Concrete Block Calculator
Count 8×8×16 cinder blocks (CMU) for a wall, plus bags of mortar and the grout or concrete to fill the cores — with the math shown.
Your estimate
189 blocks
Blocks (incl. 5% waste)
- Courses × blocks per course
- 6 × 30
- Wall area
- 160 sq ft
- Mortar bags
- 15
- Core-fill grout
- 9.6 cu ft (0.36 cu yd)
- Grout bags
- 17 × 80 lb
Show the math
- Courses = height ÷ 8 in (block + joint), rounded up = 6
- Blocks per course = length ÷ 16 in, rounded up = 30
- 6 × 30 = 180; × (1 + 5%) → 189
- Mortar = 189 blocks ÷ 13 per bag → 15 bags
- Grout = wall area × 36.1 cu ft per 100 sq ft × (8 ÷ 48) = 9.6 cu ft (0.36 cu yd)
Shopping list & cost
| Item | Spec | Qty | Cost |
|---|---|---|---|
| Concrete blocks | 8 × 8 × 16 in standard CMU | 189 | — |
| Mortar mix | 80 lb bag (≈ 13 blocks each) | 15 bags | — |
| Core-fill grout / concrete | 80 lb bag ≈ 0.6 cu ft | 17 bags for 9.6 cu ft | — |
Prices are what you enter under Advanced options. Estimates only — check stock and pricing with your supplier.
Quick answer A 40 ft wall 4 ft high built with standard 8 × 8 × 16 in blocks needs 189 blocks including 5% waste (6 courses of 30), about 15 bags of 80 lb mortar mix and 9.6 cu ft (0.36 cu yd) of grout to fill the rebar cells every 48 in. A 12 × 1.2 m wall of 390 × 190 mm blocks needs 189 blocks. Rule of thumb: 1.125 blocks per square foot of wall.
Method & assumptions
Courses = Wall height ÷ 8 in (block + joint), rounded up Blocks per course = Wall length ÷ 16 in, rounded up Blocks = Courses × Blocks per course × (1 + waste %) Mortar bags (80 lb) = Blocks ÷ 13 Grout = Wall area × 36.1 cu ft per 100 sq ft × (8 in ÷ cell spacing) (8 in CMU)
- Nominal size
- 16 × 8 in face including the ⅜ in joint; the block itself is 15⅝ × 7⅝ in
- Cell spacing
- distance between grouted cores, set by your plans or local code
- 36.1 cu ft / 100 sq ft
- NCMA TEK 4-2A grout for 8 in CMU with every cell filled, incl. 3% waste
Assumptions & limits
- Running bond with standard ⅜ in (10 mm) joints.
- Corners and ends use full or half units cut from stock — included in the waste factor.
- Mortar yield of 13 blocks per 80 lb bag is Quikrete's published figure for Mortar Mix and Mason Mix; your mason may do better or worse.
- Grout volumes are for two-core hollow units; bond beams and lintels need extra.
- Not a structural design: footings, rebar size and spacing for retaining or load-bearing walls come from code or an engineer.
Rounding: purchase quantities are always rounded up to whole units as sold. See our methodology.
Diagram
How to use this calculator
- Measure the wall length along the footing and the full height, including any courses below grade.
- Pick the block width. 8 in CMU is the standard for garden, foundation and garage walls; 6 in for screens and infill; 4 in for non-structural partitions.
- Choose how often the cores will be filled. Rebar cells are usually every 32 or 48 in; some walls are grouted solid.
- Subtract large openings under Advanced options, and raise the waste factor for walls with many corners or cuts.
- Use the mortar and grout bag counts as a shopping list — check the yield printed on the bag you buy.
Worked examples
Imperial: garden wall 24 ft × 32 in, 8 in CMU, no core fill
- Courses = 32 ÷ 8 = 4; per course = 288 ÷ 16 = 18
- Blocks = 4 × 18 = 72; +5% = 75.6 → 76 blocks
- Mortar = 76 ÷ 13 = 5.8 → 6 bags of 80 lb mix
Imperial: garage wall 30 ft × 8 ft, cells grouted every 32 in
- Courses = 96 ÷ 8 = 12; per course = 360 ÷ 16 = 22.5 → 23
- Blocks = 276; +5% = 289.8 → 290
- Grout = 240 sq ft × 36.1 ÷ 100 × 8 ÷ 32 = 21.7 cu ft ≈ 0.8 cu yd
In depth: concrete block planning notes
Counting by courses, not just by area
The quick rule of 1.125 blocks per square foot is fine for a budget, but walls are built in whole courses and whole blocks. A wall 52 in high cannot have six and a half courses: either the footing steps, the top course is cut, or the wall becomes 56 in. Likewise a 21 ft wall is 15¾ blocks long, and the last block in every course is a cut piece. That is why this calculator rounds courses and blocks per course up separately before multiplying — it matches what actually gets laid, and it is why the count is usually a few percent higher than area ÷ 0.889.
