Concrete Block Calculator

Measuring for a block wall takes about a minute. Enter the wall length (in feet) and the wall height (in feet), then pick your block size — the default is the standard 8×8×16 CMU that most home walls use. Press calculate and you'll see exactly how many concrete blocks and how many bags of mortar to buy. Need metric, a bigger waste allowance, or a price estimate? Open More options to switch to meters, bump the breakage factor, or add a price per block. That's the fastest way to answer "how many blocks do I need for a 10x20 wall" without doing the math yourself.

A concrete masonry unit (CMU), often called a cinder block, is sized by its nominal face — the space the block plus its mortar joint occupies in the wall. A standard block has a 16" × 8" face, which works out to 1.125 blocks per square foot of wall (144 in² ÷ 128 in²). The calculator multiplies your wall area by that blocks-per-square-foot figure, adds your waste factor, and rounds up to whole blocks because you can't buy a fraction of one. It also estimates mortar at roughly 3 standard bags per 100 blocks, the common rule of thumb for a 3⁄8" joint. Because nominal sizing already includes the mortar joint, you don't subtract anything for grout lines — the math is built in.

Blocks = Wall area (ft²) × blocks per ft² × (1 + waste %), rounded up.

Blocks per ft² = 144 ÷ (face width in × face height in). For a standard 16"×8" block that is 144 ÷ 128 = 1.125. A half-height 16"×4" block doubles to 2.25 per ft². Mortar bags = blocks × 3 ÷ 100, rounded up. This is the same "cinder block calculator by the square foot" method the pros use on a takeoff sheet.

A 20 ft × 10 ft garden wall, standard block: 200 ft² × 1.125 = 225 blocks, plus 5% waste = 237 blocks and about 8 bags of mortar.

A 10 ft × 8 ft shed wall: 80 ft² × 1.125 = 90 blocks × 1.05 = 95 blocks. That answers "how many cinder blocks for a 10x8 wall" in one step.

Half-height 16"×4" cap course, 40 ft long × 0.33 ft: small areas still round up to whole blocks, so always buy a few extra.

- Always keep the waste factor on. 5% covers cuts and breakage on a straight wall; use 10% if the wall has corners, openings, or you're new to laying block.
- Subtract large openings yourself. For a big door or window, measure that area and knock it off the wall length × height before entering — the calculator assumes a solid wall.
- Buy mortar and blocks together. The 3-bags-per-100 estimate is for standard joints; deep or wide joints use more.
- Order full pallets. Blocks are cheaper by the pallet (usually 90–120 units), so round your order up to the next pallet.