Nominal Dimensions vs. Actual Dimensions - Lumber and Cinder Block Dimensions


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We’ve received several phone calls from frantic customers that were standing in the lumber or masonry department of their local building supply store stating that they couldn’t find 2” x 4”s – only 1 ½” x 3 ½” boards. We have also received similar calls regarding CMU / cinder block – they could only find 15 5/8” x 7 5/8” x 5 5/8” block, not the 6” x 8” x 16” cinder blocks that are required per our Materials List. Not to worry – while we reference the NOMINAL dimensions in the Installation Manual, all measurements and CAD diagrams are based on ACTUAL dimensions.

So what’s up with Nominal and Actual Dimensions anyway? Well, when a green 2” x 4” is milled and planed, it is an ACTUAL 2” x 4”. It is then placed into an industrial sized kiln and dried for several days. Once the moisture is removed from the wood, the wood is removed from the kiln and given a 2nd plaining. The end result – 1 ½” x 3 ½”.

CMU / Cinder Block dimensions are different. While it too is placed in a large kiln to cure / harden, it starts life at 15 5/8” x 7 5/8” x 5 5/8” and stays that way for eternity – or until you cut or drop it. The 3/8” difference from the Nominal Size is the estimation of 3/8” of mortar that is placed between cinder blocks. So, for example, if you look at a cinder block house or wall, there is typically a 3/8” mortar joint between the block…

And now you know… And knowing is half the battle. GO JOE!

Firebrick measurements are ACTUAL size - 9” x 4.5” x 2.5” and 9” x 4.5” x 1.25”.

More Useless Knowledge: Cinder block has several names… Breeze Block, Besser Block, Clinker Block and Hollow Block. Most US stores stock them as CMU (Concrete Masonry Unit) block.