We use cubic yards for volume measurements, typically concrete, but also soil. For this post, we’ll just use concrete as the reference, as soil can get pretty complicated — we could spend a month on posts about soil measurements.
To determine the number of cubic yards of concrete you need for the concrete item you need, you simply determine the volume in feet and divide by 27. So, for a 4″ thick concrete patio or slab that’s 10′ by 12′, you would first convert the thickness to a foot measurement by dividing by 12 — 4/12′ = .33′. Now, multiply 10′ x 12′ x .333′ = 40 cubic feet Now, you need to divide by 27 (the number of cubic feet in a cubic yard) — 40/27 = 1.48 cubic yards.
Don’t forget to divide by 27!! It happens more frequently than you might think and then you’d end up with 40 cubic yards of concrete, rather than the 1.48 that you actually need!
For slabs, it’s pretty easy. It’s really an area measurement (just like we talked about, yesterday), multiplied by the thickness of concrete in feet, divided by 27. What about for other units that are not cubes? Nothing much changes, you still get a volume measurement in feet and divide by 27. For a column, that would be Pi (3.14) x the radius squared x the length of the column all in feet measurements and then divide by 27. I know, that’s a bit more complicated, but the formula remains the same — volume in feet divided by 27.
If you’re using a redi-mix concrete (bagged concrete mix), most of the bags will tell you how many bags you need for the concrete unit you’re building. For example, if you have a 4″ diameter post hole that’s 3′ feet deep you need X number of bags. However, when ordering concrete from a concrete company, you need to be able to order in cubic yards. As I’ve mentioned before, you also want to be able to double check your subcontractor’s quantities, too.
Tomorrow, we’ll talk about square yards.
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Great post. Even as an architect, I have to stop and remind myself about units and the right numbers to use from time to time. Material measurements and take-offs are so critical, if you're DYI, run your calcs two or three times and/or ways to make sure your covered.
Thanks for the input, David! This one in particular gave me a hard time on many estimates. You get in a hurry and forget that last step and wonder why your concrete number is so high.
As a homeowner, I definitely wouldn't want 20 concrete trucks lined up down my street when I only needed 1.
What about cubic yard measurement for gravel?
You mentioned soil as being done differently than concrete.
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