Estimate how many standard concrete blocks you need for a wall from its dimensions, with a waste factor.
blocks = wall_area_ft² × 1.125 × (1 + waste)
A standard 16×8″ nominal concrete block covers 0.888 ft² of wall, so you need about 1.125 blocks per square foot.
CMU is dimensionally regular and cut less often, so a 5% waste factor is usually enough.
A foundation wall 40 ft long and 4 ft high is 160 square feet of face. Standard 16 x 8 inch block covers 0.888 square feet each, so you need about 180 blocks before waste. CMU is dimensionally regular and cut far less than brick, so 5% waste is usually plenty — about 189 blocks.
Don't forget the mortar: at roughly 1 bag per 13-15 blocks, 189 blocks needs about 13-14 bags of mortar. Use the mortar calculator to confirm.
Block coverage assumes a 16×8″ nominal face per ASTM C90 sizing. The 1.125 blocks/ft² figure is an industry convention.
About 1.125 for standard 16×8″ nominal block.