Free Concrete Block Calculator
Use this concrete block calculator to see how many concrete or cinder blocks you need to build a wall.
Concrete blocks are laid in courses (rows). The calculator works out how many courses fit the wall height and how many blocks fit each course, then subtracts doors and windows and adds a little extra for breakage.
How the concrete block calculator works
Courses = wall height (in) ÷ block height (in), rounded up. Blocks per course = wall length (in) ÷ block length (in), rounded up. Blocks = courses × blocks per course − openings ÷ block face area, plus the extra %. Block sizes are nominal: a standard block is 7⅝ × 15⅝ in, and the ⅜ in mortar joint makes it 8 × 16.
Blocks needed for common block wall sizes
| Wall (length × height) | Courses | Blocks (no extra) | With 5% extra |
|---|---|---|---|
| 10 × 2 ft | 3 | 24 | 26 |
| 10 × 4 ft | 6 | 48 | 51 |
| 20 × 4 ft | 6 | 90 | 95 |
| 30 × 6 ft | 9 | 207 | 218 |
| 40 × 8 ft | 12 | 360 | 378 |
Before you build
- Block walls sit on a concrete footing. Use the concrete footing calculator for the concrete underneath.
- Retaining walls and walls over about 4 ft usually need engineering and a permit. Check local code.
- Cores filled with concrete or grout need extra material. Ask your supplier for fill per block.
Frequently asked questions
How many concrete blocks do I need?
With standard 8 × 16 in blocks, you need 1.125 blocks per square foot of wall. A 20 ft long, 4 ft high wall is 6 courses of 15 blocks: 90 blocks, or 95 with 5% extra.
Is this a cinder block calculator too?
Yes. Most "cinder blocks" sold today are concrete masonry units (CMU). The standard size is 8 × 8 × 16 in nominal, which already includes the 3/8 in mortar joint.
How do I calculate blocks for a block wall?
Divide the wall height in inches by 8 (rounding up) for the number of courses, and the wall length in inches by 16 (rounding up) for blocks per course. Multiply the two, subtract openings and add a little extra.
How accurate is the concrete block calculator?
It counts whole courses and whole blocks per course, which matches how a wall is built. Corners, pilasters and half blocks can change the exact mix of block types; ask your supplier when you order.