My sister told me she wanted the blanket to be around 30" x 40". Given that most fabrics come at 44" width or more, buying just one yard of each fabric was sufficient, but it never hurts to check before a purchase to be sure you have enough for your project.
For the weight you want 10% of the person's weight + 1lb. My nephew is just shy of 30 lbs, so the equation looked like this:
30lbs x .1 = 3 + 1 = 4 lbs.
Once I knew how much weight, I needed to figure out how much would go into each section, which also means I needed to know how many sections I would end up with. I decided the squares would be 4" x 4", so the blanket would end up being 32" x 40" in order to make 4" squares.
32" ÷ 4 = 8 squares; 40" ÷ 4 = 10 squares 8 x 10 = 80 total sections.
1 lb = 16 oz; 4 x 16 = 64 oz in 4 lbs.
64 oz ÷ 80 sections = .8 oz per section.
For the binding, I decided using the 2" binding would affect the corner squares too much, so I cut the binding in half lengthwise. Figuring the length was simple enough.
32" x 2 sides = 64"; 40" x 2 sides = 80"; 64" + 80" = 144"
Since one package of binding is 4.75 yards or 171" (4.75 x 36in/yd) so one package is plenty for this project.
That does it for the math, you can breathe easy now ;-)
Since the sewing on this project is done all with the right sides out, I did not leave extra for a seam allowance. To start, clip the wrong sides of the fabric together with one of my new favorite tools, Wonder Clips
(Seriously, if you haven't tried these, you want to!) Then sew a seam down the two long sides leaving the top and bottom open.
Find the center of the fabric and sew a new seam across all of the seams. This is the start of the blocks where the pellets will go.
Using a food scale, measure out the portion of weight you need for each section, in this case 0.8 oz. Pour the pellets into each channel and shake them to the seam. Sew a seam to close the 4" blocks. Repeat this until you get to the last set of squares. Fill the first two, sew one, then add the pellets to the third so you always have one you are about to sew, and one more full of pellets. This keeps them from falling out of the lower ones, and is much easier to fill than if they are near the machine foot, trust me... Then, turn the blanket and follow the same steps to complete the second half.
When you come to a corner, bend the binding 90°, then fold the edges down so the corner of the fold meets the edge of the binding. Clip this in place to hold it while you finish sewing it into place.