A standard U.S. concrete masonry unit is 7⅝ × 7⅝ × 15⅝ in; with a ⅜ in mortar joint it occupies a nominal 8 × 16 in face – 0.889 ft², or 1.125 blocks per square foot. Blocks of different widths (4, 6, 8, 10 and 12 in) have the same face size, so the count only changes with the face dimensions, but the weight goes up with width.
How to use the Concrete Block Calculator
- Wall length
- Wall height
- Openings area
- Block size
- Waste
- Blocks laid per 80 lb bag of mortar — Manufacturers quote about 12–13 standard blocks per 80 lb bag
- Price per block
- Price per bag of mortar
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the wall length and height, the area of doors and windows, choose the block size, waste and how many blocks one bag of mortar lays. The calculator divides the net area by the face area of one block, adds waste, then counts mortar bags and prices everything.
Tips for an accurate estimate
- Measure the formwork, not the drawing – forms bulge and subgrades are rarely perfectly flat, so add 5–10%.
- Below about 1 cubic yard, premixed bags are usually cheaper; above that, ready-mix saves time and gives a more consistent pour.
- Order ready-mix in quarter- or half-yard steps and ask about short-load fees for small deliveries.
- Keep fresh concrete damp for at least a week – curing matters as much as the mix for final strength.
Common mistakes to avoid
- Running short mid-pour, which leaves a weak cold joint.
- Adding extra water on site to make concrete easier to place – it lowers strength and increases cracking.
- Ignoring the volume of rebar, posts or pipes in the pour, or the gravel base under it.
Inputs explained
| Field | What to enter |
|---|---|
| Wall length | Units: ft, m, in, yd, cm |
| Wall height | Units: ft, m, in, yd, cm |
| Openings area | Units: ft², m², yd², acre |
| Block size | Choose one: 8 × 8 × 16 in (standard CMU), 6 × 8 × 16 in, 4 × 8 × 16 in, 12 × 8 × 16 in … |
| Waste | In % |
| Blocks laid per 80 lb bag of mortar | A number. Manufacturers quote about 12–13 standard blocks per 80 lb bag |
| Price per block | Units: /piece |
| Price per bag of mortar | Units: /piece |
What each result means
- Net wall area = max([Wall length]×[Wall height] – [Openings area], 0)
- Blocks needed = ceil([Net wall area]÷([Face length]×[Face height])×(1 + [Waste]÷100))
- Bags of mortar = ceil([Blocks needed]÷[Blocks laid per 80 lb bag of mortar])
- Approximate weight of blocks = [Blocks needed]×[Block weight]
- Total cost = [Blocks needed]×[Price per block] + [Bags of mortar]×[Price per bag of mortar]
Example values
How the main result changes when Wall length changes and everything else stays at the example values above:
| Wall length (ft) | Blocks needed |
|---|---|
| 20 | 189 |
| 30 | 284 |
| 40 | 378 |
| 50 | 473 |
| 60 | 568 |
| 80 | 756 |
Formula
Blocks = (wall area − openings) ÷ block face area (incl. joint) × (1 + waste)Mortar bags = blocks ÷ blocks per bagWorked example
Inputs — Wall length: 40 ft; Wall height: 8 ft; Openings area: 0 ft²; Block size: 8 × 8 × 16 in (standard CMU); Waste: 5 %; Blocks laid per 80 lb bag of mortar: 12; Price per block: $2.5 /piece; Price per bag of mortar: $8 /piece.
Results — Net wall area = 320 ft²; Blocks needed = 378; Bags of mortar = 32; Approximate weight of blocks = 14,334 lb; Total cost = $1,201.00.
Key terms
- CMU
- Concrete masonry unit – the technical name for a concrete block.
Frequently asked questions
How many blocks are in a square foot?
1.125 standard 8 × 16 in blocks per ft², or 12.5 blocks per m² for 440 × 215 mm UK blocks.
How much mortar do I need for block?
About one 80 lb bag of mortar mix for every 12–13 blocks, or 8.5 ft³ of mortar per 100 blocks.
How much does a concrete block weigh?
A standard 8 in hollow block weighs about 35–40 lb (16–18 kg); lightweight blocks about 28 lb.
What formula does the Concrete Block Calculator use?
Blocks = (wall area − openings) ÷ block face area (incl. joint) × (1 + waste).