How Many Concrete Blocks Do I Need? 8×8×16 Blocks per Wall, Mortar and Core Fill

A wall takes 1.125 standard 8×8×16 in blocks per sq ft of wall face: 112.5 per 100 sq ft, or 119 with a 5% allowance for waste and breakage, the figure in NCMA TEK 04-02A.

Each block shows 8/9 sq ft of wall face. A 20 ft × 8 ft wall with one 3 ft × 7 ft door is 139 sq ft, which takes 156.4 blocks net and 165 with waste. Mortar for those is 10 bags of 80 lb mix by the NCMA rate or 13 by the QUIKRETE label.

How do you calculate how many concrete blocks you need?

net wall area (sq ft) = length × height − openings
blocks before waste   = net wall area ÷ face area of one block   (8/9 sq ft = 0.8889 for an 8×16 face)
                      = net wall area × 1.125
blocks to buy         = ceil( blocks before waste × (1 + waste) )

1. Find the area of each wall in square feet: length × height. The square footage calculator helps when a wall has a gable or a step.
2. Subtract the area of doors, windows and other openings.
3. Multiply by 1.125, the number of 8×16 blocks in a square foot. NCMA explains it this way: the exposed surface of one unit in the wall is 8/9 sq ft, so you divide the wall area by that.
4. Add waste. NCMA’s figure includes 5% for waste and breakage. This is an editable allowance, so raise it for a wall with many corners or cut blocks.
5. Round up to whole blocks.

A block sold as 8×8×16 is a nominal size, which includes the mortar joint, so it fills 8 in of wall height and 16 in of length once laid. People also call it a cinder block, but the unit is a concrete masonry unit, or CMU. A 6 in, 10 in or 12 in thick block has the same 8 in × 16 in face, so the count per square foot is the same. The thickness changes the grout volume and the weight. Brick walls are counted the same way from a smaller face: see how many bricks per square foot.

Blocks per 100 sq ft by unit size

The count is the wall area divided by the face area of the unit, so other shapes follow from their faces.

The standard-unit row is NCMA’s. The others follow from the same method and are arithmetic on the face sizes. NCMA warns that counts vary with the mason’s skill, non-modular layouts and numerous returns and corners.

Worked example: a 20 × 8 ft wall with a door

• Gross area: 20 × 8 = 160 sq ft.
• Door: 3 × 7 = 21 sq ft, so the net area is 160 − 21 = 139 sq ft.
• Blocks before waste: 139 × 1.125 = 156.375.
• With 5% waste: 156.375 × 1.05 = 164.19, so 165 blocks.
• Mortar at the NCMA rate of 16 blocks per 80 lb bag: 156.375 ÷ 16 = 9.77, so 10 bags.
• Mortar at the QUIKRETE label of up to 13 standard blocks per 80 lb bag: 156.375 ÷ 13 = 12.03, so 13 bags.

The two mortar counts differ because NCMA states a generic rate and QUIKRETE states what its own bag lays. Use the label for the mortar you buy.

How do courses, height and length work?

You can also count by courses, the horizontal rows of block. Each course is 8 in tall, and each block is 16 in long.

courses          = wall height (in) ÷ 8
blocks per course = wall length (in) ÷ 16

The 20 ft × 8 ft wall has 96 ÷ 8 = 12 courses of 240 ÷ 16 = 15 blocks, which is 180 blocks before the door. That equals 160 × 1.125 = 180, so the two methods agree. Walls laid to the module come out even: a wall 8 ft high has 12 courses exactly, and a wall 20 ft long has 15 blocks per course exactly. A 7 ft wall is 10.5 courses and a 10 ft wall is 7.5 blocks long, so those need half-high or cut blocks. Half-length blocks are made for the ends of courses.

Quick table by wall size

These counts are for solid walls with no openings, 8×8×16 blocks and a 5% allowance. Mortar is for the net block count without waste, in 80 lb bags, at the NCMA rate of 16 blocks per bag and the QUIKRETE label rate of 13.

How many bags of mortar per 100 blocks?

NCMA’s table gives one preblended 80 lb bag per 16 conventional blocks (50 brick-size units), and it includes nominal waste. That is 100 ÷ 16 = 6.25, so 7 bags per 100 blocks. The QUIKRETE Mortar Mix No. 1102 data sheet says each 80 lb bag lays up to 13 standard 8×8×16 blocks, so 100 ÷ 13 = 7.69, or 8 bags per 100 blocks. Per 100 sq ft of wall (112.5 blocks) the two rates give 7.03 and 8.65 bags, so 8 or 9 bags. The NCMA rate assumes face-shell bedding, where mortar goes on the outer shells of the block only. Joints, skill and layout move it up or down. The guide on how much mortar you need covers brick and block together.

Core fill and footings

If the cores of a wall are filled with grout, NCMA gives the volume per 100 sq ft of wall by how far apart the filled cores are. These figures are for two-core units, include 3% waste and are in cubic feet:

For the 139 sq ft wall with cores filled every 16 in, that is 139 ÷ 100 × 18.1 = 25.16 cubic feet, or 0.93 cubic yards at 27 cubic feet per cubic yard. Which cores get filled, and whether they carry reinforcement, is set by the wall’s design. Ask your building department or an engineer. This guide only converts a spacing you have been given into a volume. Ask your supplier whether the cores are filled with grout or with a concrete mix, and use the yield printed on that product. For the volume of a footing, or of fill that comes from a concrete mix, the concrete calculator takes the sizes and the bag yield.

A strip footing under the wall is a volume too: length × width ÷ 12 × depth ÷ 12, in feet and inches. For illustration only, a 20 ft footing 16 in wide and 8 in deep holds 20 × 1.3333 × 0.6667 = 17.78 cubic feet, or 0.66 cubic yards. Footing size is not decided by the block count. It depends on soil, load, frost depth and local code.

Assumptions and limits

• Rectangular walls laid in running bond to the module, with a standard joint. Non-modular layouts, many returns and corners, bond beams, lintel blocks and corner units change the count.
• Rebar, ties, anchors and lintels are not counted here. They are engineered items, as are wall height, lateral support and mortar type. Ask your building department or a licensed engineer.
• Mortar and grout rates are for the products and assumptions named above. Use the label on the product you buy.
• Mortar and grout are cement-based. Wear gloves and eye protection.

Common questions