Cinder Block Calculator
Calculate how many cinder blocks (CMU) you need for a wall from length, height and openings, with courses, blocks per course, mortar bags and core fill.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Blocks to buy
- 284 blocks
- 270 standard 8 x 8 x 16 blocks in the wall plus 5% waste
- Courses
- 12 courses
- 8 in per course including the joint, 8 ft tall
- Blocks per course
- 22.5 blocks
- 30 ft at 16 in per block; half blocks fill odd ends
- Mortar mix (80 lb bags)
- 22 bags
- About 13 blocks per bag with 0.375 in joints
- Core fill grout (if all cells grouted)
- 2.2 cu yd
- 0.22 cu ft per block; most walls grout only the cells with rebar
Cinder Block formula
Wall area (sq ft) = Length x Height - openings
Blocks = Wall area x 1.125 x (1 + waste %) (standard block with 3/8 in joints covers 16 x 8 in = 0.889 sq ft)
Courses = Height (in) / 8, Blocks per course = Length (in) / 16
Mortar bags (80 lb) = Blocks / 13Worked example
A 40 x 6 ft foundation wall with one 3 x 7 ft door opening: wall area is 240 - 21 = 219 sq ft. 219 x 1.125 = 246.4, so 247 standard blocks, or 259 with 5% waste. The wall is 72 / 8 = 9 courses of 480 / 16 = 30 blocks each.
Mortar: 259 / 13 = 20 bags of 80 lb mortar mix. If every core is grouted, 246 x 0.22 = 54 cubic feet, or 2 cubic yards of grout.
Quick reference
| Wall length | 4 ft tall (6 courses) | 6 ft tall (9 courses) | 8 ft tall (12 courses) |
|---|---|---|---|
| 10 ft | 45 | 68 | 90 |
| 20 ft | 90 | 135 | 180 |
| 30 ft | 135 | 203 | 270 |
| 40 ft | 180 | 270 | 360 |
| 50 ft | 225 | 338 | 450 |
How to calculate concrete blocks
Multiply the wall length by its height for the face area and subtract any openings. A standard block with its mortar joint covers 16 by 8 inches, which is 0.889 square feet, so every square foot of wall takes 1.125 blocks. A 240 square foot wall needs 270 blocks, and 5 percent waste brings it to 284.
Check the layout too. Blocks stack in 8 inch courses, so an 8 foot wall is exactly 12 courses. A 7 foot wall is 10-1/2 courses, which means a cut course or a 4 inch half-high course. Along the length, each course of a 30 foot wall takes 22.5 blocks. Alternate courses end in a half block to keep the running bond, so order 6 half blocks or split stretchers with a block splitter. Every corner course takes a corner block with one square end: a 4-corner foundation 12 courses tall needs 48. They come out of the count, not on top of it, but they are a separate line on the order.
Mortar, grout and reinforcing
Block is laid with face-shell bedding, so an 80 lb bag of mortar mix lays about 13 blocks. For larger walls, site-mixed mortar from masonry cement and sand is cheaper: about 3 bags of masonry cement and 1/3 cubic yard of sand per 100 blocks. Type N (ASTM C270, 750 psi) is for garden walls, partitions and anything above grade. Foundation walls, retaining walls and anything touching soil take Type S (1,800 psi); Type N there is an inspection fail and a weak bond. Below grade the outside face also gets parged and damp-proofed (IRC R406.1), with a drain at the footing (R405.1).
Foundation walls get vertical bars from IRC Tables R404.1.1(2) through (4), set by wall height, backfill depth and soil class. A common result is #4 at 48 inches or #5 at 32 inches, doweled to the footing and lapped 40 bar diameters (20 inches for #4, 25 for #5). The top course is bond beam block with two continuous #4 bars, grouted solid, carrying the sill plate and its 1/2 inch anchor bolts at 6 feet on center (R403.1.6). Reinforced cells are filled with 8 to 11 inch slump grout (ASTM C476), not slab mix, in lifts of 4 to 5 feet with the bar held centered. Grouting only the reinforced cells takes a fraction of the volume shown for a fully grouted wall: at 48 inch spacing it is one cell in six. Ladder wire (9 gauge joint reinforcement) goes in every other course. Where the tables run out for 8 inch block, they call for 10 or 12 inch block.
What this calculator leaves out
The footing (use the concrete footing calculator), dowels and vertical rebar, bond beam block and its bars, and corner, half, lintel and cap blocks. Grout, ladder wire, anchor bolts, parging and damp-proofing below grade, the footing drain, and delivery. The open cores in the top course of a garden wall need cap block or a grouted cap, or they fill with water and split in the first freeze.
Mistakes that cost money
- Type N mortar below grade fails inspection and the bond; foundation and retaining walls take Type S (ASTM C270), Type N stays above grade.
- Slab mix ordered as core fill hangs up in the cells; grout is 8 to 11 in slump (ASTM C476), in lifts under 5 ft.
- Mortar droppings left in a rebar cell bridge it and leave the bar in a void; keep reinforced cells clean or use cleanouts.
- No bond beam: anchor bolts (1/2 in at 6 ft OC, IRC R403.1.6) need a grouted top course with two #4 bars to bite into.
- Ordering stretchers only: each corner course needs a corner block with one square end, and alternate courses end in a half block.
- A cube of 90 standard block weighs about 3,200 lb; if the boom truck cannot reach the footing you hand-carry 284 blocks.
Key facts
- A standard concrete block is 15-5/8 x 7-5/8 x 7-5/8 in; with a 3/8 in joint it lays up to a nominal 16 x 8 x 8 in.
- A wall takes 1.125 standard blocks per square foot, or 112.5 blocks per 100 sq ft.
- An 80 lb bag of mortar mix lays about 13 standard blocks; 100 blocks need about 8 bags.
- A standard hollow 8 in block weighs 35 to 38 lb; a lightweight block weighs 26 to 30 lb.
- Fully grouting an 8 in block wall takes about 0.93 cubic yards of grout per 100 sq ft of wall.
Frequently asked questions
How many cinder blocks do I need for a 100 square foot wall?
113 standard 8 x 8 x 16 blocks, or about 119 with 5% waste.
How many blocks per course?
Divide the wall length in inches by 16. A 20 ft wall is 240 / 16 = 15 blocks per course; a 30 ft wall is 22.5, so alternate courses end in a half block.
Do I have to fill the cores with concrete?
Not all of them. A garden or landscape wall needs only a cap on the top course. A foundation or retaining wall gets grout in the cells that hold rebar, typically every 32 to 48 in, plus the bond beam on top. The fill is grout at 8 to 11 in slump (ASTM C476), not bagged concrete, so it flows to the bottom of the cell.
Are cinder blocks and concrete blocks the same?
Today, yes. True cinder blocks with coal-ash aggregate are rarely made; modern concrete masonry units (CMU) come in the same nominal sizes and the names are used interchangeably.
Sources and references
- CMU-TEC-001 Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications (Concrete Masonry & Hardscapes Association)
- QUIKRETE Mortar Mix No. 1102 product data sheet (Quikrete)
- Sakrete Mortar Mix Type S product page (Sakrete)
- Chapter 4 Foundations, 2021 IRC (ICC)
Next steps for this project
The usual order for a concrete & masonry project. See all concrete & masonry calculators.
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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.