Understanding CMU Block Sizes and Their Names

Today, we’re going to discuss the sizes of concrete masonry units (CMUs) and how they get their names. On the table, we have four common sizes: 4-inch, 6-inch, 8-inch, and 12-inch blocks.

How Block Sizes Work

Block sizes are reported as nominal measurements. For example, an 8-inch block is commonly referred to as an 8x8x16 block. However, if you measure it, you’ll notice that the actual dimensions are slightly smaller. For instance, the 16-inch length is actually 15 5/8 inches. This discrepancy accounts for the mortar joint, which is typically 3/8 of an inch.

When calculating block dimensions, we include the mortar joint to arrive at the nominal measurement. Thus, an 8x8x16 block is not actually 7 5/8 x 7 5/8 x 15 5/8 inches.

Measuring a Block

Blocks are measured by their length, width, and height. The common sizes are as follows:

• 4-inch block: 4x8x16
• 6-inch block: 6x8x16
• 8-inch block: 8x8x16
• 12-inch block: 12x8x16

You can see the commonality: all blocks are 16 inches long and 8 inches tall. The variable that gives each block its name is the thickness or width of the unit itself (4, 6, 8, or 12 inches).

Why These Sizes?

The different sizes serve various structural and architectural purposes, which we will cover in another episode.

Understanding these sizes and their naming conventions helps in selecting the right block for your construction needs.