How the block count works
Blocks are sold by their nominal size: the real block plus one 3/8 in mortar joint. A standard block is nominally 16 in long and 8 in tall, so it covers 128 sq in, or 0.89 sq ft, of wall.
- Rows: wall height ÷ 8 in, rounded up.
- Blocks per row: wall length ÷ 16 in, rounded up.
- Total: rows × blocks per row, minus whole blocks that fit inside door and window openings.
- Waste: add 5 to 10% for broken blocks and cut ends.
Counting row by row gives a slightly higher number than area × 1.125, because a row that ends partway through a block still needs a whole block, which you cut to fit. The row count is what you'll actually buy.
How many blocks for common wall sizes
| Wall length | 4 ft high | 6 ft high | 8 ft high |
|---|---|---|---|
| 10 ft | 48 | 72 | 96 |
| 20 ft | 90 | 135 | 180 |
| 30 ft | 138 | 207 | 276 |
| 40 ft | 180 | 270 | 360 |
Standard 8 × 8 × 16 in blocks, no openings, before waste.
How much mortar you need
Quikrete's data sheet for its Mortar Mix says one 80 lb bag will lay up to 37 standard bricks or 13 standard (8 in by 8 in by 16 in) blocks
. "Up to" assumes no spills or thick joints, so round up and keep a spare bag or two. If you use a different product, enter its coverage in the calculator.
Joints should be about 3/8 in thick, according to Quikrete's concrete block construction guide. Thicker joints use more mortar.
Worked example
A garden wall 20 ft long and 6 ft high, standard blocks, 5% waste:
- Rows: 72 in ÷ 8 in = 9
- Blocks per row: 240 in ÷ 16 in = 15
- Blocks: 9 × 15 = 135, plus 5% = 142
- Mortar: 142 ÷ 13 = 11 bags (80 lb)
Common questions
How many concrete blocks per square foot?
A standard 8 × 8 × 16 in block covers 0.89 sq ft of wall face including the mortar joint, so you need 1.125 blocks per square foot. Half-high blocks (4 × 16 in face) need 2.25 per square foot.
How many cinder blocks do I need for a 10 × 10 ft wall?
About 120 standard blocks: 8 blocks per row (10 ft ÷ 16 in, rounded up) times 15 rows (10 ft ÷ 8 in, rounded up). Add 5 to 10% for breakage and cuts.
How much mortar do I need for concrete blocks?
Quikrete states that one 80 lb bag of its Mortar Mix lays up to 13 standard 8 × 8 × 16 in blocks. Divide your block count by 13 and round up, then buy one or two extra bags because "up to" assumes no waste.
Is a cinder block the same as a concrete block?
Most blocks sold as "cinder blocks" today are concrete masonry units (CMU). The count depends on block size, not the material, so this calculator works for both.
Do I need rebar or to fill the cores?
Retaining walls, load-bearing walls and walls in high-wind or earthquake areas often need vertical rebar and grout-filled cores. The spacing is set by your local code or engineer, so this calculator doesn't guess it.
Sources
- Quikrete Mortar Mix (No. 1102) data sheet: mortar yield per 80 lb bag
- Quikrete, Concrete Block Construction: 3/8 in mortar joints