarters. I “live a low scrap lifestyle” so I wanted to be able to incorporate as much of my fabric as possible into my quilt and have a minimal amount of scraps leftover.

Here are the assumptions I made to figure this out:

  • I want to cut squares to make half square triangles.
  • I want my finished half square triangle units to be able to be evenly combined into a 12 inch block.

Here’s how I did the math to calculate how many pieces I could cut in each size from a fat quarter:

Take my 12 inch block and divide by 2, 3, 4, and 6 for potential finished half square triangle unit sizes.

  • 36 patch = 2”
  • 16 patch = 3”
  • 9 patch = 4”
  • 4 patch = 6”

Calculate the cut size of each square by adding 7/8 inch seam allowance to each of those measurements.

  • 36 patch = 2 7/8”
  • 16 patch =  3 7/8”
  • 9 patch = 4 7/8”
  • 4 patch = 6 7/8”

Figure out how many strips I can cut per fat quarter. I calculated this by taking the usable width of a fat quarter (20 inches to leave plenty of room for selvedge), dividing by the cut size, and rounding down to the nearest whole number since I can’t use partial strips.

  • 36 patch = 6
  • 16 patch = 5
  • 9 patch = 4
  • 4 patch = 2

Figure out how many pieces I can sub cut from each of those strips. I calculated this by taking the usable length of fat quarter (16 inches to leave some wiggle room), dividing by the cut size, and rounding down to the nearest whole number since I can’t use partial pieces.

  • 36 patch = 5
  • 16 patch = 4
  • 9 patch = 3
  • 4 patch = 2

Then I got really fancy because I also wanted to know how much fabric I would have leftover as scraps if I cut my pieces each of these sizes.

Calculate how much leftover/scrap fabric I have for each potential finished half square triangle unit size.

Leftover width = usable width of a fat quarter (20”) - (cut width x number of strips)

  • 36 patch = 2 3/4”
  • 16 patch = 5/8”
  • 9 patch = 1/2”
  • 4 patch = 6 1/4”

Leftover length = usable length of a fat quarter (16”) - (cut length x number of pieces per strip)

  • 36 patch = 1 5/8”
  • 16 patch = 1/2”
  • 9 patch = 1 3/8”
  • 4 patch = 2 1/4”

Total leftover/scrap fabric = (usable width of a fat quarter x leftover width) + (usable length of a fat quarter x leftover length)

  • 36 patch = 81”
  • 16 patch = 20 1/2”
  • 9 patch = 32”
  • 4 patch = 161”

I went with the 16 patch blocks because that cut size left the smallest amount of leftover/scrap fabric.

Working with Fat Quarters or 1/4 Yard cuts of yardage

If you want to use all the same fabric for the print/color portion of each block, you will be able to cut 2 blocks worth of pieces from each fat quarter or 1/4 yard cut of yardage.

If you want to mix prints/colors between blocks you can 2 1/2 blocks worth of pieces cut per fat quarter or 1/4 yard.

This is the amount you need per fabric group so, for example, you would need to purchase this much print/color fabric plus this much white fabric.

Working with Fat Eighths or 1/8 Yard cuts of yardage

If you want to do each block in a different print/color fabric, eighth yards are ideal.
Either a fat eighth or a 1/8 yard cut of yardage will allow you to cut all of the pieces you need for one block.

For my sample, I purchased fat quarters but I treated them as fat eighths by using half for the quilt top and half for the backing.

This is the amount you need per fabric group so, for example, you would need to purchase this much print/color fabric plus this much white fabric.

Working with Precuts

If you want to minimize how much cutting you need to do before you can start sewing, you can also work with precuts. Precuts lend themselves to different block sizes than the 12 inch blocks I created using fat quarters, fat eighths, and yardage.

Jelly Rolls

A Jelly Roll is a precut bundle that contains strips of fabric that are 2 1/2” wide x 42” long. A Jelly Roll usually contains 40 strips.

Let’s figure out what size quilt we can make with a Jelly Roll. 

Our strips are already cut to 2 1/2”. We need 7/8” for seam allowance. That means our half square triangle units will be 1 5/8” finished (heads up: that is pretty tiny).

We need to sub cut each strip into squares.
42” / 2 1/2” = 16 4/5”

We need to round down so we have a whole number of squares which means we can cut 16 squares from each strip.

Next, we need to figure out how many squares we can cut total from the Jelly Roll.

40 strips x 16 pieces per strip = 640 pieces

Each block is going to take 16 pieces to make which means we can make a maximum of:
640 pieces / 16 pieces per block = 40 blocks

The width and length of each finished block will be
4 x 1 5/8” = 6 1/2”

If we are using 6 1/2” finished blocks then we need…

  • Craft 4 blocks x 4 blocks = 16 blocks total
  • Crib  5 blocks x 5 blocks = 25 blocks total
  • Throw 8 blocks x 8 blocks = 64 blocks total

Oh, look at that. We can only make 40 blocks with a Jelly Roll so a throw size quilt is too big. That means a crib size quilt is the biggest we can make with one Jelly Roll.

We could also use 2 Jelly Rolls (this quilt would be really easy and awesome if you bought one Jelly Roll that was an awesome print collection and one all white Jelly Roll).

