Concrete Block Calculator
This concrete block calculator works out how many blocks a wall needs, the mortar that goes with them, the concrete that fills the cores, and what the whole thing costs.
Block Wall, Blocks & Mortar
Block Fill, Core Grout Volume
Block Wall Cost
How to use the Concrete Block Calculator
Enter the wall’s length and height, pick the block size, and subtract any door or window openings. The calculator returns the block count, the mortar that goes with it, and the courses the wall will run to.
Use the Core Fill tab when the cells are being grouted, and the Cost tab once you have supplier prices. Prices there are placeholders, replace them with real quotes.
Concrete Block formula
Block counting works off nominal dimensions, which already include the 3/8 in mortar joint. A standard block is 8 × 16 in nominally, so it covers 0.889 sq ft of wall:
blocks per sq ft = 144 ÷ (nominal height × nominal length)- Standard 8 × 16 in block:
144 ÷ 128 = 1.125 blocks per sq ft - Half-height 4 × 16 in block:
144 ÷ 64 = 2.25 blocks per sq ft
Then:
wall area = (length × height) − openingsblocks = wall area × blocks per sq ft × (1 + waste)courses = wall height ÷ nominal block height
Mortar follows published masonry coverage: one 80 lb bag lays about 12 blocks, one 60 lb bag about 8.8.
Core fill uses grout yields per block:
| Block width | Grout per block | Blocks per cubic yard |
|---|---|---|
| 6 in | 0.225 ft³ | 120 |
| 8 in | 0.270 ft³ | 100 |
| 10 in | 0.338 ft³ | 80 |
| 12 in | 0.415 ft³ | 65 |
Worked example
A 20 ft × 8 ft garden wall in standard 8 in block, no openings, 5% waste:
- Wall area: 20 × 8 = 160 sq ft
- Blocks: 160 × 1.125 = 180, plus 5% = 189 blocks
- Mortar: 189 ÷ 12 = 16 bags of 80 lb
- Courses: 96 in ÷ 8 in = 12 courses of 15 blocks
Solid grouting all 189 cores would take 189 ÷ 100 = 1.89 cubic yards, ready-mix territory, not bags.
What the result means
The block count already includes your waste allowance, so it is the number to order. Blocks are sold by the unit and most suppliers take returns on full cubes, so rounding up costs little.
Mortar bags are the bigger source of error: coverage swings with joint thickness, block texture and how much gets dropped. Treat the bag count as a starting point and keep a spare bag or two on site.
Common mistakes
- Using actual dimensions instead of nominal. A standard block measures 7 5/8 in, not 8. The missing 3/8 in is the mortar joint, and nominal sizing already accounts for it. Counting on 7 5/8 overestimates by about 5%.
- Forgetting openings. A single 3 ft × 7 ft doorway removes 21 sq ft, roughly 24 blocks.
- Assuming every core gets filled. Most residential walls only grout the cells with vertical rebar. Solid grouting can cost several times more.
- Ignoring the bond beam and lintels. These need different units and extra grout, and are not in the plain block count.
Frequently asked questions
How many concrete blocks are in a square foot?
A standard 8 by 16 inch block covers 0.889 square feet, so you need 1.125 blocks per square foot of wall. Half-height blocks at 4 by 16 inches need 2.25 per square foot.
How many blocks can I lay with one bag of mortar?
About 12 blocks per 80 lb bag, or roughly 9 bags per 100 blocks. A 60 lb bag lays about 8.8 blocks. Coverage varies with joint thickness and how rough the block face is, so keep a spare bag on hand.
How much concrete does it take to fill a block wall?
One cubic yard fills about 100 standard 8 inch blocks, 120 six inch blocks, 80 ten inch blocks, or 65 twelve inch blocks. That is roughly 1.12 cubic yards per 100 square feet of 8 inch wall when every core is grouted.
What size is a standard concrete block?
Nominally 8 by 8 by 16 inches. The block itself measures 7 5/8 inches in each direction, with the remaining 3/8 inch taken up by the mortar joint. Estimating always uses the nominal size.
How many blocks high can a wall go without reinforcement?
That depends on wall thickness, soil, wind load and whether the wall is retaining anything, and it is set by your local building code rather than a rule of thumb. Any freestanding or retaining wall past a few courses needs vertical rebar and grouted cells specified by an engineer.
Do I need to fill concrete blocks with concrete?
Not always. A non-structural garden or screen wall is often left hollow. Load-bearing walls, retaining walls and anything carrying a bond beam need the cells with rebar grouted, and sometimes every cell. Follow the engineered drawings for the job.
Sources and assumptions
Block counts are exact geometry from nominal dimensions, which include the standard 3/8 in mortar joint.
Mortar coverage follows published masonry estimating tables: 9 bags of 80 lb or 12 bags of 60 lb per 100 standard blocks. Grout yields per block (0.225 / 0.270 / 0.338 / 0.415 ft³ for 6 / 8 / 10 / 12 in) come from block supplier grout charts, and cross-check against the 1.12 cubic yards per 100 sq ft figure for solid-grouted 8 in wall.
Coverage figures are averages. Joint thickness, block texture, temperature and site waste all move them. This is an estimating tool, confirm quantities with your supplier and follow your local building code for reinforcement, footing and wall height.