Cement or concrete blocks are available in several sizes and many shapes, and have a wide range of physical properties. They may be hollow or solid, made from lightweight cement or be formed from very dense aggregate. Specialist cement blocks have additional benefits such as high thermal and sound insulation properties. Calculating the quantity of blocks needed for a building is a straightforward process that does not require any previous building or architectural skills.
Establish the length and height of every wall to be built from cement blocks. Refer to the building plans for the dimensions of height and length, and then ascertain the area of each wall using the equation surface area = length x height. Combine the areas of all the walls to obtain a total wall area. For example, a wall 30 feet long and 20 feet high has a surface area of 600 square feet — 30 x 20 = 600 — so four identical walls have a total surface area of 2,400 square feet — 4 x 600 = 2,400.
Refer to the plans to identify gaps in walls such as doorways and windows. Calculate the area of each gap using the same formula — area = length x height — and then add together the surface areas of all the gaps. The result is the total area not requiring blocks. For example, a wall with a surface area of 600 square feet and a door opening measuring 4 feet wide and 7 feet high requires sufficient blocks to cover 572 square feet — 600 - (4 x 7) = 572.
Deduct the total area not requiring blocks from the total wall area. The result is the surface area requiring blocks. For example, a square building with a total wall area of 2,400 square feet containing 300 square feet of door and window openings requires sufficient blocks to cover 2,100 square feet — 2,400 - 300 = 2,100.
Determine the surface area of a cement block. Measure the length and height of the block in inches and then substitute the values in the equation square feet of block = (length of block x height of block) / 144. For example, a standard cement block measures 16 x 8 inches, so it covers 0.89 square feet — (16 x 8) / 144 = 0.89.
Calculate the number of blocks required by substituting the area to be built and the size of the block in this equation: total area of walls / area of a block = number of blocks required. To conclude the example, building 2,100 square feet of wall from 0.89-square-foot blocks requires 2,360 blocks — 2,100 x 0.89 = 2,359.55.
The 16 x 8 standard cement block is actually a little smaller than the nominal size. The difference between actual and nominal size allows for mortar between blocks. This is why it is not necessary to allow for mortar in the calculations.
Consider purchasing additional blocks to allow for on-site breakages, unavoidable changes to plans and building errors. An additional 5 percent is often recommended, but it may be more financially efficient to wait until near the end of the build before buying additional blocks. They may not be required.
Some block suppliers will refund you for unused pallets of blocks at the end of the build.
Unless you are competent, consult a professional quantity surveyor when calculating the materials required for complex buildings.
David Robinson has written professionally since 2000. He is a Fellow of the Royal Geographical Society and the Royal Meteorological Society. He has written for the "Telegraph" and "Guardian" newspapers in the U.K., government publications, websites, magazines and school textbooks. He holds an honors Bachelor of Arts in geography and education and a teaching certificate from Durham University, England.