Work out how many blocks you need. This free concrete block calculator divides wall area by block face, subtracts doors and windows, adds waste, and estimates mortar bags – in feet or metres.
Enter the wall size and pick your block. Subtract the area of any doors or windows, and add a waste allowance (5-10% typical) for cuts and breakage.
Blocks = net wall area / block face area, rounded up after waste. A standard 16 x 8 in block covers about 0.89 ft2, so roughly 1.125 blocks per square foot. Mortar is a rough guide (about one 60 lb bag per 30-35 blocks).
How the Concrete Block Calculator Works
This tool works out how many concrete masonry units (CMU) you need to build a wall, based on the wall’s size, the block you’re using, and any doors or windows you need to leave out.
Formula: Net wall area = (Wall length × Wall height) − Openings area. Raw block count = Net wall area ÷ Block face area. Blocks needed = ROUNDUP(Raw block count × (1 + Waste% ÷ 100)). Mortar bags = ROUNDUP(Blocks needed ÷ 32).
- Wall length — the total length of the wall to be built, in feet or metres
- Wall height — the height of the wall, in the same unit as length
- Block size (face) — the block’s face dimensions, chosen from presets (e.g. 16 × 8 in standard CMU) or entered as a custom width and height
- Openings area — the combined area of doors and windows to subtract from the wall, optional
- Waste — the extra percentage added for cut blocks and breakage, typically 5–10%
Example Scenarios
| Wall length (ft) | Wall height (ft) | Block face | Openings (ft²) | Waste | Blocks needed |
|---|---|---|---|---|---|
| 20 | 8 | 16 × 8 in | 0 | 10% | 198 |
| 30 | 8 | 8 × 8 in | 0 | 10% | 595 |
| 15 | 8 | 12 × 8 in | 20 | 5% | 158 |
| 40 | 8 | 16 × 8 in | 40 | 10% | 347 |
| 10 | 4 | 8 × 8 in | 0 | 10% | 100 |
| 25 | 8 | 16 × 8 in | 0 | 15% | 259 |
Concrete Block Calculator FAQ
How many concrete blocks do I need per square foot of wall?
It depends on the block size. A standard 16 × 8 in CMU covers about 0.89 ft² of face area, which works out to roughly 1.125 blocks per square foot before waste. Smaller blocks, like 8 × 8 in, need about 2.25 blocks per square foot.Should I subtract doors and windows from the wall area?
Yes — enter their combined area in the openings field so the calculator only counts the block wall you actually need to lay. Skipping this step overstates your block count, sometimes significantly on walls with several openings.How much waste allowance should I use for block walls?
5–10% is typical, covering cuts at corners and openings plus the occasional broken block. Walls with more corners, angles or openings tend to need cutting more blocks, which can justify waste toward the higher end of that range.How many bags of mortar do I need for a block wall?
This calculator estimates roughly one 60 lb bag of mortar per 32 blocks, rounded up. Actual mortar use depends on joint thickness, block size and how much is wasted while mixing, so treat this as a starting estimate rather than an exact order quantity.What’s the difference between the block size presets and custom size?
The presets (16 × 8 in, 8 × 8 in, 12 × 8 in, and a metric UK size) cover common CMU face dimensions. If your block is a different size, select Custom and enter its exact face width and height so the block face area is calculated correctly.Does this calculator include the mortar joint width in the block count?
No — it calculates blocks from the wall area and the block’s face dimensions alone, without adjusting for mortar joint thickness between blocks. The built-in waste percentage is meant to absorb small discrepancies like this.Related Calculators
If your block wall sits on a poured footing, size that footing first with the concrete calculator. Building with brick instead of block? The brick quantity calculator uses the same area-based approach, and if you’re enclosing a yard rather than a structure, the fence calculator covers post-and-panel layouts instead.