Concrete Block Calculator
Count concrete blocks (CMU) for a wall with standard 3/8-inch mortar joints, plus mortar bags and core-fill grout if you fill the cores.
How many concrete blocks do I need?
Block walls are counted by area, using the block's module: the space one block occupies in the finished wall, including its share of mortar. A standard block actually measures 7⅝ × 15⅝ inches, and with the standard ⅜-inch mortar joint it covers exactly 8 × 16 inches of wall — which is why the trade figure of 1.125 blocks per square foot works out so cleanly.
Blocks = wall area ÷ module area · Standard module = 8 in × 16 in = 0.889 sq ft (1.125 blocks per sq ft)
Example: a 10 ft long, 8 ft high wall is 80 sq ft × 1.125 = 90 blocks exactly — 99 blocks with the 10% allowance for cuts and breakage, laid with about 9 bags of 80-lb mortar mix.
Mortar and core fill
| Material | Planning rule used here |
|---|
| Mortar mix, 80-lb bags | ≈ 8–9 bags per 100 standard blocks (about 12 blocks per bag — the manufacturers' published rate) |
| Core fill (grout), all cores | ≈ 0.22 cu ft per standard block ≈ 0.81 cu yd per 100 blocks |
| Core fill in 80-lb bags | A bag of concrete mix yields ≈ 0.6 cu ft — about 37 bags per 100 blocks |
Most garden and partition walls leave the cores empty or fill only the reinforced vertical cores (typically every 4 ft and at corners, with rebar). Filling every core is a structural choice for retaining and load-bearing walls — the calculator shows the all-cores figure so the size of that choice is visible; scale it down in proportion for verticals-only filling. Openings (doors, gates) come off the wall area before you start: measure them separately and enter the net wall as length × height sections if the shape is awkward.
Frequently asked questions
How many concrete blocks are in 100 square feet?
112.5 — call it 113 — for standard 8 × 16 in blocks with ⅜ in mortar joints. The same figure scaled: 1.125 blocks per square foot, before waste. With the standard 10% allowance, a 100 sq ft wall wants about 124 blocks on the pallet.
Why is the block 7⅝ in if it is called an 8-inch block?
The name is the module, not the block: 7⅝ in of block plus a ⅜ in mortar joint lays up to exactly 8 in of wall height per course. Same for the length — 15⅝ in of block plus the joint makes 16 in. Counting with nominal sizes alone is how walls come out a course short.
How much mortar do I need per block?
About 1/12 of an 80-lb bag of mortar mix per standard block — roughly 8–9 bags per 100 blocks, jointing both the bed (horizontal) and head (vertical) joints at ⅜ in. That is the bag manufacturers' published laying rate (an 80-lb bag lays about 12 standard blocks), and it is the same figure the mortar calculator here uses. Scratch and parge coats are separate work and separate material.
Do I need to fill the cores with concrete?
Only where the design calls for it: retaining walls, structural walls, and cores carrying rebar. A simple garden wall usually fills just the reinforced vertical cores or none at all. If you fill, the grout figure here assumes every core, so partial filling scales straight down.
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