Concrete Block Calculator

1. Enter the wall length and height in feet.
2. Enter the total openings area in square feet to subtract doors and windows, if any.
3. Choose the block size and a waste allowance of about 7 to 10 percent.
4. Read the result: blocks needed, mortar bags, and the courses and blocks per course to lay it out.

Here is what each result line means:

A concrete block calculator turns the size of a wall into the number of concrete masonry units, or CMUs, and the bags of mortar it takes. Unlike a poured wall, a block wall is built from stacked units, so it is estimated by counting blocks by area rather than by volume.

It works for foundation walls, garden and boundary walls, garages and retaining-wall stems built from block. The count rests on a simple fact: a standard nominal 8 by 8 by 16 inch block, with its mortar joint, covers 1.125 blocks per square foot of wall. If you are instead pouring a solid concrete wall, size that by volume, and to estimate the concrete to fill block cores use the concrete mix calculator or the cement calculator for the binder.

Because block is laid in a grid, the number of units is fixed by the wall area and the face size of one block. A standard block shows a 16 by 8 inch face once the mortar joint is added, which is 0.888 square feet, so it takes about 1.125 blocks to cover a square foot.

1. Multiply length by height for the gross area and subtract the openings.
2. Multiply the net area by the blocks per square foot for the block count, then add the waste allowance.
3. Divide the block count by 100 and multiply by three for the mortar bags.
4. Divide the height by the block height for courses, and the length by the block length for blocks per course.

For a solid grouted or bond-beam wall, the grout and any reinforcement are extra; a concrete weight calculator helps with the filled weight.

Suppose you are building a 20 foot by 8 foot wall from standard 8 by 8 by 16 inch block, with no openings and 7 percent waste.

Calculation: the area is 20 x 8 = 160 sq ft. Blocks = 160 x 1.125 = 180 before waste, and with 7 percent that is 193 blocks. Mortar = 193 / 100 x 3 = about 6 bags. The wall is 12 courses high and 15 blocks per course.

The lesson: the raw count comes straight from the area, and the waste allowance covers the cuts and breakage that every real wall has.

Standard 8 by 8 by 16 Block

The workhorse unit covers 1.125 blocks per square foot of wall, nominal, with a 3/8 inch mortar joint. Width (6, 8, 10 or 12 inch) changes the wall thickness, not the face count.

Half-high 8 by 4 by 16 Block

A half-height block shows a 16 by 4 inch face, so it takes 2.25 per square foot, twice the standard count, often used for a finer coursing or a cap.

Why the Face, Not the Width

Block is counted by the face it shows on the wall. A 6 inch and a 12 inch block of the same 16 by 8 face both count at 1.125 per square foot; the extra width is more concrete per block, not more blocks.

These are nominal estimating figures. Actual coursing, special units and bond patterns can shift the count a little.

Wall Area

The count follows the net area directly, so length and height drive it. Subtract large openings so you do not buy block for space that is not there.

Block Face Size

A half-high block doubles the count for the same wall, while width changes the concrete per block, not the number. Pick the face that matches your coursing.

Waste and Breakage

Cutting blocks at corners, ends and openings breaks some, so a 7 to 10 percent allowance keeps you from a second trip for a handful of units.

Bond Pattern and Specials

Corner, bullnose and half units, and a non-running bond, shift the count a little. For a plain running-bond wall the area method is close.

A block wall and a poured concrete wall solve the same job differently, and are estimated in different units.

For a poured wall, size it by volume instead; for the delivered cost of any concrete you add to fill block cores, the concrete cost calculator in the related tools prices it by the yard.

Ordering Block and Mortar

Turn the wall size into a block count and mortar bags before you buy, so you order close to a full pallet and enough mortar.

Laying Out the Courses

The courses and blocks per course help you plan the coursing, the corners and where a cut block falls.

Pricing a Block Job

The block and mortar counts are what a supplier prices against, so they let you budget the wall and compare quotes.

1. Counting by Block Width

Block is counted by its face, not its width, so a 12 inch block covers the same 1.125 per square foot as an 8 inch one. Width changes thickness, not the count.

2. Forgetting the Mortar

Blocks without mortar is half an order. Three 80 pound bags per hundred blocks is the common planning figure, which the tool adds for you.

3. Skipping the Waste Allowance

Corners, ends and openings mean cut and broken blocks. Leaving out a 7 to 10 percent allowance leaves you a few blocks short.

4. Ignoring Grout and Rebar

A reinforced wall has grout-filled cores and steel that this count does not include. Work the grout and rebar out separately from your plans.

5. Treating the Estimate as a Design

Block count is not structural design. Block strength, reinforcement and the design of a load-bearing or retaining wall come from a code or an engineer, not from this tool.

The counting is exact for a plain running-bond wall; the mortar and waste are standard planning figures, so treat them as estimates.

Multiply the wall area by the blocks per square foot, about 1.125 for a standard 8 by 8 by 16 block, then add 7 to 10 percent waste. A 20 by 8 foot wall is 160 x 1.125 = 180 blocks, or about 193 with waste. Enter your own wall size above.

A standard nominal 8 by 8 by 16 inch block covers 1.125 blocks per square foot of wall with a 3/8 inch mortar joint. A half-high 8 by 4 by 16 block shows half the face, so it takes 2.25 per square foot.

About three 80 pound bags of mortar per 100 blocks is the common planning figure, so a 193 block wall needs roughly 6 bags. The exact amount depends on the mortar type, the joint size and whether you bed the full block or just the face shells.

No. Block is counted by the face it shows, so a 6, 8, 10 or 12 inch wide block of the same 16 by 8 face all count at 1.125 per square foot. Width changes the wall thickness and the concrete per block, not the number of blocks.

Divide the wall height by the block height. A standard block is 8 inches high with its joint, so an 8 foot wall is about 12 courses; a half-high block is 4 inches, so it doubles the courses. The tool shows the courses for your wall.

About 7 percent for an experienced crew, and up to 10 percent for a wall with many corners and openings, where more blocks are cut. The allowance covers breakage and cuts so you are not a few blocks short.

No. It counts the blocks and mortar. Grout-filled cores, bond beams and reinforcing steel in a structural wall are separate; work them out from your plans or an engineer.

It depends on the job, the labour and the region. Block needs no formwork but more hand labour; a poured wall needs forms but goes up fast. Estimate block by count with this tool and a poured wall by volume, then compare.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• Titan America block estimating guide (Titan America; 1.125 block per sq ft and about 3 bags of mortar per 100 block).
• American Cement Association (American Cement Association; concrete masonry basics).
• National Ready Mixed Concrete Association (NRMCA; concrete and masonry materials).