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Enter the wall size to get the number of concrete blocks (CMU) and an approximate mortar estimate.
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Estimates, not quotes. Check product packaging for exact coverage before ordering.
Divide wall area by the face area of one block, then add waste. A standard 8 by 16 inch concrete block has a nominal face of 0.888 sq ft, so about 1.13 blocks cover one square foot. Mortar is approximate: plan roughly one bag per 100 blocks, and confirm with your supplier.
A 20 by 4 ft wall is 80 sq ft. At 0.888 sq ft per block that is about 90 blocks without waste, or 99 with 10 percent. A wall with corners, openings or a pilaster needs more, and the mortar estimate is about one bag for this size.
A block wall is more than blocks: it needs a footing below frost depth, reinforcement and often an engineer for retaining or tall walls. Check local code before buying material, because the footing and grout can cost more than the blocks.
Concrete masonry units (CMU) come in nominal 8 by 16 inch faces and several thicknesses. Standard 8 inch block is used for walls and foundations; 6 inch and 4 inch block are used for garden walls and partitions. Cinder block is lighter and weaker than concrete block, so check the spec for load-bearing walls.
Block walls need a footing, and most need reinforcement or grout in the cores. Retaining walls and tall walls usually need an engineer.
| Wall | Area | 8x16 blocks (10% waste) |
|---|---|---|
| 20 x 4 ft | 80 sq ft | 99 |
| 10 x 3 ft | 30 sq ft | 38 |
| 30 x 6 ft | 180 sq ft | 223 |
| 40 x 8 ft | 320 sq ft | 397 |
Add blocks for pilasters and corners, and check the mortar estimate with your supplier.
A block wall starts with a concrete footing below frost depth and wider than the wall. Vertical rebar is set into the footing and grouted into the cores, and horizontal reinforcement is laid in the mortar joints.
Grouting fills the cores and ties the wall together. Skipping it is the most common reason a block wall cracks or leans. The footing and grout can cost more than the blocks themselves.
Mortar is mixed to a workable consistency and the first course is set level and square, because everything above it follows. Keep the courses level, the joints consistent and the corners plumb.
Plan the course height before buying: a wall that is not a multiple of the block height needs cutting, which wastes blocks. A string line and a level are the two tools that keep a wall straight.
A low garden wall is a common DIY project for someone comfortable with mortar. Foundations, retaining walls, structural walls and anything that needs a permit are mason work, and often need an engineer.
Get the footing and reinforcement details in writing before comparing block prices, because cheap blocks can hide expensive foundation work.
Block walls are quick to lay and cheaper than brick for garden and foundation walls. Poured concrete is stronger for retaining walls and basements. Brick costs more but gives a finished face that does not need rendering.
The block count is only the start: the footing, reinforcement, grout and finish decide the real cost of a wall. Compare complete builds, not block prices.
Mortar should not be laid in freezing weather, and blocks need protection from rain while the mortar cures. Keep the wall covered for the first day and avoid backfilling a retaining wall until the mortar has set.
Check plumb and level every course, not at the end. A wall that leans by the top is expensive to correct and usually has to come down.
You need a mixing tub, a trowel, a jointer, a level, a string line and a masonry hammer. Mix mortar in small batches so it does not set before it is used.
The first course is the whole wall: set it level, square and straight, and let it set before building up. Everything above follows that line.
Garden walls below a certain height may not need a permit, but retaining walls, walls near a boundary and anything structural usually do. Retaining walls also need drainage behind them and often an engineer.
Ask the building department before buying blocks. The footing, drainage and permit are the expensive parts, and they are cheaper to plan than to fix.
About 1.13 blocks per sq ft for a standard 8 by 16 inch block, because each covers 0.888 sq ft.
About 90 blocks without waste, or 99 with a 10 percent allowance.
Roughly one bag per 100 blocks as a starting estimate; confirm with your supplier and wall type.
Yes, block walls need a proper footing, and many need reinforcement or engineering.
Small garden walls are common DIY projects; structural and retaining walls need engineering and permits.
About 1.13 blocks per sq ft for an 8 by 16 inch block, because each covers 0.888 sq ft.
About 90 blocks without waste, or 99 with a 10 percent allowance.
Yes, block walls need a proper footing, and many need reinforcement or engineering.
Roughly one bag per 100 blocks as a starting estimate; confirm with your supplier.
Block face areas are nominal: 8 x 16 in covers 0.888 sq ft, 6 x 16 in 0.666 sq ft, 4 x 16 in 0.5 sq ft. Mortar estimates are approximate; confirm with your supplier. · Last updated October 30, 2026 · Report a correction