Free Live Calculators

Concrete Block Calculator (CMU & Cinder Block)

Calculate blocks needed for any wall — includes mortar bags, courses, door/window openings, waste factor, and cost. Works for CMU and cinder blocks.

Openings (doors & windows)

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How to Use This Calculator

  1. 1

    Enter wall dimensions

    Enter wall length and height in feet. For multiple walls (e.g., all four sides of a room), add lengths together and enter the combined total. Enter just one height value — most walls share the same height.

  2. 2

    Select block size and mortar joint

    Choose your block size from the dropdown (Standard 8×8×16 is most common for structural walls; Partition 4×8×16 for interior; Half Block 8×8×8 for filler courses). Select 3/8-inch mortar joint for standard masonry; 1/2-inch for thicker joints or period aesthetics.

  3. 3

    Set waste factor

    10% is the recommended default for most projects. Use 5% for simple straight walls with no cuts; 15% for walls with many openings, corners, or curves. The calculator adds waste to the raw block count automatically.

  4. 4

    Enter door and window openings

    Enter the number of doors and windows and their dimensions (width × height in feet). The calculator subtracts opening areas from the wall area before computing blocks. Standard exterior door: 3×7 ft. Standard window: 3×4 ft.

What Each Value Means

Courses (rows)
The number of horizontal rows of blocks in the wall. Courses = wall height ÷ (nominal block height + mortar joint). For an 8-ft wall with standard 8-inch blocks and 3/8-inch joints: 96 ÷ 8.375 = 11.46 → 12 courses. Each course adds one block height plus one mortar joint to the wall height.
Waste Factor (percent)
Additional blocks ordered beyond the calculated minimum to account for cuts at openings and wall ends, breakage during delivery and laying, and irregular dimensions. Standard recommendation: 10% for typical walls, 15% for complex walls with many openings or curves.
Mortar Bags (80 lb) (bags)
Pre-mixed mortar in 80 lb bags. Coverage: approximately 8.5 bags per 100 standard blocks, accounting for bed joints (horizontal, full-length) and head joints (vertical, at block ends). Coverage varies slightly by mortar joint thickness and mason technique. Pre-mixed bags include Portland cement and sand — just add water.

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Frequently Asked Questions

How many concrete blocks do I need per square foot?
Standard 8×8×16 concrete blocks (CMU) with a 3/8-inch mortar joint cover 0.889 square feet each, so you need approximately 1.125 blocks per square foot. For a 100 sq ft wall: 100 × 1.125 = 112.5, rounded up to 113 blocks. Always add 10% waste for cuts, breakage, and irregular edges, bringing the total to about 125 blocks.
What is the difference between a cinder block and a concrete block?
Cinder block is an older term for what are now called concrete masonry units (CMU). Original cinder blocks used coal cinders as aggregate — these are no longer produced commercially. Today, virtually all blocks sold as 'cinder blocks' are actually concrete blocks made with portland cement and aggregates like gravel or sand. The two terms are interchangeable in modern use. This calculator works for both.
How many mortar bags do I need for a concrete block wall?
For standard 8×16 CMU blocks with 3/8-inch mortar joints, plan for approximately 8.5 bags of 80 lb pre-mixed mortar per 100 blocks. A 200-block wall needs about 17 bags. This accounts for both bed joints (horizontal) and head joints (vertical). Pre-mixed mortar bags simplify estimating; if mixing from scratch, 1 bag of masonry cement plus 3 bags of sand covers roughly the same area as 3–4 pre-mix bags.
How do I calculate the number of courses in a block wall?
Courses (horizontal rows) = wall height in inches ÷ (block height + mortar joint). For a standard 8-inch block with 3/8-inch joint: courses = wall height in inches ÷ 8.375. A 96-inch (8 ft) wall = 96 ÷ 8.375 ≈ 11.46 → 12 courses. In practice, this means the finished height rounds to the next full course. The calculator shows courses automatically.
Should I add a waste factor to my block count?
Yes — always add at least 10% for a standard concrete block wall. Cuts at corners, door/window frames, and irregular dimensions require partial blocks. Breakage during transport and laying adds more. Experienced masons use 5–10% waste for straight walls and 10–15% for walls with many openings, curves, or corners. It's cheaper to order extra and return them than to short-order and wait for a second delivery.