Concrete Block Calculator
Work out how many concrete masonry units (CMU) and bags of mortar you need for a wall, foundation or retaining wall – with doors, windows and waste taken into account.
Formulas shown on this page · Last reviewed October 2026
How the calculation works
net wall area = length × height − openings
blocks = net wall area × blocks per sq ft × (1 + waste %)
Block sizes are nominal: an 8×8×16 block actually measures 7⅝ × 7⅝ × 15⅝ inches, and the 3/8-inch mortar joint brings it up to a full 8 × 16 face. That is why one block covers exactly 128 square inches, or 0.889 sq ft.
Blocks per square foot by size
| Nominal size | Face area | Blocks per sq ft |
|---|---|---|
| 8 × 8 × 16 in | 0.889 sq ft | 1.125 |
| 8 × 4 × 16 in | 0.444 sq ft | 2.25 |
| 8 × 8 × 12 in | 0.667 sq ft | 1.5 |
Tips before you order
- Corners and ends need corner or bullnose blocks – count one per course at each free end.
- Load-bearing and retaining walls usually need rebar and grout fill; check local code.
- Order 5–10% extra for breakage and cuts, more if the wall has many openings.
Frequently asked questions
How many concrete blocks are in a square foot?
A standard 8×8×16 inch block covers 0.889 sq ft of wall including the 3/8-inch mortar joint, so you need 1.125 blocks per square foot.
How many blocks do I need for a 10 × 8 ft wall?
An 80 sq ft wall needs 90 standard blocks, or about 95 with 5% waste.
How much mortar do I need per block?
A common rule of thumb is one 80 lb bag of Type S or N mortar mix for every 28–32 blocks. Professionals mixing their own use about 3 bags of masonry cement per 100 blocks.
How much does a concrete block wall cost?
Standard gray CMU blocks cost about $1.75–$3 each at US home centers. Installed block walls typically run $10–$25 per square foot including labor, footing and grout.