A twin size quilt would need 81 blocks which is just one too many for 2 Jelly Rolls (40 blocks per Jelly Roll x 2 = 80 blocks).

That means a throw sized quilt made with 2 Jelly Rolls would be ideal.

Charm Squares

Charms Squares are fabric that comes cut into 5” squares. Yay, squares! Minimal cutting required. Charm Square packages usually come with 42 Charm Squares per pack.

We need to leave 7/8” in each direction for seam allowance which means when working with Charm Squares, our finished half square triangle units will be:
5” - 7/8” = 4 1/8”

Our 16 patch blocks will be 4 half square triangle units wide by 4 half square triangle units tall which means our finished blocks will be:
4 x 4 1/8” = 17”

If we are using 17” finished blocks then we need…

  • Craft 1 block x 2 blocks = 2 blocks total
  • Crib  2 blocks x 3 blocks = 6 blocks total
  • Throw 3 blocks x 3 blocks = 9 blocks total
  • Twin 3 blocks x 5 blocks = 15 blocks total
  • Double 5 blocks x 5 blocks = 25 blocks total
  • Queen 5 blocks x 5 blocks = 25 blocks total
  • Super Queen 5 blocks x 6 blocks = 30 blocks total
  • King 6 blocks x 6 blocks = 36 blocks total

We need 16 Charm Squares to make each block which means we need a total of…

  • Craft 2 blocks x 16 Charm Squares = 32 Charm Squares total
  • Crib  6 blocks x 16 Charm Squares = 96 Charm Squares total
  • Throw 9 blocks total x 16 Charm Squares = 144 Charm Squares total
  • Twin 15 blocks total x 16 Charm Squares = 240 Charm Squares total
  • Double 25 blocks total x 16 Charm Squares = 400 Charm Squares total
  • Queen 25 blocks total x 16 Charm Squares = 400 Charm Squares total
  • Super Queen 30 blocks total x 16 Charm Squares = 480 Charm Squares total
  • King 36 blocks total x 16 Charm Squares = 576 Charm Squares total

Each pack contains 42 Charm Squares which means we need a total of…

  • Craft 32 Charm Squares total / 42 per pack = round up to 1 pack
  • Crib  96 Charm Squares total / 42 per pack = round up to 3 packs
  • Throw 144 Charm Squares total / 42 per pack = round up to 4 packs
  • Twin 240 Charm Squares total / 42 per pack = round up to 6 packs
  • Double 400 Charm Squares total / 42 per pack = round up to 10 packs
  • Queen 400 Charm Squares total / 42 per pack = round up to 10 packs
  • Super Queen  480 Charm Squares total / 42 per pack = round up to 12 packs
  • King  576 Charm Squares total / 42 per pack = round up to 14 packs

Take the number of packs you need and divide by 2 again. Get that many packs in each of your distinct fabric groupings (prints and/or colors + white is easy). For craft and crib size quilts which require an odd number of packages, round up to the next even number so you can split your fabrics into 2 distinct groups.

Layer Cakes

Layer Cakes are bundles of fabric that come cut into 10 inch squares. They usually come with 42 squares. Layer cakes are awesome if you want minimal cutting and minimal sewing (thanks to the larger size).

We need to leave 7/8” in each direction for seam allowance which means when working with Layer Cakes, our finished half square triangle units will be:
10” - 7/8” = 9 1/8”

Our 16 patch blocks will be 4 half square triangle units wide by 4 half square triangle units tall which means our finished blocks will be:
4 x 9 1/8” = 36 1/2”

If we are using 36 1/2” finished blocks then we need…

  • Crib 1 block x 1 block = 1 block total
  • Twin 1 blocks x 2 blocks = 2 blocks total
  • Double 2 blocks x 2 blocks = 4 blocks total
  • Super Queen 2 blocks x 3 blocks = 6 blocks total
  • King 3 blocks x 3 blocks = 9 blocks total

Since Layer Cake squares are so large, some quilt sizes would use the same number of blocks. I’ve selected the smaller batting size in those cases so that you can save material.

We need 16 squares to make each block which means we need a total of…

  • Crib 1 block x 16 squares = 16 squares total
  • Twin 2 blocks x 16 squares = 32 squares total
  • Double 4 blocks x 16 squares = 64 squares total
  • Super Queen 6 blocks x 16 squares = 96 squares total
  • King 9 blocks x 16 squares = 144 squares total

Each pack contains 42 squares which means we need a total of…

  • Crib 16 squares total / 42 per pack = round up to 1 pack
  • Twin 32 squares total / 42 per pack = round up to 1 pack
  • Double 64 squares total / 42 per pack = round up to 2 packs
  • Super Queen 96 squares total / 42 per pack = round up to 3 packs
  • King 144 squares total / 42 per pack = round up to 4 packs

Take the number of packs you need and divide by 2 again. Get that many packs in each of your distinct fabric groupings (prints and/or colors + white is easy). For crib, twin, and super queen size quilts which require an odd number of packages, round up to the next even number so you can split your fabrics into 2 distinct groups. Really though, buying 2 Layer Cake packages to make a crib size quilt isn’t going to be practical. These giant squares are going to be ideal for large quilts.