Concrete block calculator

Standard 8 by 8 by 16 inch block is sized so each block plus its mortar joint covers 16 by 8 inches of wall, which is 1.125 blocks per square foot. Multiply the wall length by the height, subtract doors and windows, multiply by 1.125 and add a few percent for breakage and cuts. A wall 40 feet long and 8 feet tall is 320 square feet, or 360 blocks before extra.

With 8 inch nominal block, each course is 8 inches tall including the joint, so there are 1.5 courses per foot of height. An 8 foot wall is 12 courses, and a 4 foot footing wall is 6.

Block is sold by nominal size. The actual unit is 3/8 inch smaller in each direction so that one 3/8 inch mortar joint brings it back to a clean 8 by 16 inch module. That is why wall lengths and openings are laid out on 8 inch increments.

It depends on the mortar type, whether you mix from masonry cement and sand or use a premixed bag, the joint thickness and waste on the board. Use the yield printed on the bag or the supplier's guidance for your mix rather than a single rule of thumb.

Spacing is set by the engineer or the code for the wall's height and load, and it is often somewhere from 16 to 48 inches on center, with bars at corners, wall ends and beside openings. Cores with rebar are filled with grout. Follow the plans.