Concrete Masonry Units (CMU), or Concrete Blocks, come in a variety of sizes.  CMUs are identified by their depth - i.e. the thickness of the wall they create.  For instance, a 6" CMU is nominally 6" deep.

As with brick, CMUs have actual dimensions and nominal dimensions.  The nominal dimension of a CMU is the actual dimension plus the width of the mortar joint.  Typical CMU mortar joints are 3/8".  The nominal dimension works within the 4" grid that other construction materials follow.

Nominal Dimensions of an 8" CMU
Nominal Dimensions of an 8" CMU" alt="Nominal Dimensions of an 8" CMU
Actual Dimensions of an 8" CMU
Actual Dimensions of an 8" CMU" alt="Actual Dimensions of an 8" CMU

4" CMU Full Block

Actual Size: 3 5/8" x 7 5/8" x 15 5/8"

Nominal Size: 4" x 8" x 16"

4" CMU Half-Block

Actual Size: 3 5/8" Deep x 7 5/8" High x 7 5/8" Long

Nominal Size: 4" Deep x 8" High x 8" Long

6" CMU Full Block

Actual Size: 5 5/8" Deep x 7 5/8" High x 15 5/8" Long

Nominal Size: 6" x 8" x 16"

6" CMU Half-Block

Actual Size: 5 5/8" Deep x 7 5/8" High x 7 5/8" Long

Nominal Size: 6" Deep x 8" High x 8" Long

8" CMU Full Block

Actual Size: 7 5/8" Deep x 7 5/8" High x 15 5/8" Long

Nominal Size: 8" x 8" x 16"

8" CMU Half-Block

Actual Size: 7 5/8" Deep x 7 5/8" High x 7 5/8" Long

Nominal Size: 8" Deep x 8" High x 8" Long

10" CMU Full Block

Actual Size: 9 5/8" Deep x 7 5/8" High x 15 5/8" Long

Nominal Size: 10" x 8" x 16"

10" CMU Half-Block

Actual Size: 9 5/8" Deep x 7 5/8" High x 7 5/8" Long

Nominal Size: 10" Deep x 8" High x 8" Long

12" CMU Full Block

Actual Size: 11 5/8" Deep x 7 5/8" High x 15 5/8" Long

Nominal Size: 12" x 8" x 16"

12" CMU Half-Block

Actual Size: 11 5/8" Deep x 7 5/8" High x 7 5/8" Long

Nominal Size: 12" Deep x 8" High x 8" Long

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