Updated · Built and checked by the PlinzoTools team, formulas tested against published sources
Calculate how many CMU blocks and bags of mortar you need for a wall from its area and any openings.
This is a block-counting tool, not a poured-concrete volume calculator -- a genuinely different calculation from the other tools in the Concrete family. It estimates how many standard concrete masonry units (CMU), commonly called cinder blocks, a wall needs based on its area, plus roughly how many bags of mortar to lay them.
Enter the total wall area in square feet, and subtract door or window openings separately (the calculator nets those out automatically). The advanced section lets you adjust the blocks-per-square-foot ratio and mortar-bags-per-100-blocks ratio if you're using a non-standard block size.
Blocks = (Wall Area − Openings) × 1.125
Mortar Bags = (Blocks ÷ 100) × 3
The 1.125 blocks-per-square-foot figure (equivalently, "112.5 blocks per 100 square feet") is a standard masonry-industry rule of thumb for the common 8in x 8in x 16in nominal CMU laid with a 3/8-inch mortar joint -- it comes from that block's actual face area including its mortar joint allowance, not a rough guess. The mortar ratio (about 3 bags of Type N or S mortar per 100 blocks) is a similarly standard trade estimate.
Use this for a block foundation wall, a freestanding block garden wall, or a block garage/shed wall. If you're building the same kind of wall but from brick instead of block, see the Brick Calculator, which uses the same area-based logic with brick-specific coverage numbers.
The 1.125 blocks/sqft figure is specifically for the standard 8x8x16in nominal CMU with a 3/8in mortar joint -- if you're using a different block size (a 4-inch or 12-inch block, for example), adjust the "Blocks per sqft" advanced setting to match your block's actual coverage.
A quick way to trust the estimate is to count courses. A nominal 8in x 8in x 16in block laid with a 3/8in joint covers 16 inches of length and 8 inches of height on the wall face. Picture a hypothetical garden wall 24 feet long and 4 feet high. That is 288 inches of length, or 18 blocks per course, and 48 inches of height, or 6 courses. Multiplying gives 108 blocks, which matches the calculator: 96 sqft times 1.125 is 108. If your own hand count and the calculator disagree by a lot, a measurement or unit entry is usually the culprit.
The result is a plain area-based count, so it does not know about corners, half blocks, bond beams, lintel blocks, or cap blocks. Cutting a block to fit a wall end or a jamb usually leaves a piece you cannot reuse. Many builders add a margin of roughly five to ten percent for breakage and cuts, and more for walls with many corners or openings. Decide on a margin before you order, because blocks from a different production lot can vary slightly in color and size.
The mortar figure covers only the bed and head joints between blocks. It does not include grout for filling cores around rebar, which is a separate material quantity and often required by code in foundation walls. It also does not include footing concrete beneath the wall. For the footing, size the pour separately with a concrete tool. Keep the mortar type your plans call for, since structural and below-grade walls often specify a particular type.
If you switch to a 4in or 12in thick block with the same 8in x 16in face, the face area is the same, so the blocks-per-square-foot figure stays the same even though the block is thinner or thicker. The number changes only if the face dimensions change, for example with half-height units. In that case work out the face area including the joint, divide 144 by it in square inches, and type the result into the advanced setting.
Retaining walls, foundation walls, and any wall carrying loads are engineered elements. Reinforcement, footing depth, drainage, and permits depend on local codes and soil. Use this tool for material budgeting, and rely on plans or a licensed professional for the structure itself.
About 1.125 blocks per square foot for the standard 8x8x16in CMU with a 3/8in mortar joint -- equivalently, 112.5 blocks per 100 square feet.
Roughly 3 bags of mortar mix per 100 blocks is a standard trade estimate, though it varies with joint thickness and mason technique.
Yes -- enter the total area of any openings and the calculator subtracts them from the wall area before counting blocks, since you don't lay block across an opening.
Open the "Advanced" section and adjust the blocks-per-square-foot figure to match your specific block's actual face coverage.
No. It counts only the field of the wall from the area you enter. Cap or coping units are a separate line item measured along the top length of the wall.
Yes, buy whole bags and round up. Running short mid-wall is worse than having a partial bag left, and mortar mixing habits vary between masons.
Hand counts depend on whole blocks and whole courses, while the calculator uses an average area ratio. Small differences, especially on short walls, are normal. Big differences point to a measurement error.