Concrete Slab Calculations

One of our Building Designers asked me the other day if a 10% “shrink factor” should be used when advising how much concrete it takes to pour a concrete slab on grade. This particular Building Designer “in a previous life” had been a building contractor.  It had been his practice to always order 10% more concrete for a pour, than calculated! In my neck of the woods, concrete is pretty darn expensive. I only want to order what is actually needed to do the job.

First, let’s talk about concrete slab thickness. Concrete floors and slabs on grade are called out by their “nominal” thickness. Just like a 2×4, a four inch thick slab is only 3-1/2” thick. This is so the edges can be formed by using a 2×4 and the area to be prepared can be graded off the same way, by using a 2×4. The same goes for a six inch thick slab, being actually 5-1/2” thick.

Second, let’s do the math. Concrete is purchased by the cubic yard. A cubic yard would be three feet in all directions, or 27 cubic feet. If pouring a four inch thick slab, we need to spread this one yard cube, across four inches of thickness. As four goes into 12 three times, we can multiply 27 by three and get 81 square feet four inches thick.

For a nominal four inch thick pour, I would divide the square footage of the total pour, by 81 to get the number of yards required. Let’s consider a 24’ x 36’ pole building. 864 square feet of area divided by 81 equals 10.67. Round up to the nearest whole yard and order 11 yards.

Hold it…..but a four inch thick concrete slab, is only going to really be 3-1/2” deep in real life!

In an “ideal perfect world” a 3-1/2” nominal thickness floor would allow 92.57 square feet of area to be covered by a yard of concrete. However, there is just no prepared site which is perfectly level and compacted so tightly as to not have some of the concrete going into making it level or filling voids in the fill area. The concrete mixture also includes water, which will be absorbed into the surface below or evaporate as the concrete cures.

The summary is – for a nominal four inch thick pour, divide area by 81. For a nominal five inch pour divide by 65, six divide by 54. As long as a good grading and compaction processes have been followed, these numbers work every time to give you “just enough” for your concrete slab.

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