How to Figure Mortar for Block

Accurate mortar estimates for block construction save money and time.

Masonry mortar joins concrete masonry units or blocks. Standard-sized blocks are roughly 8 inches tall by 16 inches wide by 8 inches deep. Mortar joints are usually 3/8 inches thick. The amount of mortar needed for a project depends on the number of blocks needed. Online mortar calculators are a convenient alternative to calculating mortar amounts by hand. When a computer is not accessible, it helps to know the conventional method. Calculating mortar amounts depends on whether the project will use premixed bags or mortar mixed from components on-site.

Things You'll Need

• Tape measure
• Calculator
• Paper
• Pencil

Instructions

1. Measure and write down the length and height of the block walls needed for the project in linear feet. Multiply the length by the height to get the project's net area square footage. Multiply this number by 0.12 to account for cuts and waste in the field and get the overall square footage.

2. Multiply the overall square footage by 1.125 to get the number of standard-sized blocks needed for the project. For example, 100 square feet of wall area needs roughly 112.5 blocks.

3. Write down the number of blocks needed for the project if purchasing premixed bags. Divide the number of blocks by the manufacturer recommended number of premixed mortar bags recommended per block. Multiply this number by 0.10 to account for breaking, spilling or errors in the field.

4. Write down the amount of bulk cement and sand needed based on Rule 38, where 38 cubic feet of cement and sand make 1 cubic yard of mortar if mixing from components. Use the recommended mortar mix proportions. A 1:3 mix is 9.5 cubic feet cement and 28.5 cubic feet sand, which add together to 38 cubic feet.

Tips & Warnings

• Apply Type N mortar, in general, on interior walls, above-grade exterior walls under normal loading conditions, and veneer applications and Type S on above-grade block construction.

• Read the premix mortar manufacturer's recommendations for bags per block, which can range from one bag for every 10 blocks up to one bag per 40 blocks.

• Mix 18 shovelfuls or 200 lbs. of fine masonry sand to every 80 lb. bag of masonry cement as an alternative for mixing components in the field.

• Read local building codes or specifications for recommended mortar mix proportions and their applications.

• Use Rule 38 for small projects or for quick estimates on larger constructions project, but not for accurate quantity estimates on large masonry construction projects.

References

• JLC Online; Block-Laying Basics; John Carroll; November 2006
• FindArticles; Tools of the Trade: Block Work for Dummies: Tips for Working with Block and Mortar; John Carroll; August 2011
• American Concrete Products: Calculating the Required Number of Standard Construction Blocks
• Ace Hardware: Laying Concrete Block

Resources

• Masonry Contractors Association of America; Choosing the Right Mortar; John Melander; September 2000
• Integrated Publishing: Estimating Mortar