Concrete Block Calculator

How many concrete blocks do I need?

This concrete block calculator divides the wall’s face area by 0.888 square feet, the space one standard 8 × 16 inch block fills in a wall once you include its mortar joint. That is 1.125 blocks per square foot. The formula is blocks = (length × height) ÷ 0.888, rounded up. A 20 ft wall that is 8 ft tall needs 181 blocks.

Enter the wall length and height in feet and your price per block. Tick the 10% waste box to cover breakage and cuts.

Why 0.888 sq ft per block

A standard concrete masonry unit (CMU) is 15-5/8 × 7-5/8 inches on its face. Add a 3/8-inch mortar joint and it takes up a clean 16 × 8 inch rectangle in the wall. That is 128 square inches, and 128 ÷ 144 = 0.888 sq ft. Turn it around and 1 ÷ 0.888 = 1.125 blocks for every square foot of wall.

The same face size is used for 4, 6, 8, 10, and 12 inch thick blocks. Wall thickness changes the weight and the grout volume, not the block count.

Worked examples

1. Garage wall: 20 ft × 8 ft, $2.50 per block

• Area: 20 × 8 = 160 sq ft
• Blocks: 160 ÷ 0.888 = 180.2, rounded up to 181 blocks, costing $452.50
• With 10% waste: 180.2 × 1.10 = 198.2, rounded up to 199 blocks, costing $497.50

2. Crawlspace foundation: 100 ft perimeter, 4 ft tall

• Area: 100 × 4 = 400 sq ft
• Blocks: 400 ÷ 0.888 = 450.5, rounded up to 451 blocks, costing $1,127.50
• With 10% waste: 495.5, rounded up to 496 blocks, costing $1,240.00

Check by courses: 4 ft is 6 courses, and 100 ft × 0.75 is 75 blocks per course. That gives 75 × 6 = 450 blocks, which matches.

Concrete block reference

Wall height	Courses (8 in each)	Blocks per linear foot of wall
2 ft	3	2.25
4 ft	6	4.50
6 ft	9	6.75
8 ft	12	9.00
10 ft	15	11.25

Nominal block (W × H × L)	Common use
4 × 8 × 16 in	Veneer, partitions, cap courses
6 × 8 × 16 in	Interior partitions, some screen walls
8 × 8 × 16 in	Foundations, garages, most exterior walls
10 × 8 × 16 in	Taller basement walls, heavier loads
12 × 8 × 16 in	Retaining walls, deep basements

The thickness you need is set by wall height, soil load, and reinforcement. Check local code or your engineer.

How to measure a block wall

• Measure in 8-inch steps. Block walls work best when the length and height land on 8-inch multiples. A 20 ft 4 in wall is 30.5 blocks long, so you end up cutting blocks. Adjust the layout early if you can.
• Subtract openings. Doors and windows come off the area. Lintels and bond beam blocks over the openings are ordered separately.
• Count special blocks. Corner, end (bullnose or plain-end), half, and bond beam blocks replace some standard units. They are sold by count, so list them at each corner and opening.
• Measure from the footing up. Start at the top of the footing, not grade, and include courses that will end up below ground.

Common block-wall mistakes

• Treating thickness as a dimension. Entering the 8 inch thickness instead of the height gives a nonsense count.
• Forgetting the footing. Blocks need a level concrete footing that is wider than the wall. Size it with the concrete footing calculator.
• Skipping grout and rebar. Foundation and retaining walls are usually reinforced and partly or fully grouted. The core fill grout calculator estimates the concrete for the cells.
• Buying mortar by guesswork. Use the mortar calculator so you do not run out mid-course.

For smaller decorative walls, compare the brick calculator or the cinder block calculator.

Load-bearing and retaining walls should be designed or checked by a licensed engineer or approved by your local building department before you build.