Blocks, mortar and core fill for a CMU wall — the three orders that come off one wall area, counted separately.
Concrete Block Calculator
Inputs
Total area of openings. A single garage door is about 112 ft².
Solid-grouted walls are 100. Filling only the cells with rebar at 48 in spacing is roughly 17.
Blocks arrive chipped and get cut. 5% is normal, 10% on a wall with many openings.
Results
Blocks needed
378 blocks
blocks = ft² × 1.125 × (1 + waste); fill = blocks × 0.224 ft³ × % filled
Calculated with BuildCalculators.com
Calculated with BuildCalculators.com · buildcalculators.com/concrete-block-calculator
Estimates are for planning only — confirm quantities with your supplier or contractor before ordering.
A block wall is three separate orders that all come off the same wall area: the blocks, the mortar that joins them, and — on most structural walls — the grout that fills their cores. People reliably order the first and get surprised by the other two. A 40 ft wall 8 ft high is 360 blocks, about 28 bags of mortar, and if it is solid-grouted, close to 4 cubic yards of concrete on top of that.
The block count itself is the easy part, because a standard block presents an 8 by 16 inch face including its mortar joints. That is 0.889 ft², so every square foot of wall takes 1.125 blocks — a figure that holds for 6, 8, 10 and 12 inch block alike, since only the depth changes. What the block width changes is the core fill, and that changes it a lot: a 12 inch block swallows nearly twice the grout of a 6 inch one.
A = (length × height) − openingsA × 1.125 × (1 + waste)height in inches ÷ 8blocks ÷ 13 bags of 80 lbblocks × ft³ per block × % of cells filledA "standard 8 inch block" actually measures 7 5/8 by 7 5/8 by 15 5/8 inches. The missing 3/8 inch on each dimension is the mortar joint, and adding it back gives the nominal 8 by 8 by 16 that all estimating is done in. This is not sloppiness — it is what makes block coursing work out to whole numbers. Eight inches of nominal height means three courses to two feet, and every opening height in a block wall is designed around it.
Pre-mixed mortar comes with the sand already in the bag and lays about 13 blocks per 80 lb bag. Masonry cement comes as cement only, gets mixed with sand you buy separately, and works out at roughly 3 bags per 100 blocks plus a cubic yard of sand per 7 bags of cement. Pre-mixed is far easier for a small wall and considerably more expensive by volume; past about 300 blocks, cement and sand is the obvious choice. The calculator gives you both figures so you can price the two routes against each other, and the mortar-calculator takes it further — joint width changes consumption more than anything else, and every published rule of thumb quietly assumes 3/8 in.
Most garden and screen walls are laid dry-cored and stand fine. Retaining walls, walls above about 4 feet, and anything carrying a floor or roof load need grouted cells with vertical rebar — and how many cells depends on your plan, not on preference. Rebar at 48 inch spacing fills roughly one cell in six, or about 17% of the wall. Solid grouting is 100%. That is why the calculator asks for a percentage instead of guessing: the difference between the two on a 40 ft wall is more than three cubic yards of concrete.
A block core is a tall narrow void with obstructions. Standard ready-mix carrying 3/4 inch stone bridges across the neck and leaves voids you cannot see and cannot fix. Ask the plant for a fine-aggregate grout — pea gravel or sand only, and a slump around 8 to 10 inches so it flows and consolidates under its own weight. If you are bagging it, core-fill grout is sold as its own product for exactly this reason.
Nothing here covers what the wall stands on, and a block wall on an inadequate footing is a wall that cracks along its bed joints. The footing wants to be at least twice the wall thickness wide and to sit below the frost line — size and price the footing before you order a single block.
A plain wall with no openings, dry-cored, laid with pre-mixed mortar.
378 blocks and 28 bags of mortar
The 5% is not padding. Blocks arrive with chipped corners, and every wall needs cuts at openings and corners — the half you cut off is rarely the half you need next.
Now a structural wall, all cells filled, 8 in block.
3.6 yd³ of grout — a truck, not bags
3.6 cubic yards is 162 bags of 80 lb mix. That is over six tons to carry, mix and place before the first batch sets. This is the point where a short-load delivery stops being a luxury.
Only the cells carrying vertical bar get filled — about one in six.
9.5 ft³ — about 16 bags of 80 lb
Small enough to bag comfortably. Note how much the block width matters: the same wall in 6 in block would need barely half this. A retaining wall also needs drainage behind it and that is not optional.
