Concrete Block Calculator — blocks for a wall

How it works

To find how many concrete blocks (CMUs) a wall needs, multiply the wall area by the blocks that cover one square foot, adding a waste allowance:

blocks = ⌈area × (1 + waste) × blocks-per-sq-ft⌉

A standard 8 × 8 × 16 inch block, laid with a mortar joint, covers about 1.125 blocks per square foot of wall face (its 8 × 16 inch face is 128 square inches, and with mortar each block accounts for roughly 0.89 square feet). Different block sizes change this, so the blocks-per-square-foot is an input. A 10% waste allowance covers breakage and cuts, and the result rounds up.

Measure the wall face area (length × height). Subtract large openings, or leave them in as a buffer. Because blocks are large, a wall needs far fewer of them than the same wall in brick.

Worked example

A 100 sq ft wall at 1.125 blocks per sq ft with 10% waste needs 100 × 1.1 × 1.125 = 124 blocks. A 200 sq ft foundation wall takes 248 blocks, and a 400 sq ft garage wall needs 495 blocks.

Frequently asked questions

How many concrete blocks do I need?

Multiply the wall area by the blocks per square foot, add a waste allowance, and round up: blocks = ⌈area × (1 + waste) × blocks-per-sq-ft⌉. A 100 sq ft wall at 1.125 blocks/ft² with 10% waste needs 124 blocks.

How many blocks are in a square foot?

About 1.125 for a standard 8 × 8 × 16 inch block laid with a mortar joint — its 8 × 16 inch face is 128 square inches, and with the mortar joint each block accounts for close to 0.89 square feet, so a little over one block per square foot. Half-height or larger blocks differ.

How is a block wall different from a brick wall?

Blocks are much larger than bricks, so the same wall needs far fewer — about 1.125 blocks per square foot versus about 7 bricks. Block walls go up faster and are common for foundations and structural walls; brick is often a thinner veneer.

How much waste should I allow for block?

About 10% covers breakage in handling and cuts at corners and openings. Walls with many openings, curves or piers may need 15%. Round up to whole blocks and keep a few spares.

Do I subtract door and window openings?

You can, for a tighter count — work out each opening's area and take it off the wall area. Leaving small openings in provides a margin, since cuts around them waste block. For a large opening, subtracting it is sensible.

Does this include mortar or rebar?

No — it counts the blocks only. Mortar is estimated from the block count (roughly three 80-pound bags per 100 blocks), and reinforcement (rebar, fill) from the wall design. Calculate those separately.