How Many Concrete Blocks Do I Need?
Concrete block (cinder block / CMU) is estimated by area: about 112.5 standard blocks per 100 square feet of wall. Here is the figure for every block size, tables by wall area and wall size, the mortar to go with it, and a worked example.
By the Calculate My Reno Team / Published
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Concrete block - whether your supplier calls it cinder block, concrete block or CMU - is estimated by area, exactly like brick. The headline figure is 112.5 standard blocks per 100 square feet of wall, and everything else follows from the block size and the joint.
Skip the math
Enter your wall sizes, take off the openings, pick the block size and joint, and the concrete block calculator returns the block count, the mortar and the sand. This guide is the method and the tables behind it.
The quick formula
blocks per area = 1 ÷ ((block length + joint) × (block height + joint))
net wall area = sum of (wall length × height) − doors & windows
blocks = ceil( net area × blocks/area × waste )
A standard 8 × 8 × 16 in block (actual 7-5/8 × 7-5/8 × 15-5/8 in) on a 3/8 in joint fills a nominal 8 × 16 in face, which is exactly 112.5 blocks per 100 sq ft (1.125 per sq ft). A metric 390 × 190 × 190 mm block on a 10 mm joint is 12.5 blocks per m². Add 5% waste for a clean wall, 10% where there are lots of openings and cuts.
Blocks by wall area
Standard 8 × 8 × 16 in block, 1.125 per sq ft, with 5% waste:
| Net wall area | Blocks (8×8×16) | Metric block (12.5/m²) |
|---|---|---|
| 100 sq ft (9.3 m²) | 119 | 122 |
| 200 sq ft (18.6 m²) | 237 | 244 |
| 300 sq ft (27.9 m²) | 355 | 366 |
| 500 sq ft (46.5 m²) | 591 | 610 |
| 1,000 sq ft (93 m²) | 1,182 | 1,220 |
Blocks by wall size
A few everyday block jobs, standard block, 5% waste:
| Wall | Area | Blocks |
|---|---|---|
| Retaining / garden 20 × 4 ft | 80 sq ft | 95 |
| Garage wall 24 × 8 ft | 192 sq ft | 228 |
| Foundation 40 × 4 ft | 160 sq ft | 190 |
| Shed base course 32 ft × 1 course | ~21 sq ft | 25 |
For a stacked retaining-wall block (interlocking, no mortar), the count is by course and face size instead - see the dedicated retaining-wall method rather than the per-area figure here.
Block sizes and the per-area count
The face (length × height) sets the count; the depth (6, 8, 12 in) sets the wall thickness and the mortar - see concrete block sizes for the full dimensions and how much a block weighs for the handling weight:
| Block (nominal) | Per 100 sq ft | Per m² |
|---|---|---|
| 6 × 8 × 16 in | 112.5 | ~12.1 |
| 8 × 8 × 16 in | 112.5 | ~12.1 |
| 12 × 8 × 16 in | 112.5 | ~12.1 |
| 390 × 190 × 190 mm | ~116 | 12.5 |
| 440 × 215 × 100 mm (UK) | ~92 | ~9.9 |
Worked example
A 24 ft × 8 ft single-skin block garage wall, less a 9 ft × 7 ft door, 5% waste:
- Gross wall area: 24 × 8 = 192 sq ft.
- Deduct the door: 9 × 7 = 63 sq ft, so net = 129 sq ft.
- Blocks: 129 × 1.125 = 145, × 1.05 waste = 153 blocks.
Round up, and order the mortar to suit - see how much mortar do I need. If you are laying brick instead, how many bricks do I need uses the same area method.
Don’t forget the footing, caps and rebar
The block count is only the wall itself. Three things sit around it:
- Footing. A block wall needs a concrete footing under it - size the pour with the concrete calculator.
- Capping. Finish a freestanding garden or boundary wall with a capping course (besser caps or solid cap blocks) along the top. Caps are counted by the wall’s length, not its area - the concrete block calculator has a per-wall cap option that counts and prices them.
- Rebar and grout. Taller, retaining or load-bearing walls usually need vertical rebar in grouted cores (and often a bond-beam course) - the concrete block calculator has an optional reinforcement section that estimates the bars, the grout to fill the cores, and their cost from your spacing (a quantity estimate, not a structural design). Step the mortar up to Type S below grade, and see concrete block sizes for the bond-beam and cap units.
Frequently asked questions
How many concrete blocks are in 100 square feet?
About 112.5 standard 8 × 8 × 16 in blocks cover 100 square feet of wall with a 3/8 in joint. That is one block per nominal 8 × 16 in (128 sq in) face, so 14,400 sq in ÷ 128 = 112.5. The depth of the block (6, 8 or 12 in) does not change this - it changes the wall thickness and the mortar, not the count per area.
How many cinder blocks do I need for a wall?
Multiply the wall area (length times height, minus openings) by 1.125 blocks per square foot for standard 8 × 8 × 16 in block - or divide by 100 and multiply by 112.5. So a 200 sq ft wall is about 225 blocks before waste. Cinder block and concrete block are estimated identically. Add 5 to 10% for cuts and round up.
How many blocks high is a wall?
A standard block is 8 in high with the joint (7-5/8 in block plus 3/8 in bed), so divide the wall height in inches by 8. An 8 ft (96 in) wall is 12 courses; a 4 ft wall is 6. A metric 190 mm block on a 10 mm joint is 200 mm per course, so a 2.4 m wall is 12 courses.
How much mortar do I need for concrete block?
About 4 cubic feet of mortar per 100 standard blocks (0.11 m³) with face-shell bedding - roughly 8 bags of premixed 80 lb mortar mix, since a bag lays about a dozen blocks, or about 3 bags of masonry cement plus sand if you site-mix. Full (solid) bedding or fuller joints use up to twice as much, so buy a little over. The calculator estimates it from your block size, joint and bag size and gives the bags and sand.