Measure the height and width of the area you need the cinder blocks to fill and then multiply these numbers together. This gives you the total area that needs to be covered in cinder blocks.
Find the type of cinder block you want to use and measure the height and width of an individual block. Multiply these numbers together as well.
Divide the area of your desired location by the area of the block. This total is the number of blocks you will need to complete your project.