Concrete blocks, also called concrete masonry units or CMUs, are sold by the piece or the pallet, so the first step in any block project is a count. This calculator turns the length and height of a wall, minus doors and windows, into the number of blocks to buy, plus the courses (rows) and the blocks in each course so you can set out the first row.
How to use the concrete block calculator
- Enter the wall length and height. Each box has its own unit menu.
- Subtract openings. Enter the total area of doors, windows and other gaps.
- Choose the block face. The standard block is 8 inches high and 16 inches long (nominal). Half-height 4 × 16 and half-length 8 × 8 units are also common.
- Add a waste allowance for breakage and cuts, then read the order quantity.
Concrete block formula
Net wall area = length × height − openings
Courses = ⌈ height ÷ block height ⌉ · Blocks per course = ⌈ length ÷ block length ⌉
Block sizes are quoted as nominal dimensions, which include the 3/8-inch mortar joint. A standard block is therefore planned as 8 × 16 inches (128 sq in or 0.889 sq ft), even though the unit itself measures 7-5/8 × 15-5/8 inches. That is why there are 1.125 blocks per square foot of wall.
Worked example: a 30 × 6 ft wall with a door
- Gross area: 30 × 6 = 180 sq ft. A 3 × 7 ft door removes 21 sq ft, leaving 159 sq ft.
- Blocks: 159 ÷ 0.8889 = 178.9.
- Add 10% waste: 178.9 × 1.10 = 196.8, rounded up to 197 blocks.
Standard block counts for common walls
Standard 8 × 16 in blocks, with a 10% waste allowance and no openings.
| Wall (length × height) | Area (sq ft) | Blocks (no waste) | Blocks with 10% |
|---|---|---|---|
| 8 × 4 ft | 32 | 36.0 | 40 |
| 10 × 4 ft | 40 | 45.0 | 50 |
| 12 × 6 ft | 72 | 81.0 | 90 |
| 20 × 6 ft | 120 | 135.0 | 149 |
| 20 × 8 ft | 160 | 180.0 | 198 |
| 30 × 8 ft | 240 | 270.0 | 297 |
| 40 × 8 ft | 320 | 360.0 | 396 |
| 50 × 10 ft | 500 | 562.5 | 619 |
Block sizes
- 8-inch block (7-5/8 × 7-5/8 × 15-5/8 actual): foundation walls, retaining walls and load-bearing walls.
- 4-inch and 6-inch blocks: partitions and veneers; the face is the same, but they are thinner.
- Half blocks: used at the ends of courses in running bond so that joints stagger.
The face dimensions drive the count, and the thickness does not affect the number of blocks, only the weight and the mortar and grout required.
Ordering tips
Blocks are delivered on pallets, and the quantity per pallet depends on the block type, so ask your supplier. Plan ahead for corner blocks, bond beam blocks at the top and any lintel units over openings. These are counted separately from the field blocks. To estimate the mortar, use the Block Mortar Calculator, and to fill the cores with grout or concrete use the Concrete Block Fill Calculator. For footings under the wall see the Concrete Footing Calculator, and for a retaining wall use the Retaining Wall Calculator.
Frequently asked questions
How many 8×8×16 blocks are in 100 square feet?
About 112.5. A standard block with its mortar joint covers 16 × 8 inches = 128 sq in = 0.889 sq ft of wall, so 100 ÷ 0.889 = 112.5 blocks before waste.
How many blocks do I need for a 10 × 10 ft wall?
A 10 ft by 10 ft wall is 100 sq ft, which is about 113 blocks, or 124 blocks with a 10% waste allowance.
What is the actual size of an 8×8×16 block?
Its real size is 7-5/8 × 7-5/8 × 15-5/8 inches. The “nominal” 8 × 8 × 16 size includes the 3/8-inch mortar joint, so you plan the wall with the nominal dimensions.
Should I add extra blocks for waste?
Yes — a few percent covers breakage and cuts. 5% is reasonable for simple walls; 10% for walls with many openings, corners or cut blocks.
Do openings really reduce the block count?
Yes. Subtract the area of doors and windows, but keep in mind that blocks around openings must be cut, so do not trim the waste allowance too far.