First lets make one assumption: You will be using a 12" block. The standard nominal size would be; 12"deep x 8"high x 16"long. 12x8x16. Lets keep this simple. First figure your total linear feet for the foundation. Example: 40feet long by 28feet wide. 40 + 28 + 40 + 28 = 136 linear feet. Multiply 136 X .75 = 102 That means you need 102 block per course. This is just a basic figure. You will have to consider the number of specialty blocks needed. These would include: L-corners, sash block, half jambs, double ends, half highs, bond beams ect.. a phone call to your local concrete products supplier would be the best thing to do. They would be able to figure out exactly what you need. Good luck.