For me, I can't quilt without the math. I'll be happy to walk you through the math, but I don't have enough information for the entire quilt.
1 - how many blocks are you going to make for your quilt?
2 - will there be sashing, cornerstones, borders? If so, how big?
There may be more pieces needed, but I'll get you started.
There are many ways to make HST. The one with the least waste says add 7/8" to your finished square to find out the size of square to cut. Draw a line down the center and sew 1/4" on either side of the line. However, I round up to the full inch for three reasons:
#1 - if I only cut my square 4 7/8" my finished HST will not be correct -- I have yet to sew that perfect 1/4" seam or more importantly cut a perfect 4 7/8" square.
#2 - it is so much easier to cut a 5" square - the waste is minimal when compared to my frustration of working with 4 7/8" square
#3 - the math is so much easier with 5".
You'll need 1 - 5" square for every 2 HST - 2 HST make one pieced square.
6 squares of the first color, 5 of the 2nd color, 4 of the third color and 1 of the last color.
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So, if you are making a quilt which has 12 blocks and no sashing, you then multiple the # of squares needed by 12.
6 x 12 = 72 - color 1
5 x 12 = 60 - color 2
4 x 12 = 48 - color 3
1 x 12 = 12 - color 4
Since yardage varies widely, and I've seen some really wide selvages, I figure my yardage on 40" of usable fabric. I'd rather have too much than not enough.
So, you can cut 8 - 5" squares from 40" ( 40/5 = 8)
The next step is to determine how many strips you need.
Color 1 the math is 72 squares / 8 per strip = 9 strips
9 strips * 5" = 45" with no extra
Color 2 = 60 squares / 8 per strip = 7.5 strips -- you have to round up
8 strips * 5" = 40"
Color 3 = 48 squares / 8 per strip = 6 strips
6 strips * 5" = 30"
Color 4 = 12 squares / 8 per strips = 1.5 strips - round up
2 strips * 5" = 10"
The amounts are the bare minimum I would buy.
Hope this helps. If you have any questions, don't hesitate to ask.