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.
Sets typical shell/web defaults below — both editable.
Many walls grout only rebar cells and corners.
Optional — vertical bars displace a little grout; 1–2% is plenty.
From your bag’s label — an 80 lb bag commonly yields ~0.5 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.
[ Ad slot — replace with AdSense / Ezoic code ]
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. The fill volume is computed from the block’s shell-and-web geometry (see the core-fill section below); a standard 8″ block works out to roughly a quarter of a cubic foot of void. Tick the core-fill box for the volume and bag count.
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.
Concrete block fill calculator (core fill)
How much grout fills a block wall’s cores is pure geometry, and the calculator now computes it from the block itself rather than a fixed rule of thumb. A hollow block’s void is what’s left after the two face shells and (typically) three webs:
core length = 15.625″ − 3 × web · core width = actual width − 2 × shell void per block = core length × core width × 7.625″ fill = blocks laid × void × share filled × (1 − rebar displacement) × (1 + waste)
With typical shell and web thicknesses, that geometry gives about 0.17 ft³ of void for a 6″ block, 0.26 for an 8″, 0.34 for a 10″ and 0.41 for a 12″ — figures that land within a few percent of the fill volumes block suppliers quote, because it’s the same geometry they’re measuring. Pick the nominal width and the shell/web fields preload typical values; edit them to match your actual unit if you’ve measured it. Vertical rebar displaces a little grout (1–2% covers it), and the grout bag count uses your bag’s labeled yield, since products vary. Grouting only the rebar cells and bond beam? Set the share-filled percentage accordingly — fully grouted walls are the exception, not the rule.
[ Ad slot — replace with AdSense / Ezoic code ]
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. The calculator computes the void per block from shell-and-web geometry — about 0.26 ft³ for a standard 8″ unit — and converts the fill to bags using your bag’s labeled yield. For ready-mix yardage on big fills, the concrete calculator handles the pour itself.
How much grout per block by width?
From shell-and-web geometry with typical dimensions: ~0.17 ft³ per 6″ block, ~0.26 per 8″, ~0.34 per 10″, ~0.41 per 12″ when filled solid — so 100 fully grouted 8″ blocks take roughly 26 ft³ (about a yard). The shell and web fields are editable if your unit differs.
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.