Concrete Block Calculator

What Is the Concrete Block Calculator?

The Concrete Block Calculator estimates the number of concrete masonry units, usually called CMU or cinder blocks, needed to build a wall. It also reports the number of courses, the blocks in each course and the bags of mortar required.

Concrete block is used for foundations, basement walls, retaining walls, garages, sheds and garden walls. Blocks are large and heavy, so the count is smaller than for brick, and each one represents a meaningful cost and weight. An accurate estimate helps plan delivery and placement of pallets as well as the budget.

How the Calculator Works

A standard block is nominally 8 inches high and 16 inches long. Its actual size is 7 5/8 by 15 5/8 inches, which leaves room for a 3/8 inch mortar joint. With the joint, each block occupies exactly 8 by 16 inches of wall face, or 0.889 square feet.

The calculator adds the joint thickness to the block length and height, multiplies them, and divides 144 by the result to get the number of blocks per square foot. For standard block this is 1.125.

It multiplies the wall length by its height to get the area, multiplies by blocks per square foot, adds the waste percentage, and rounds up. Mortar is estimated at one 80 pound bag of premix per 12 blocks.

The Concrete Block Formula

Wall area = Length × Height

Blocks per sq ft = 144 ÷ ((Block length + Joint) × (Block height + Joint))

Blocks = Wall area × Blocks per sq ft × (1 + Waste % ÷ 100), rounded up

Mortar bags = Blocks ÷ 12, rounded up

A quick rule

For standard 8 × 16 inch block, multiply the wall area in square feet by 1.125. A 100 square foot wall needs 113 blocks before waste.

Step-by-Step Explanation

1. Measure the wall length and height in feet.
2. Multiply them to get the area in square feet.
3. Multiply the area by 1.125 for standard block, or calculate blocks per square foot for other sizes.
4. Add 5 to 10 percent for breakage and cuts.
5. Round up to a whole block.
6. Divide by 12 for the number of 80 pound mortar bags.

Worked Example

A garage wall is 20 feet long and 8 feet high, built with standard block.

• Area: 20 × 8 = 160 square feet.
• Blocks: 160 × 1.125 = 180.
• With 5 percent waste: 189 blocks.
• Courses: 96 inches ÷ 8 = 12.
• Blocks per course: 240 inches ÷ 16 = 15.
• Mortar: 189 ÷ 12 = 15.75, so 16 bags.

You can check the count another way: 12 courses of 15 blocks is 180, which matches. If the wall has a 3 by 7 foot door, subtract 21 square feet, or about 24 blocks, and add a lintel above the opening.

How to Use the Calculator

1. Enter the wall length and height.
2. Leave the block dimensions at the defaults for standard 8 × 16 block, or enter the actual size of a different unit.
3. Enter the mortar joint thickness.
4. Set a waste allowance.
5. Press Calculate.
6. Note the block count, courses and mortar.

Block walls sit on a poured footing, which you can estimate with the Concrete Calculator. The Mortar Calculator gives bag counts for different bag sizes. If you are facing the wall with brick, the Brick Calculator counts the veneer. Drainage stone behind a retaining wall can be sized with the Gravel Calculator, and core fill grout is a volume you can work out with the Cubic Yard Calculator.

Practical Tips

• Plan wall dimensions in multiples of 8 inches where you can. It avoids cutting blocks.
• Order half blocks and corner blocks for ends and openings. Each end of a wall in running bond needs a half block on alternate courses.
• Have pallets placed close to the work. A standard block weighs 30 to 38 pounds.
• Lay the first course in a full bed of mortar on the footing and take care to get it level. Every course above follows it.
• Build corners up first, then stretch a line between them.
• Use face-shell bedding for normal courses, with mortar on the outer edges only.
• Place horizontal joint reinforcement every second or third course as specified.
• Cover the top of unfinished walls to keep rain out of the cores.

Common Mistakes

Using actual size without the joint. Calculating with 7 5/8 by 15 5/8 inches and no joint overstates the count by about 7 percent.

Forgetting special units. Corners, lintels, bond beams and half blocks are separate items.

Ignoring reinforcement. Many block walls need vertical rebar and grouted cores. That is additional steel and concrete.

An undersized footing. The footing is normally twice the wall width and at least as deep as the wall is thick.

Not accounting for wall thickness. Block comes in 4, 6, 8, 10 and 12 inch widths. The face count is the same, but price and weight are not.

Skipping drainage on retaining walls. Water pressure behind a wall is the main cause of failure.

Block Sizes

The face of a standard block is nominally 8 by 16 inches. Thickness varies: 4 inch block is used for veneers and partitions, 6 inch for light walls, 8 inch for most structural walls, and 10 or 12 inch for foundations and tall or retaining walls. Half-high blocks, 4 inches tall, are also made. Since the face size is the same, the count per square foot does not change with thickness.

Core Fill and Reinforcement

Hollow blocks have two cores. Where the design calls for it, vertical rebar is placed in certain cores and they are filled with grout. As a rough guide, fully grouting an 8 inch block wall takes about 1.1 cubic yards of grout per 100 square feet. Grouting only the reinforced cores at 32 or 48 inch spacing uses much less. An engineer or the local code sets the requirement.

Foundations and Retaining Walls

Foundation walls must resist soil pressure and carry the house. Retaining walls taller than 3 to 4 feet usually need engineering and a permit. Both need waterproofing or damp-proofing on the soil side, drainage stone, and a perforated drain pipe at the base.

Laying Out the First Course

Snap chalk lines on the footing for both faces of the wall and dry-lay the first course with 3/8 inch gaps to check how the blocks fall at corners and openings. Adjust the joints slightly if the length is a little off, so that you avoid a sliver cut at one end. Once the layout works, lift the blocks, spread a full mortar bed and set the corner blocks first. A storey pole marked every 8 inches helps keep each course at the right height all the way up.

Finishing a Block Wall

Bare block is porous. Exterior walls are commonly finished with stucco, a parge coat, masonry paint or a stone or brick veneer. Interior basement walls are often furred out with framing, insulated and covered with drywall. Tool the mortar joints concave for weather resistance if the block will stay exposed, and cap the top course with solid cap blocks so water cannot enter the cores.

Estimating Cost and Delivery

Blocks are priced per piece, with a discount for full pallets. A pallet of standard 8 inch block usually holds 72 to 90 units. Divide the block count by the pallet quantity to plan the delivery. Add mortar, joint reinforcement, rebar and grout where specified, plus cap blocks for the top course. Because a pallet weighs well over a ton, decide where each one will be set down before the truck arrives, ideally along the length of the wall so that blocks do not have to be carried far.