Count the concrete or cinder blocks for a wall — with courses, mortar bags, door and window openings deducted, and an optional core-fill volume. This tool counts blocks; for poured volume it hands off to the concrete calculator.
Units
Nominal face = actual block + 3/8″ mortar joint. 16×8 ≈ 1.125 blocks/ft².
5% for simple walls, 7–10% with many cuts/corners.
Rule of thumb ~30 (80 lb). Edit to match your bag.
Many walls fill only rebar cells/corners. 100% = solid-filled. Void per standard block ≈ 0.13 ft³.
Block count and mortar are estimates from the block face plus joint; mortar yield and fill are editable rules of thumb that vary by product, joint thickness and how solidly cells are grouted. Bond beams, lintels, jambs and corner units can change counts. Brands referenced as facts; cost fields are examples, not a quote.
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Filling the cores or pouring something else?
Core fill is poured concrete, so once you have the fill volume above, take it to the Concrete Calculator for the bag count and cost. Putting gravel behind a retaining wall or under a slab? The Gravel Calculator gives it in tons and yards.
What this concrete block calculator does — and its lane
This tool counts blocks: it turns a wall’s length and height into a number of concrete masonry units (CMU) or cinder blocks, plus courses and mortar. That’s a different job from figuring poured concrete — if you need cubic yards of a slab or footing, use the concrete calculator instead. The one place the two meet is core fill: the concrete grout poured into hollow block cells. This block wall calculator gives you the fill volume, then sends it across to the concrete tool for bags and cost.
How the block count works
A block wall is counted by area. Take the wall area, subtract any door and window openings, then divide by the area of one block face — and the face that matters is the nominal face, the actual block plus its mortar joint, because that’s the footprint each block occupies in the wall.
blocks = (wall area − openings) ÷ block face area × (1 + waste) standard CMU face 16″ × 8″ = 0.889 ft² ≈ 1.125 blocks/ft²
The 16×8 figure already includes a 3/8″ joint; the bare block is 15⅝″ × 7⅝″. Picking the right block face is the whole game, so the calculator offers standard and jumbo CMU, a 4″-high split-face unit, paving blocks, and a fully custom face for anything else.
Courses, mortar and core-fill
Courses — rows of block up the wall. Course count = wall height ÷ (block height + joint). A standard 8″-nominal block gives 12 courses in an 8′ wall.
Mortar — the bedding between units. A common rule is about 3 bags per 100 blocks, i.e. one 80 lb bag per ~30 blocks; the calculator uses an editable blocks-per-bag figure so you can dial in your product.
Core fill (grout) — concrete poured into the hollow cells, usually only where rebar sits or at corners. A standard block has roughly 0.13 ft³ of void; one 80 lb grout bag yields ~0.5 ft³ and fills ~4–5 cores. Tick the core-fill box for the volume.
CMU, cinder block, pavers and metric
CMU is the modern term; cinder block is the older name for the same wall unit and calculates identically. Paving blocks (paver wall blocks) just use a different face size, which you can enter. Metric users can switch units at the top — wall dimensions in metres, block face in centimetres — and the count comes out the same way.
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Frequently asked questions
How many concrete blocks per square foot?
A standard CMU nominal face (16″×8″, block plus 3/8″ joint) is 0.889 ft², so about 1.125 blocks per square foot of wall. Multiply wall area by 1.125 and add waste. The calculator also counts courses and deducts openings for partial walls.
CMU vs cinder block — what’s the difference?
CMU (concrete masonry unit) is the modern term for the hollow concrete wall block. “Cinder block” is an older name from when coal cinders were the aggregate. Today they’re effectively the same and used interchangeably, and they calculate identically — use the same block face for either.
How many courses in an 8-foot wall?
A standard block is 8″ tall nominal (7⅝″ block + 3/8″ joint). 96″ ÷ 8″ = 12 courses. Course count depends on block height plus joint, not wall length, so a 4″-high unit doubles the courses for the same wall height.
How many mortar bags per 100 blocks?
About 3 bags per 100 blocks as a rule of thumb — roughly one 80 lb bag per 30–35 blocks with a 3/8″ joint. It varies by product and joint, so the calculator uses an editable blocks-per-bag value. Buy a little extra.
How do I calculate core-fill concrete?
Core fill (grout) is concrete in the hollow cells, often only at rebar or corners. A standard block has ~0.13 ft³ of void; an 80 lb grout bag yields ~0.5 ft³ (~4–5 cores). Tick the core-fill box here for the volume, then use the concrete calculator for the fill’s bags and cost.
Should I deduct doors and windows?
Yes — openings can be a big share of a wall and leaving them in over-orders blocks. Enter each opening’s width and height and the area is subtracted before counting. Keep a waste allowance anyway, since cuts around openings make offcuts you can’t always reuse.