About this calculator
Concrete masonry units (CMU), often called cinder blocks, are sized so that one block plus a ⅜ in mortar joint takes up exactly 16 × 8 in of wall. That makes counting them straightforward: 1.125 blocks per square foot of wall. This calculator does the count, adds a breakage allowance and estimates the mortar you’ll need.
How to use it
- Enter the wall length and height.
- Subtract the area of openings (doors, windows) in square feet.
- Choose the block face size. The block’s width (4, 6, 8 or 12 in thick) doesn’t change how many you need, only their weight and price.
- Keep 5% extra for broken blocks and cuts.
The formula
Blocks per sq ft = 144 ÷ (16 × 8) = 1.125
Blocks = (wall area − openings) × blocks per sq ft × (1 + extra%)
Mortar bags ≈ blocks ÷ 13 (80 lb bags, 8×8×16 block)
Block dimensions are nominal: an “8×8×16” block is actually 7⅝ × 7⅝ × 15⅝ in, and the ⅜ in joint brings it to 8 × 16 in on the wall. The mortar estimate uses the manufacturer’s figure of about 13 standard blocks per 80 lb bag of mortar mix.
Worked example
These are the calculator’s starting values and the results they produce:
| Wall length | 20 ft |
|---|---|
| Wall height | 4 ft |
| Area of openings to subtract | 0 sq ft |
| Block face size | Standard 16 in long × 8 in high |
| Extra for breakage and cuts | 5 % |
Tips before you buy
- Lay out walls in multiples of 8 in (length and height) to minimize cutting.
- Corners use the same number of blocks as straight runs, but you may want corner or bond-beam blocks for specific courses.
- Retaining walls and foundations usually need rebar and grout-filled cores — ask your supplier or engineer for quantities.
- An 8 in hollow block weighs roughly 35–40 lb; plan deliveries and staging accordingly.
Frequently asked questions
How many concrete blocks are in a square foot?
1.125 standard 8×8×16 blocks per square foot of wall (one block per 128 sq in including the mortar joint). For 100 sq ft of wall you need 113 blocks before waste.
How many bags of mortar per concrete block?
About one 80 lb bag of mortar mix for every 13 standard blocks, according to Quikrete. That’s roughly 8 bags per 100 blocks.
How many blocks do I need for a 20 ft × 4 ft wall?
The wall is 80 sq ft, so 90 blocks, or 95 with 5% extra, plus about 8 bags of mortar mix.