Walls 8 ft high, no openings, before breakage allowance.
| Wall | Area | Blocks | Mortar bags (80 lb) |
|---|---|---|---|
| 10 × 8 ft | 80 ft² | 90 | 7 |
| 20 × 8 ft | 160 ft² | 180 | 14 |
| 30 × 8 ft | 240 ft² | 270 | 21 |
| 40 × 8 ft | 320 ft² | 360 | 28 |
| 60 × 8 ft | 480 ft² | 540 | 42 |
| 100 × 8 ft | 800 ft² | 900 | 70 |
Published grout volumes by nominal block width.
| Block width | Per block | Per 100 blocks | As 80 lb bags |
|---|---|---|---|
| 6 in | 0.224 ft³ | 0.83 yd³ | 38 |
| 8 in | 0.270 ft³ | 1.00 yd³ | 45 |
| 10 in | 0.332 ft³ | 1.23 yd³ | 56 |
| 12 in | 0.416 ft³ | 1.54 yd³ | 70 |
| Vertical bar spacing | Cells filled | Share of wall |
|---|---|---|
| Every cell (solid) | All | 100% |
| 16 in on centre | 1 in 2 | 50% |
| 32 in on centre | 1 in 4 | 25% |
| 48 in on centre | 1 in 6 | 17% |
| None | None | 0% |
Standard 8 inch block runs $1.80–$3.00 each in 2026, so the blocks in a 40 by 8 ft wall are $680–$1,130. Laid by a mason, the same wall is $10–$20 per square foot finished — $3,200–$6,400 — because block work is slow, heavy and skilled, and labour is most of the bill.
Mortar at $7–$12 a bag, grout or ready-mix for the cores, vertical and horizontal rebar, ladder or truss wire every other course, and the footing underneath. On a structural wall the concrete going into the cores frequently costs more than the mortar and rebar combined — which is why volume in cubic yards and the fill percentage matter as much as the block count.
The first course is the whole job. It has to be dead level, dead straight and square, because every course above inherits its errors and a block wall gives you no way to correct them later. Experienced masons spend a disproportionate share of the day on course one. If you are laying your own wall, lay the first course, walk away, and check it again cold before you go higher.
For a foundation wall, poured is usually faster and often cheaper once forming is priced, and it is monolithic — price the poured version before deciding. Block wins on access: no truck or pump needed, material carried in by hand, and the work can happen in stages by one person. Below about 4 ft, block is almost always the practical choice; above 8 ft with soil pressure behind it, get an engineer either way.
The block count is the smallest surprise in a block wall. Mortar, grout, rebar, wire and the footing together often exceed the block cost on a structural wall. Price the whole assembly before deciding block is the cheap option.
Using 15 5/8 by 7 5/8 inches instead of 16 by 8 inflates the block count by about 5% — you buy blocks you do not need, and worse, your coursing arithmetic stops landing on whole courses at door and window heights.
Three-quarter inch stone bridges in a block core and leaves voids around the rebar, which is precisely where the wall needed the grout. Specify a fine-aggregate core-fill grout at a high slump, and consolidate it.
Every wall needs cut blocks at corners and openings, and pallets arrive with damage. Running out three courses from the top means a second delivery charge for a handful of blocks, and a colour difference across the wall if the batch has changed.
A block wall concentrates a lot of weight on a narrow line. On a footing that is too narrow for the soil, it settles differentially and cracks follow the bed joints — the one crack pattern in masonry you cannot repair invisibly. Size the footing for the soil and the load before the wall exists.
Multiply the wall area in square feet by 1.125. A standard block presents an 8 by 16 inch face including its mortar joints, which is 0.889 ft², so a 40 by 8 ft wall of 320 ft² takes 360 blocks before breakage. Add about 5% for cuts and damage.
About 13 standard blocks per 80 lb bag of pre-mixed mortar — a manufacturer "up to" figure, so plan nearer 11 or 12 in cold weather or with wide joints. Buying masonry cement and sand separately instead, the rule is roughly 3 bags of cement per 100 blocks, plus a cubic yard of sand per 7 bags of cement.
Per 100 blocks solid-grouted: 0.83 yd³ for 6 inch block, 1.0 for 8 inch, 1.23 for 10 inch and 1.54 for 12 inch. Most walls are not solid — rebar at 48 inch spacing fills about one cell in six, or 17% of that figure.
Not always. Garden and screen walls are routinely laid dry-cored. Retaining walls, walls over about 4 feet, and anything carrying a floor or roof load need grouted cells with vertical rebar, and how many cells is set by your plan or local code rather than by preference.
Nominally 8 by 8 by 16 inches; actually 7 5/8 by 7 5/8 by 15 5/8. The missing 3/8 inch on each face is the mortar joint. Always estimate with the nominal size — the actual size gives a block count about 5% too high and wrecks your coursing arithmetic.
It is a bad idea. Standard ready-mix carries 3/4 inch stone that bridges across the narrow core and leaves voids exactly where the rebar needed grout around it. Ask for a fine-aggregate core-fill grout at a high slump so it flows and consolidates on its own.
Slabs, footings and columns in cubic yards and bags
Bag counts for base, finish and lath
Installed cost by finish, material shown separately
Grid bar counts, length and weight by size
New or replacement driveway, installed and itemised
Embed the Concrete Block Calculator anywhere — it is free, stays in sync with this page, and sizes itself to fit. The only condition is the credit line in the snippet, which links your readers to the full version with the formulas and worked examples.
This calculator uses published constants from named industry sources, each asserted in an automated test that runs on every build. The formula is printed with the result above, and any correction we make is logged on the changelog.
It is a planning estimate. It does not replace an engineer, a site visit or a written quote — confirm quantities with your supplier before ordering.