How to use the concrete block calculator
- Measure the wall. Enter the total length of the wall and its finished height in feet. For a foundation or a garden wall with several sides, add the lengths together.
- Choose the block size. Most walls use the standard 8 × 8 × 16 in concrete masonry unit (CMU). The 6 in and 12 in wide blocks have the same face size, so they need the same count. Half-high 8 × 4 × 16 blocks need twice as many.
- Subtract openings. Enter the area of doors and windows in square feet so you do not buy blocks for holes.
- Keep some waste. 5% covers broken blocks and cuts. Use 10% for walls with many corners or openings.
Concrete block formula
A standard block is 15 5/8 × 7 5/8 in. With a 3/8 in mortar joint it covers a nominal 16 × 8 in, which is 128 square inches, or 0.889 sq ft. That gives 1.125 blocks per square foot of wall.
Blocks = wall area × 1.125 (8 in high blocks)
Blocks to buy = blocks × (1 + waste %)
Mortar bags = blocks ÷ 30
Blocks per square foot
| Block (W × H × L) | Face with joint | Blocks per sq ft | Blocks per 100 sq ft |
|---|---|---|---|
| 8 × 8 × 16 in | 16 × 8 in | 1.125 | 112.5 |
| 6 × 8 × 16 in | 16 × 8 in | 1.125 | 112.5 |
| 12 × 8 × 16 in | 16 × 8 in | 1.125 | 112.5 |
| 8 × 4 × 16 in | 16 × 4 in | 2.25 | 225 |
Worked example
A 40 ft long, 6 ft high block wall with no openings has 240 sq ft of face. At 1.125 blocks per sq ft that is 270 blocks: 9 courses of 30 blocks. Add 5% waste and buy 284 blocks. At about 30 blocks per 80 lb bag of mortar mix you need 9 bags of mortar. If you also fill the cores with concrete or grout, use our concrete calculator for the fill volume.
Mortar, rebar and footings
- Mortar: one 80 lb bag of mortar mix lays roughly 28 to 35 standard blocks, depending on joint thickness and how much drops. Type S mortar is the usual choice for block walls and foundations.
- Footing: a block wall needs a solid concrete footing, usually twice as wide as the block and at least 8 in deep, below the frost line where you live.
- Rebar and grout: retaining and load-bearing walls usually need vertical rebar in grouted cores. Follow your local building code or an engineer for spacing.
- Garden walls: for a short landscape wall built from segmental blocks, the retaining wall calculator is a better fit.
Frequently asked questions
How many concrete blocks are in a square foot?
A standard 8 × 8 × 16 in block covers 16 × 8 in with its mortar joint, so you need 1.125 blocks per square foot, or 112.5 blocks per 100 square feet.
How many cinder blocks do I need for a 10 x 10 wall?
A 10 ft long, 10 ft high wall is 100 sq ft, which needs 112.5 standard blocks. With 5% waste, buy 119 blocks.
How many blocks does one bag of mortar lay?
An 80 lb bag of mortar mix lays about 30 standard 8 × 8 × 16 in blocks. Thicker joints or a lot of droppings lower that number.
Are cinder blocks and concrete blocks the same?
People use the names for the same thing. Modern blocks are concrete masonry units made with sand and gravel; older cinder blocks used coal cinders as aggregate. Both come in the same sizes, so the count is the same.
How many courses of block are in an 8 foot wall?
Standard blocks are 8 in high with the joint, so an 8 ft (96 in) wall has 12 courses.
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Last updated October 2026.