Plan the wall on the block module if you can. Lengths that are multiples of 16 in (8 in for half blocks) and heights in multiples of 8 in mean fewer cuts, stronger corners and less waste. Corners need either corner blocks with a finished end or a half block on alternate courses to keep the running bond; openings need lintels or bond-beam blocks above them. Count those specialty units separately when you order — most yards price them differently.
Mortar, grout and reinforcement
Mortar holds the units together and seals the joints; grout fills the cores around reinforcement and turns a hollow wall into a solid, reinforced one. They are different products. Type S mortar is the usual choice for walls below grade and garden walls, Type N for above-grade, non-load-bearing work. Grout is a fluid mix poured into the cells, often in lifts of 4–5 ft with the bars already in place; a stiff concrete with large stone can bridge across the narrow cells and leave voids.
How many cells get grout depends on the design. Garden and screen walls often have a vertical bar every 32–48 in with only those cells filled; foundation and retaining walls are commonly grouted at closer spacing or solid, and need horizontal bond beams. NCMA estimates 36.1 cu ft of grout per 100 sq ft for an 8 in wall filled solid; at 48 in spacing that falls to about 6 cu ft. Anything that retains soil, supports a floor or exceeds roughly 4 ft is structural — the footing, bar size and spacing should come from your local code or an engineer, and this calculator then turns that design into a shopping list.
Reference table
| Block | Blocks per 100 sq ft · grout, every cell |
|---|---|
| 8 × 8 × 16 in (standard) | 112.5 · 36.1 cu ft |
| 6 × 8 × 16 in | 112.5 · 25.6 cu ft |
| 10 × 8 × 16 in | 112.5 · 47.0 cu ft |
| 12 × 8 × 16 in | 112.5 · 58.9 cu ft |
| AU 390 × 190 mm (per m²) | 12.5 blocks · 0.11 m³ (190 mm) |
Key facts
- 8 × 8 × 16 actual size
- 7⅝ × 7⅝ × 15⅝ in
- Blocks per sq ft of wall
- 1.125
- 80 lb mortar mix lays
- up to 13 standard blocks
- Typical 8 in block weight
- ≈ 30–40 lb (lightweight to normal-weight)
Tips & common mistakes
- Lay the first course in a full mortar bed on a level footing — every error in the first course grows on the way up.
- Mark rebar cells on the footing before you start so the dowels line up with the cores.
- Use a pourable grout or a small-aggregate "core-fill" mix; ordinary concrete with ¾ in stone can bridge in narrow cells and leave voids.
- Keep unused blocks off the ground and covered — wet blocks shrink as they dry and can crack the wall.
Frequently asked questions
How many cinder blocks do I need for a 10 × 10 wall?
A 10 ft long, 10 ft high wall is 100 sq ft, which is 112.5 standard 8 × 8 × 16 blocks — 15 courses of 8 blocks, so 120 blocks laid in practice, or about 126 with 5% waste.
How many blocks are in a square foot?
One standard 16 × 8 in block face covers 0.889 sq ft including the joint, so you need 1.125 blocks per square foot, or 112.5 per 100 sq ft.
How much mortar do I need for concrete blocks?
Quikrete rates one 80 lb bag of Mortar Mix or Mason Mix at up to 13 standard blocks. Mixing your own, one 70 lb bag of masonry cement with sand lays about 30 blocks (NCMA).
How much concrete does it take to fill a cinder block?
The two cores of an 8 × 8 × 16 block hold about 0.22 cu ft. Including waste, NCMA estimates 36.1 cu ft of grout per 100 sq ft of wall when every cell is filled.
Are cinder blocks and concrete blocks the same?
In everyday use, yes. True cinder blocks used coal cinders as aggregate; modern CMU uses sand, gravel or lightweight aggregate, but most people still call them cinder blocks.
Sources
- CMHA TEK 4-2A – Estimating concrete masonry materials — accessed : “Including a 5 percent allowance for waste and breakage, this translates to 119 units per 100 ft² (9.29 m²) of wall area.”
- Quikrete – Mortar Mix (No. 1102) product data sheet — accessed : “Each 80 lb (36.2 kg) bag of QUIKRETE Mortar Mix will lay up to 37 standard bricks or 13 standard (8 in by 8 in by 16 in) blocks.”
- Concrete Masonry & Hardscapes Association – CMU sizes — general reference — not cited for a specific figure
Default values (yields, coverage rates and densities) are typical figures from manufacturer literature. Always check the label on the product you buy.
Related calculators
- Brick CalculatorCalculate bricks and mortar for a wall using US modular, UK/EU or Australian brick sizes, single or double skin, with openings and waste.
- Retaining Wall CalculatorPlan a segmental block retaining wall: blocks with a buried course, caps, compacted base gravel, drainage stone, drain pipe and fabric.
- Rebar CalculatorWork out how many 20 ft rebar sticks a slab needs: grid bar count, total length, lap splices, weight, chairs and tie wire.
Method, waste and worked examples: Brick and block counts, joints and mortar bags.
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DIYCalcLab. “Concrete Block Calculator.” Oct 19, 2026. https://diycalclab.com/masonry/concrete-block-calculator/
Estimates only. Verify quantities with your supplier and local building codes. For structural, electrical or safety-critical work, consult a licensed professional. Full disclaimer.