How to calculate mortar

Updated February 21, 2017

Mortar is a mixture of lime, cement, sand and water. It is used to hold cement blocks together between the joints. It is also commonly used on brick and stone. When you are installing any of these materials, the first step in the installation process is to calculate how much mortar will be needed for installation. To determine how many bags of mortar you need, figure out how many blocks you are using.

Measure the length, width and height of the area in inches. Run a tape measure across the length and record the measurement. Measure the width or depth of the area and write down that measurement. Measure the height of the wall and record that measurement also.

Calculate what size blocks you will be using. The standard size blocks are 8 by 8 by 16. These blocks are 8 inches wide, 8 inches tall and 16 inches long. Calculate how many blocks you are using for the installation process.

Divide the length of the area in inches by 16. If the length was 320 inches, then divide that number by 16 to get 20 blocks for the length. Divide the depth or width of the blocks by 8. If the depth was 32 inches, then divide 32 by 8 to get 4. Divide the height by 8. If the height is 48, then divide 48 by 8 to get 6.

Multiply all three measurements together to get the number of blocks necessary. 16 multiplied by 4 equals 64. 64 multiplied by 6 equals 384. This is the number of blocks necessary for installation.

Enter this number into a mortar calculator or look on a bag of mortar to figure how much mortar is needed. For example, 320 blocks that are 8 by 8 by 16 inches in size require 36 bags of 60-pound mortar and 27 bags of 80-pound mortar.


The standard thickness of a mortar joint is 3/8 inch. The number of bags needed is based on that thickness of mortar between the joints. This number can vary, depending on the thickness you desire. In general, every 100 blocks will require nine bags of mortar.

Things You'll Need

  • Tape measure
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About the Author

Alexander Callos began writing in 2005 for "The Lantern" at The Ohio State University and has written for various websites, including Bleacher Report, Top Ten Real Estate Deals and Columbus Sports. He has published articles for CBS Sports, and other websites. He graduated in 2007 from The Ohio State University with a bachelor's degree in public affairs journalism.