Estimate the number of concrete blocks for a wall from its measured dimensions and openings. This area-based count is a purchasing estimate; it does not lay out each course or automatically calculate every special unit, mortar joint, or grouted core.
Measure net wall area
Enter the overall wall length and height, then subtract door, window, or other full-thickness openings. The remaining net wall area must be greater than zero. If openings are irregular or the wall has separate heights, break it into measured rectangles and subtract the openings from the regions they actually occupy. The calculator’s basic method counts by face area, so it assumes a regular field of blocks rather than drawing a detailed bond pattern.
Use nominal module dimensions for the default count. A common nominal 8 × 8 × 16 in CMU has a face module of 8 × 16 in when the nominal dimensions include the intended mortar-joint spacing. That face area is 128 in², or 8/9 ft². Do not enter actual block measurements and then add the mortar joint again unless you deliberately configure a nonstandard module; doing both would double count the spacing.
Calculate a block count and allowance
The theoretical count is net wall area divided by one block’s face-module area. The purchase estimate applies the separate block allowance and rounds upward to a whole block: blocks = ceil(net area ÷ module area × (1 + allowance)). The default 5% allowance is an adjustable planning choice for breakage, cuts, and handling. It is not a masonry rule and it does not promise that the same spare quantity fits every bond, corner, opening, or delivery condition.
For a 20 ft long by 8 ft high wall, the gross area is 160 ft². A 3 ft by 7 ft opening removes 21 ft², leaving 139 ft². Dividing by 8/9 ft² per nominal module gives 156.375 theoretical blocks. Applying 5% gives 164.19, which rounds up to 165 blocks. The fractional theoretical value is useful for showing the area math; the order count must be a whole unit.
Optional mortar estimate
Mortar is optional because a wall may use a different mortar product, installation method, or takeoff. If you select QUIKRETE Mason Mix No. 1136 and its stated coverage applies, the manufacturer estimates up to about 13 standard 8 × 8 × 16 in blocks per 80 lb bag. In the example above, an independent 10% mortar allowance applied to the 156.375 theoretical blocks gives ceil(156.375 × 1.10 ÷ 13) = 14 bags. The block purchase allowance is not applied again to this mortar calculation.
This is a named product estimate, not a recommendation that Type S mortar is appropriate for every wall. Confirm mortar selection against the project documents, manufacturer instructions, and local requirements. Coverage can change with joint size, workmanship, block geometry, and waste. The mortar result should be labeled with the selected product and coverage assumption. If no applicable product is selected, the page should leave the mortar quantity unestimated rather than inventing a universal bags-per-wall conversion.
Keep grout separate from mortar and concrete
Mortar bonds units at the joints; masonry grout fills designated CMU cores or cavities. A hollow block is not a solid rectangular concrete volume. A grout estimate needs a supported core-fill volume or a product-specific fill value and the number of cores being filled. The tool should not treat every block as fully solid, infer which cells are reinforced, or apply an ordinary concrete bag yield to all masonry grout needs.
If you already know the total grout volume, keep it as a separate quantity and use a product whose documentation covers the intended application. The CMHA grout guidance distinguishes masonry grout from mortar and other concrete materials. Without a verified product yield and a defined fill pattern, a precise bag count would be false precision. Grout quantity also does not tell you where reinforcement belongs or whether a wall is structurally sufficient.
What area counting leaves out
An area count is a quick material estimate, not a course-by-course layout. Corners, intersections, bond beams, lintels, half blocks, end units, control joints, pilasters, and cuts can alter the order. Some cuts produce reusable pieces while others become waste; the basic allowance cannot decide that for the mason. For a wall with many openings or several returns, use a takeoff that identifies each course and special unit, then compare it with the area estimate as a reasonableness check.
The module size should match the product you expect to purchase. The CMHA dimensional note describes common nominal and specified dimensions, while its TEK area method supports block estimates by net wall area. Unusual textures or shapes may not follow the common module. Check the manufacturer’s product data for actual dimensions and special units before placing an order.
Plan a useful order
Use the calculated whole-block count as a starting point for a supplier quote. Confirm availability, pallet quantities, delivery access, handling, and return rules. The page’s result should show the theoretical count, the selected allowance, and the rounded purchase count together. That makes it easier to distinguish mathematical coverage from the number you intend to buy. If you revise a wall opening, only the affected net area needs to change; the calculation should preserve the rest of your dimensions.
Before building, rely on approved wall details for foundation support, reinforcement, lintels, grout placement, ties, drainage, and required materials. This calculator does not approve a wall height or load. If you only need nominal versus actual dimensions or product weights, open the corresponding guide; if you need a concrete fill quantity for a separate pour, use a volume tool. Keeping blocks, mortar, grout, and concrete in separate material lines prevents a wall takeoff from obscuring what each product is meant to do.
Frequently asked questions
How many blocks cover one square foot?
A common nominal 8 × 8 × 16 in CMU uses an 8 × 16 in face module, equal to 8/9 ft². That is 1.125 blocks per square foot before openings, special units, or waste.
Do I subtract windows and doors?
Yes. Subtract their measured face area from the gross wall area, as long as each opening is inside the wall boundary and is not counted twice.
Does the result include mortar?
No, block count and mortar are separate. An optional estimate can use the selected QUIKRETE Mason Mix No. 1136 coverage when its stated product conditions apply.
Is masonry grout the same as mortar?
No. Mortar bonds adjacent masonry units; grout fills designated cores or spaces. Their products, functions, and quantity methods differ.
Does a 5% allowance cover corner blocks and cuts?
It is a general adjustable planning allowance only. A detailed layout may need specific corner, half, lintel, bond-beam, or cut units beyond an area-based count.
Can the calculator tell me how many cores to fill?
No. The filling pattern and reinforcement requirements must come from project details. A grout volume also needs applicable product coverage before it can be converted to bags.