How many concrete blocks are in a square foot?

For a standard 8" x 8" x 16" block, you need 1.125 blocks per square foot of wall area. This accounts for the standard 3/8" mortar joint. So, a 100 sq ft wall requires roughly 113 blocks.

How much mortar do I need for 100 blocks?

A standard rule of thumb is that one 80lb bag of premixed mortar lays about 12 to 15 standard blocks (8x8x16). For 100 blocks, you would need approximately 7-8 bags of mortar.

Do I need to fill the block cores with concrete?

It depends on the structural requirements. For retaining walls or load-bearing walls, it is common to fill cores with concrete (grout) and insert rebar for strength. For simple partition walls, cores are often left empty or only filled at corners and openings.

What is the actual size of a concrete block?

A standard '8x8x16' block is actually 7-5/8" x 7-5/8" x 15-5/8". The 3/8" difference allows for the mortar joint, making the installed module exactly 8x8x16 inches.

How do I calculate rebar for a block wall?

Rebar is typically placed vertically every 24 to 48 inches and horizontally every 2 to 3 courses. This calculator estimates rebar based on a standard spacing of 32" OC vertical and every 3rd course horizontal.

🧱 CMU Block Calculator

Estimate Concrete Blocks, Mortar, and Rebar for Walls

Wall Length (ft)

Wall Height (ft)

Block Size

Opening Area (sq ft)

Total area of doors/windows to deduct

🏗️ How to Estimate a Block Wall

Estimating a concrete masonry unit (CMU) wall involves calculating the wall area and dividing by the area of a single block.

1. Calculate Wall Area

Multiply Length × Height to get the total square footage. Subtract any openings (doors, windows).

2. Determine Blocks per Sq Ft

A standard 8x8x16 block (including mortar joint) covers 0.89 sq ft. Therefore, you need 1.125 blocks per square foot of wall area.

3. Estimate Mortar

A standard 80lb bag of premixed mortar will lay approximately 12 to 15 blocks. If mixing your own (1 part cement : 3 parts sand), you'll need about 1 ton of sand for every 1,000 blocks.

💡 From Tony Martinez, Concrete Contractor (19 years):

"Always order 5-10% extra blocks. You WILL break some during cutting or handling. Also, don't forget the half-blocks for the ends of the wall! If your wall height isn't a multiple of 8 inches, you'll need to cut the top course or adjust your footing depth. It's much easier to plan your footing so the wall lands on a full block height."

⚠️ Common Mistakes

1. Ignoring the Modular Dimension: Blocks are designed to work on an 8-inch module. Try to design your wall length in multiples of 8 inches (e.g., 20' 0", 20' 8") to minimize cutting.

2. Forgetting Rebar Displacement: If you are filling every cell with concrete (grout), remember that the rebar takes up some space, but more importantly, the block webs take up about 50% of the volume. Don't order enough concrete to fill a solid box the size of your wall!

3. Not wetting the blocks: In very hot, dry weather, dry blocks can suck the moisture out of the mortar too quickly, weakening the bond. (Note: This is controversial; some modern mortars don't require this, but it's a classic tip for standard mortar).

Reviewed by Tony Martinez

Licensed Concrete & Masonry Contractor, 19 years specializing in retaining walls and foundations.

🏗️ How to Estimate a Block Wall

1. Calculate Wall Area

2. Determine Blocks per Sq Ft

3. Estimate Mortar

⚠️ Common Mistakes

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