Enter the total wall length and height. Deduct openings below.
Nominal face size width x height. Mortar joint is included in nominal size.
A standard 80 lb bag of mortar lays roughly 33 standard blocks.
Enter wall dimensions to see your block estimate.
Seven Common Mistakes DIYers Make with Concrete Blocks (And How to Avoid Them)
You’ve got your plans drawn up and you’re ready to start building your retaining wall, garden bed, or other outdoor project. Before you start stacking blocks, it’s important to make sure your measurements and material estimates are accurate. Small mistakes at the beginning of a project can lead to extra trips to the store, wasted materials, and problems with the finished wall.
Here are some of the most common mistakes people make when working with concrete blocks and what you can do to avoid them.
Confusing Nominal and Actual Block Dimensions
An 8 x 8 x 16 concrete block is not actually 8 inches by 8 inches by 16 inches. Its actual dimensions are typically about 7 5/8 inches x 7 5/8 inches x 15 5/8 inches. The nominal size accounts for the mortar joint.
When planning your layout and calculating the number of blocks you need, use the nominal dimensions if your project includes standard mortar joints. When measuring and laying out the actual wall, keep the block dimensions and mortar joints in mind. Small differences can add up over a long wall and affect your final measurements.
Forgetting to Add Extra Blocks for Waste
Ordering exactly the number of blocks your calculation calls for can leave you short once construction begins. Blocks can crack while being cut, arrive damaged, or need to be cut to fit corners and ends.
Adding about 5 to 10 percent to your calculated quantity gives you some room for cuts, breakage, and other unexpected issues. For projects that require a lot of cutting, staying closer to 10 percent may make sense.
Forgetting to Subtract Openings
If your wall includes doors, windows, gates, or other openings, subtract those areas when calculating the amount of material you need.
Measure the width and height of each opening, calculate its square footage, and subtract that amount from the total wall area before determining how many blocks to purchase.
Assuming the Entire Area Measures the Same
Taking one measurement and assuming it applies to the entire project can cause problems, particularly when you’re working on uneven ground or an existing surface.
Take measurements at several points across the project area. For a small project, a good tape measure is usually enough. A laser distance measurer can make measuring larger areas faster and easier.
Starting With an Improper Base
A concrete block wall is only as stable as the surface beneath it. Setting blocks directly on loose soil, sand, or uncompacted gravel can allow the base to settle and cause the wall to shift or lean.
For a small garden or landscape wall, remove loose soil and organic material before preparing the base. Install an appropriate layer of compactable aggregate and compact it thoroughly. Take your time leveling the base from side to side and front to back. Getting the first course level makes the rest of the project much easier.
The exact base depth and construction method will depend on the height of the wall, the type of block being used, soil conditions, drainage, and the manufacturer’s installation requirements.
Mixing Mortar to the Wrong Consistency
If your project uses mortar, getting the consistency right is important. Mortar should be workable enough to spread easily without becoming watery.
Too much water can weaken the mixture and contribute to shrinkage, while a mixture that is too dry can be difficult to work with and may not bond properly. If you’re using a bagged mortar mix, follow the manufacturer’s directions for the recommended amount of water rather than estimating.
Rushing Before the Mortar Has Cured
Once the wall is built, give the mortar enough time to set and cure before putting significant pressure on it. Backfilling, loading, or disturbing the wall too soon can affect the finished project.
Curing time varies depending on the materials used, temperature, humidity, and weather conditions, so follow the instructions provided with your mortar or masonry product. A little patience at the end of the project can prevent having to repair it later.
Where to Go From Here
Now that you understand the difference between nominal and actual block dimensions, how waste allowances and openings affect your total, why consistent measurements and proper base preparation matter, and how mortar consistency and cure time affect your finished wall, use our Concrete Block Calculator above to get an accurate blocks-and-mortar count for your wall. You’ll know exactly what to enter because you understand why each factor matters.
Frequently Asked Questions About Concrete Block Projects
How many concrete blocks do I need for my project?
Start by calculating the total square footage of the wall and subtracting any openings. For a standard 8 x 8 x 16-inch block, you’ll need about 1.125 blocks per square foot. Add another 5 to 10 percent for cuts, damaged blocks, and waste.
Do I need a concrete footing under a block wall?
It depends on what you’re building. A small landscape or garden wall may have different base requirements than a structural or retaining wall. Wall height, soil conditions, drainage, and the type of block all matter. For retaining or structural walls, check local building requirements and the block manufacturer’s specifications before deciding how to build the base.
Do concrete blocks need to be filled with concrete?
Not always. Some block walls are built with hollow cells, while others require reinforcement and grout in some or all of the cells. If the wall is structural or retaining soil, the requirements can be very different from those of a simple garden wall.
How much mortar do I need for a concrete block wall?
The amount depends on the number and size of your blocks, the thickness of the joints, and the mortar product you’re using. Calculate your block quantity first and then check the coverage information on the mortar you plan to use.
Can I build a concrete block wall without mortar?
Some retaining wall systems are specifically designed to be dry stacked. Standard concrete masonry blocks are generally installed with mortar unless you’re using a system designed for another installation method. If you want a mortarless wall, use blocks made for that type of construction.