How to Calculate Concrete Blocks From a Building Plan

The take-off formula

The whole method is one formula:

Blocks = (gross wall square feet - openings) / 0.89, then add waste.

One standard 8x8x16 block covers 0.89 square feet of wall face, with the 3/8 inch mortar joint included in the nominal size. So 100 square feet of wall is 100 / 0.89 = 112 blocks before waste, and 112 x 1.10 = 124 blocks with the standard 10% allowance. Memorize that 100 square feet takes about 124 blocks and you can sanity-check any take-off in your head.

Step 1: Pull wall lengths and heights off the plan

Start with the floor plan for lengths and the elevations or foundation plan for heights. For a foundation, the floor plan gives you the perimeter walls; the foundation section shows the wall height from the footing to the sill. Read each wall as a rectangle: length times height. Mark every wall on a sketch as you go, because it is easy to count a shared corner twice or skip a short return wall.

If the plan is dimensioned, use the dimensions, not a ruler on the paper; drawings shrink when printed. If only a scale bar is available, measure the bar, confirm the scale math, and then measure the walls.

Step 2: Worked example, 24x30 foundation

Take a simple 24 by 30 foot foundation with 4 foot tall block walls, no openings:

Perimeter: 2 x (24 + 30) = 108 linear feet.
Gross wall area: 108 x 4 = 432 square feet.
Blocks: 432 / 0.89 = 485.4, so 486 blocks before waste.
With 10% waste: 486 x 1.10 = 534.6, rounded up to 535 blocks.

Order 535 (or 540 for a round number from the yard). The extra blocks absorb the corners, the cut at the top course, and anything that arrives chipped.

Step 3: Subtract the openings

Plans show every door, window, crawl-space vent, and access hatch. Measure each one as a rectangle and subtract its area before dividing by 0.89. Example: a 3 by 7 foot walk-out door is 21 square feet, which is 21 / 0.89 = 24 blocks you do not need. Two basement windows at 3 by 4 feet are another 24 square feet, or 27 blocks. For the 24x30 foundation above, that takes 486 down to 435 before waste, and the 10% allowance to 479 blocks.

Do not over-subtract. A 1-foot-square vent is 1 block; counting it exactly saves you nothing after rounding. Subtract anything a block or larger, and skip the tiny stuff.

Step 4: Handle corners, angles, and odd heights

The 10% waste allowance is sized for a simple rectangular plan. Raise it to 15% when the plan has many corners, jogs, or angles, because every turn creates cut blocks. Angled walls (45-degree corners, bay shapes) burn the most blocks, since each cut wastes half a block.

Check heights against the 8-inch module. A 4-foot wall is exactly 6 courses, which is clean. A 4-foot-4-inch wall is 6.5 courses, which means cutting every block in the top course or ordering half-high block for it. Spot this on the plan before you order, not when the crew is staring at a half course.

Quick reference table

For standard 8x8x16 block, 10% waste included:

50 sq ft: 62 blocks.
100 sq ft: 124 blocks.
200 sq ft: 248 blocks.
300 sq ft: 371 blocks.
400 sq ft: 495 blocks.
500 sq ft: 618 blocks.

Run your plan take-off first, then use the table as a cross-check: if your take-off for a 300 square foot wall says 250 blocks, something went wrong, because the table says you should be near 371.