Concrete Masonry Unit (CMU) Block Sizes - Architecture Dictionary

Concrete Masonry Unit (CMU) Block Sizes

Concrete Masonry Units (CMU), also known as concrete blocks, are used extensively in construction due to their durability, cost-effectiveness, and versatility. Understanding the sizes and dimensions of CMUs is essential for selecting the right block for your project.


CMU SizeNominal Dimensions
(D x H x L)
Actual Dimensions
(D x H x L)
4" CMU (Full Block)4" x 8" x 16"3 ⅝" x 7 ⅝" x 15 ⅝"
4" CMU (Half Block)4" x 8" x 8"3 ⅝" x 7 ⅝" x 7 ⅝"
6" CMU (Full Block)6" x 8" x 16"5 ⅝" x 7 ⅝" x 15 ⅝"
6" CMU (Half Block)6" x 8" x 8"5 ⅝" x 7 ⅝" x 7 ⅝"
8" CMU (Full Block)8" x 8" x 16"7 ⅝" x 7 ⅝" x 15 ⅝"
8" CMU (Half Block)8" x 8" x 8"7 ⅝" x 7 ⅝" x 7 ⅝"
10" CMU (Full Block)10" x 8" x 16"9 ⅝" x 7 ⅝" x 15 ⅝"
10" CMU (Half Block)10" x 8" x 8"9 ⅝" x 7 ⅝" x 7 ⅝"
12" CMU (Full Block)12" x 8" x 16"11 ⅝" x 7 ⅝" x 15 ⅝"
12" CMU (Half Block)12" x 8" x 8"11 ⅝" x 7 ⅝" x 7 ⅝"

Nominal vs Actual Dimensions

The sizes of CMU blocks are typically described in terms of nominal and actual dimensions. The 'nominal' dimension is the size of the block including the thickness of the mortar joint, while the 'actual' dimension is the physical size of the block itself. For example, a CMU block described as 8" x 8" x 16" (nominal dimension) will actually measure 7.625" x 7.625" x 15.625".

Common CMU Block Sizes

There are several standard CMU block sizes, each suitable for specific uses:

• 8" x 8" x 16" Block: This is the most commonly used block size for structural walls. It's strong and versatile, making it ideal for both load-bearing and non-load-bearing walls.
• 12" x 8" x 16" Block: This size provides additional structural capacity and is often used for foundation walls and retaining walls where higher loads may be expected.
• 4" x 8" x 16" Block: This smaller block is primarily used for non-structural partitions and veneer walls.

Choosing the Right CMU Block Size

The size of the CMU block you choose depends on the requirements of your project. For structural walls, 8" x 8" x 16" or 12" x 8" x 16" blocks are generally used, depending on the load requirements. For non-load bearing walls or partitions, smaller sizes like 4" x 8" x 16" can be used. Additionally, different surface finishes can provide aesthetic variations, making CMUs a versatile choice for various architectural styles.

Remember, it's crucial to consult with a qualified professional when selecting CMU block sizes for your project to ensure safety and adherence to local building codes.


ASTM DesignationCMU Type
ASTM C55Concrete Brick
ASTM C73Calcium Silicate Face Brick
ASTM C90Load Bearing Concrete Masonry Units
ASTM C139CMUs for construction of Catch Basins and Manholes
ASTM C744Prefaced Concrete and Calcium Silicate Masonry Units
ASTM C936Solid Interlocking Concrete Paving Units
ASTM C1372Segmental Retaining Wall Units

CMU manufacturers must conform to ASTM standards when fabricating concrete blocks.

The following table provides the ASTM standards that apply to concrete masonry units. ASTM C90 covers most conventional CMU construction and is the standard referenced by the ICC codes.

Surface Finishes

Burnished/Polished CMU

Also referred to as Ground Face CMU, concrete blocks can be burnished or polished to expose the natural aggregates in the concrete mix. It is important to work with the manufacturer to select appealing aggregate and to test the polishing process to ensure you get the aesthetic you are looking for.

Colored CMU

There are two ways a concrete masonry unit can have color. The first is by adding color pigments can be to either the concrete block or the mortar, or both. This method will have some slight variations in the color you choose due to the nature of concrete mixture. The second way to add color is by painting the block and/or the mortar.

Glazed CMU

Glazed concrete masonry units have a tile-like glazed finish made from a resin-based coating. This type of coating offers a wide variety of color as well as some faux material patterns. These surfaces may offer improved resistance to graffiti and chemicals. Glazed CMU is covered via ASTM C744, but the blocks still need to adhere to the ASTM C90 standard for load bearing CMU.

Raked CMU

Raked (also called Striated) units have vertical rake marks introduced during the molding process. The rake marks are not as deep as the scored or ribbed blocks. You can also apply rake marks to scored or ribbed blocks.

Sandblasted CMU

Sandblasting concrete masonry units also exposes the aggregate, but also blasts away some of the sand and cement to create a rougher appearance. Some people believe this creates a natural weathered look instead of the smoother look of burnished or polished CMU.

Soft-Split CMU

Soft-Split CMU is made with special molds that create a split-faced appearance instead of mechanically splitting the blocks. The effect is a bit less rough than split faced. In addition, the mold means that aggregates are not exposed.

Split Faced CMU

Split faced CMU is manufactured by molding two units side-by-side and then mechanically splitting them apart after firing. This creates a very rough texture that some people think looks like stone, especially since the aggregates in the block are also split or exposed.

Real-World Applications

Residential Construction

In many homes, 8" x 8" x 16" CMU blocks are commonly used for foundational walls due to their structural integrity and strength. These blocks can support significant weight, which makes them suitable for building a sturdy and resilient home structure. Additionally, they're used for external walls, given their resistance to elements like wind, fire, and moisture.

Commercial Buildings

For commercial buildings, which often require more load-bearing capacity, the larger 12" x 8" x 16" CMU blocks are frequently used. The added width provides extra strength, making them ideal for high-rise buildings or structures that support heavy machinery. These are commonly found in factories, warehouses, and other industrial settings.

Landscaping and Exterior Projects

CMU blocks also find extensive use in landscaping and exterior construction projects. For example, 8" x 8" x 16" blocks are often used for constructing garden walls, retaining walls, or outdoor fireplaces. Their durability and resistance to weather conditions make them suitable for these outdoor applications.

Interior Partitions and Veneer Walls

For interior non-structural walls, 4" x 8" x 16" blocks are often the preferred choice. These blocks are lighter and easier to handle, making them perfect for creating partitions in offices or residential homes. Furthermore, due to their smooth surfaces, they can also be used for veneer walls where aesthetics are a priority.

Concrete Masonry Unit Shapes

CMU Shapes are fundamental to the versatility and applicability of concrete blocks in various construction projects. While the standard rectangular block is the most recognized, CMU shapes go well beyond that. There are numerous specialized shapes designed to meet specific construction needs. This includes "U" shaped blocks for controlled fills, "L" shaped units for corners, and rounded shapes for decorative applications. Some CMUs are designed with hollow cores to reduce weight and provide space for rebar and concrete, while others are solid for structural integrity. Additionally, there are interlocking or "A" shaped blocks that facilitate alignment and mortarless construction. From load-bearing walls to decorative features, the diverse array of CMU shapes plays an essential role in making them suitable for a wide range of architectural and engineering applications.

Bond beam




Control joint


Double corner / plain end


Double open end


Kerf / All purpose


Knockout bond beam




Open end / "A" shaped










Single Corner