Calculation method
Net wall area = length × height − opening area. Divide by the nominal 8 × 16 in face (8/9 ft²), add the block allowance and round up. Mortar bags = ceiling(ordered blocks ÷ label blocks per bag × (1 + mortar allowance/100)). Mortar for all ordered blocks is an intentionally conservative basis. The nominal module already includes the joint.
Worked example: input to purchase quantity
A 20 × 8 ft wall (6.096 × 2.4384 m) minus 20 ft² of openings (1.8580608 m²) has 140 ft² net area. That is 157.5 block equivalents; with 5% extra, order 166 blocks. At up to 13 standard blocks per 80 lb bag of QUIKRETE Mortar Mix No. 1102, Type N, the estimate is 13 bags before extra mortar allowance.
Estimate standard CMU from the wall face
A conventional concrete masonry wall is ordered in blocks, while a drawing often provides only length, height, and openings. This calculator converts the net face area into equivalent standard 8 × 8 × 16 nominal concrete masonry units, adds an explicit block allowance, and rounds upward to whole blocks. It then estimates full bags of premixed mortar from an entered blocks-per-bag yield. This is an area takeoff for a single wall thickness, not a course drawing or a structural design.
The block's nominal thickness identifies the product family, while its eight-by-sixteen face controls this estimate. That face occupies 128 square inches, or eight ninths of a square foot. Therefore one square foot contains 1.125 nominal block faces before any allowance. The thickness does not multiply the wall face count. A wall with a second wythe, a different face module, decorative units, or a veneer assembly needs a separate takeoff rather than a casual adjustment to this result.
Nominal dimensions already include the joint module
QUIKRETE's block-wall instructions explain that a typical unit measures about 15⅝ inches long and 7⅝ inches high, leaving room for the conventional three-eighths-inch mortar joint. The familiar sixteen-by-eight dimensions describe the installed module. This calculator uses that nominal module directly. Adding another joint to sixteen inches and eight inches would count joint space twice and reduce the estimated quantity incorrectly. Using the bare face without joints would increase the count for a different reason.
Confirm the selected block before applying this convention. Some architectural units, metric blocks, specialty shapes, and proprietary systems do not use the same module. Entering metric measurements here still estimates the US nominal module: its exact face dimensions convert to 40.64 by 20.32 centimetres. It does not silently substitute a 400 by 200 millimetre product. Ask the supplier which physical unit matches the plans and use another takeoff method if the module differs.
Subtract openings without deleting their support needs
Measure the gross wall length and height to the intended masonry limits. Add door and window rough-opening areas and enter that total as the opening deduction. Avoid subtracting the same overlap twice. Openings cannot exceed the gross wall face; equal areas produce zero blocks and zero mortar bags. That zero result means the stated face has no masonry area left. It does not mean a wall surrounding a large opening can be built without lintels, jamb units, or supporting elements.
The area method treats cuts as equivalent block faces. It does not arrange running bond, corners, half blocks, end blocks, lintel units, pilasters, or bond beams. Draw courses around openings and identify special units separately. A small leftover area may still consume a whole cut block, and an offcut is useful only if its size and orientation fit another location. The block allowance is an ordering choice for cuts and breakage, not a substitute for that course-by-course reconciliation.
Make mortar yield a product input
The default thirteen blocks per bag comes from QUIKRETE Mortar Mix No. 1102: an eighty-pound bag is described as laying up to thirteen standard blocks. Its standard formulation is Type N; this reference does not select mortar for your wall. Follow the specified mortar type and the current label. Mortar yield changes with the unit, joints, bedding method, working losses, and product. A different bag weight or product can require a different blocks-per-bag input even if the packaging looks similar.
The bag estimate conservatively uses all ordered blocks, including the block allowance, then applies the separate mortar allowance and rounds upward. Unlaid spare blocks will not actually consume mortar, so this is an ordering budget rather than an exact placed volume. Avoid adding the same waste percentage again in a supplier worksheet. Footing concrete, core-fill grout, reinforcement, anchors, drainage, waterproofing, caps, and labor are outside this calculation. Keep these quantities separate, verify the wall design, and compare the rounded order with a supplier takeoff.
Product and method reference
Reference limits and default yields are identified separately from user-entered values; verify the adopted local code and current product label. QUIKRETE Mortar Mix No. 1102 technical data.
Calculation and editorial review: November 3, 2026. Arithmetic runs in your browser; project inputs are not submitted to TwoCut.