Concrete Block Calculator

How the block count is worked out

Three numbers decide the order: the square footage of the wall, the square footage you take out for doors and windows, and the face size of the block. Everything else is arithmetic on top of those three.

A concrete block is sold by its nominal size, and the nominal face of a standard unit is 8 inches by 16 inches. That face is 128 square inches, or 0.889 square feet. One square foot divided by 0.889 square feet per block gives 1.125 blocks per square foot of wall. The number is the same for a 4 inch, 6 inch, 8 inch or 12 inch thick block, because the thickness changes the wall, not the face.

The order of operations matters when the wall has openings. Work out the gross area first, subtract the openings, then multiply by the coverage rate. If you multiply the gross area first and subtract blocks for the openings afterwards, the waste allowance ends up applied to the wrong number and the order runs short.

• Gross area = wall length x wall height
• Net area = gross area minus the area of every opening
• Blocks = net area x blocks per square foot
• Order = blocks x (1 + waste percentage)

The result is rounded up, never down. A wall that needs 171.4 blocks needs 172, because half a block on site is a hole in the wall.

The true size of a standard block

A block sold as 8 by 8 by 16 inches does not measure 8 by 8 by 16 inches. It measures 7 5/8 by 7 5/8 by 15 5/8 inches. The difference is the mortar joint: the 3/8 inch joint plus the 7 5/8 inch actual height adds up to the 8 inch nominal height, and the same holds along the length. The Concrete Masonry and Hardscapes Association spells this out in its modular layout guidance for concrete masonry, and the units themselves are made to ASTM C90, the standard specification for loadbearing concrete masonry units.

This is why the 1.125 figure works without measuring a single joint. The nominal face already carries one half of a joint on each side, so a block plus its share of mortar covers exactly 8 by 16 inches of finished wall.

A half block covers half the face, so it takes twice as many to cover a square foot. Nobody builds a wall from half blocks, but the number is worth knowing when a supplier quotes a mixed pallet.

Worked example: a 24 by 8 foot garage wall

A detached garage wall 24 feet long and 8 feet high, with one 3 by 7 foot service door and one 4 by 5 foot window. The door takes 21 square feet and the window takes 20, so the openings total 41 square feet. Round the openings up to 42 if you prefer a margin, and the block count moves by one unit.

• Gross area: 24 x 8 = 192 square feet
• Openings: 21 + 20 = 41 square feet
• Net area: 192 - 41 = 151 square feet
• Blocks: 151 x 1.125 = 169.9, so 170 blocks
• With 7% waste: 170 x 1.07 = 181.9, so 182 blocks

The calculator on this page uses 40 square feet of openings as its default, which gives a net area of 152 square feet, 171 blocks before waste and 183 blocks to order. Both answers are right; the difference is one square foot of window.

Now the vertical check. An 8 foot wall is 96 inches, and each course of block plus its joint rises 8 inches, so the wall is 12 courses high. A 24 foot wall is 288 inches, and each block plus joint runs 16 inches, so a full course holds 18 blocks. Twelve courses of 18 blocks is 216 blocks for a solid wall, and 216 minus the 45 blocks that the 40 square feet of openings would have used leaves 171. The two methods agree, which is the point of running both.

At $2.85 a block, 183 blocks come to $521.55 before tax and delivery. That is the number to hold against a mason's quote, which will also carry mortar, reinforcement and labor.

Openings, corners and the waste allowance

Waste is not a markup. It covers the blocks that get cut at corners, around openings and at the top course, plus the ones that crack in handling. A straight run with few cuts wastes less than a wall with six windows and four corners.

The percentage applies to the net block count, not to the wall area. On the 152 square foot wall the net count is 171, so 7% is 11.97 blocks, which rounds to 12, and 171 plus 12 is 183. Doing it the other way, 152 x 1.07 x 1.125, gives 182.97, and the two land in the same place because multiplication is commutative. Where they diverge is rounding: round the block count up first, then add waste, and you never order fewer than the wall needs.

Corners deserve their own line in the plan. A corner in running bond needs a half block on the return, so a rectangular building with four outside corners needs about 24 half blocks over 12 courses. Order those as half blocks or cut them from full units, and count the leftover halves as waste if they cannot be used on the next course. That is why a garden wall with four corners and a gate often needs the 10% allowance even though it looks simple.

Wall sizes at a glance

These counts assume 8 by 16 inch nominal faces, no openings, and a 7% waste allowance. Use them to sanity check the calculator, not to replace it.

The pattern is easy to carry in your head: 1.125 blocks per square foot, then about 7% more. A 100 square foot wall is 113 blocks, a 1,000 square foot wall is 1,125 blocks, and a 2,000 square foot wall is 2,250. Once the area passes a few hundred square feet, the waste allowance is worth more than any rounding error, which is why it belongs in the order and not in your head.

What the count leaves out

A block count is one line of a materials list. These items are separate purchases, and each one can change the order:

• Mortar. Type S or Type N mortar mix in bags, plus the sand and lime if you mix from scratch. The 3/8 inch joints in a block wall use far more mortar than a brick wall of the same area.
• Reinforcement. Horizontal joint reinforcement every other course, or rebar in grouted cells for a retaining wall.
• Grout. Cells are grouted solid at corners, at openings and wherever a wall carries a load. Grout is sold by the cubic foot or by the bag, and a fully grouted wall can take a third of a cubic yard for a small garage.
• Lintels. A block lintel, a steel angle or a precast beam over every door and window opening. The opening removes blocks but adds a lintel.
• Bond beam and cap. The top course is often a U-shaped bond beam block filled with grout and rebar, and an outdoor wall usually gets a cap block or a concrete cap on top.
• Footing. A block wall sits on a concrete footing, poured wider than the wall. That concrete is not in the block count.

If the wall is a retaining wall, the block count is the least of it. Water pressure behind the wall, drainage gravel, a perforated pipe and the footing design decide whether the wall stands, and a 12 inch block is not automatically strong enough for a wall holding back four feet of soil. That is a question for an engineer or the local building department, not a calculator.

Ordering, delivery and cost

Blocks arrive on a pallet, and pallets are sold in fixed counts that vary by manufacturer and by block weight. A supplier who quotes 183 blocks will usually round to the nearest pallet and deliver a little extra, which is fine: leftover blocks store well and the next project can use them. Ask for the pallet count before you order, and check whether the price is per block or per pallet.

Delivery is the line people forget. A pallet of 8 inch block is heavy enough that it needs a truck with a lift gate or a forklift on site, and a driveway with a low wire or a steep grade can turn a delivery into a hand-carry job. If the blocks have to be moved by hand from the curb, the labor is real and it is not in the block price.

Prices move with region, weight class and quantity. Lightweight block costs more per unit and saves labor; normal weight block costs less and is harder to carry. The calculator takes whatever price you type, so put in the quote you were given rather than a national average, and add sales tax and delivery on top. The block subtotal is the floor of the project cost, never the whole of it.

This is a planning estimate, not a takeoff. Confirm the block size, pallet count and local code requirements with your supplier and building department before you order.