Just a rough guestimate... a fat quarter is just a bit larger than a block... decide on the ratio of each color in the block. Say the block is 3 colors & the background color is 1/2 of the block & the other 2 colors are 1/4 each. So you're making 12 blocks... I would guess the background fabric to be approximately 6 ft qtrs or 1.5yd. The other two fabrics would be approx 3/4yds ea. Then you need to add fabric for sashing & boarders. This is all rough on the spot guestimate... then I get 1/4 - 1/2yd extra just in case.