Cinder Block Measurement Mystery

Cinderblock Construction

Building with cinder block is the most versatile way to construct both a functional structure and a structure with great strength and integrity.  Cinder block, otherwise known as a concrete masonry unit (CMU), can be purchased at most home improvement stores such as Home Depot and Lowes.  The prices for CMU at these big box home improvement stores are most of the time very comparable to local block vendors.

Close to Backyard Flare, LLC here in Tucson, Arizona, is a block manufacturer/vendor named AZ Block.  At the same location where they manufacture the block, you can enter the facility to buy direct instead of dealing with a middleman.  This at times can make the block more cost effective for the DIY builder.

Picture of 8x8x16 cinder blocks.
Common 8x8x16 cinder block with voids. Great block for fantastic strength and structural integrity.

The most common block used to build the fireplaces is referred to as a 8x8x16.  This is literally the size of the block in inches.  Now there is a small “but…”” involved though. The block is not exactly 8x8x16.  In fact, the block measures 7 5/8” x 7 5/8” x 15 5/8”.  Notice that 3/8” is subtracted from each measurement. This is comparable to a 2×4 piece of wood not actually measuring 2” x 4”.  They are actually 1 ½” x 3 ½”.

So now the meat and potatoes for why the block nomenclatures are referred as one measurement and their actual measurements differ.  When you build a CMU structure, you will be using mortar between both the horizontal and vertical surfaces of the blocks.  This mortar effectively holds the blocks in place and gaps them appropriately.

The Use of Mortar

A perfect mortar joint should be 3/8” then?  In a perfect world, yes, but you will see that a bit more or a bit less over the length of a built structure is common.  As long as your overall structure measurements stay close to what was designed or planned, you’ll be just fine.  Don’t struggle with trying to make every mortar joint a perfect width.  You’ll only drive yourself crazy and not enjoy the construction as much.

Picture of cinderblock wall in running bond pattern.
Wall built with a staggered pattern. This is referred to as a “running bond”.

Because you won’t use just 8x8x16 blocks to build your structure, you will be buying other sizes as well.  A few of the widely-used sizes are 8x8x8, 8x4x16, and 8x2x16.  These are just a few of the 8” variety but there are so many different sizes to choose from.  Each one of these blocks will have measurements different from what the nomenclature will dictate.

I sure hope this helps you understand a little bit about cinder blocks and why they are manufactured with certain measurements.

Author: DIYoutdoorfireplaces

I am the founder and owner of Backyard Flare, LLC in Tucson, Arizona. We design outdoor fireplaces, outdoor kitchens, and pizza ovens and provide the construction plans needed by DIY savvy homeowners to build their own backyard structures.

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