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   Actual Dimensions of an 8" CMU

4" CMU

Actual Size
D x H x L (inches)
Nominal Size
D x H x L (inches)
3 5/8 x 7 5/8 x 7 5/84 x 8 x 8
3 5/8 x 7 5/8 x 15 5/84 x 8 x 16

6" CMU

Actual Size
D x H x L (inches)
Nominal Size
D x H x L (inches)
5 5/8 x 7 5/8 x 7 5/86 x 8 x 8
5 5/8 x 7 5/8 x 15 5/86 x 8 x 16

8" CMU

Actual Size
D x H x L (inches)
Nominal Size
D x H x L (inches)
7 5/8 x 7 5/8 x 7 5/88 x 8 x 8
7 5/8 x 7 5/8 x 15 5/88 x 8 x 16

10" CMU

Actual Size
D x H x L (inches)
Nominal Size
D x H x L (inches)
9 5/8 x 7 5/8 x 7 5/810 x 8 x 8
9 5/8 x 7 5/8 x 15 5/810 x 8 x 16

12" CMU

Actual Size
D x H x L (inches)
Nominal Size
D x H x L (inches)
11 5/8 x 7 5/8 x 7 5/812 x 8 x 8
11 5/8 x 7 5/8 x 15 5/812 x 8 x 16
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