Concrete Cylinders for Dummies

concrete cylinder

Our staffers often find humor in the application of literature degrees to mathematics. And why wouldn't they? So many freelancers with BA degrees in the humanities are woefully deficient in math skills: just look at some of their work some time. With the exception of one error, however, Lit major Emily Taylor was able to pull off a simple geometry exercise in the Hunker.com article, "How to Calculate Concrete Needed for a Cylinder." Where's Emily came up short was in her logic...

Taylor trotted out the standard formula for the volume of a cylinder, v = πr²h (or as Emily put it, "pi X r^2 X h"). She pounded out 287 words in 8 steps for calculating the volume of a 12-inch cylinder 40 inches tall, including converting the volume to cubic feet because as Emily explained (not once but twice),

"Concrete is most often measured in cubic feet..."

...which is funny, because we thought it was measured in yards. We guess cubic feet is for people who buy concrete by the sack. But that wasn't our point. Apparently Emily performed all her calculations on a calculator that carried π out to more decimal places than her claim of 3.142. That's probably why her math bombed when she told her readers how to convert the volume of her example cylinder to ft³,

"For example, a volume of 4523.48 cubic inches multiplied [sic] by 1728 is 2.6183 cubic feet."

There are two things wrong with that statement, Emily: first, to convert from cubic inches to cubic feet, you divide by 1728, not multiply; and second, 4523.48 divided by 1728 is  2.6178. Oops...

Of course, Emily's biggest problem is that she failed logic: it's well within the realm of probability that the OQ wanted to make a cylinder with an open center. In that case, the amount of concrete you'd need will be

V = πr₁²h₁ - πr₂²h₁ + πr₁²h₂

where r₁ is the outside diameter of the cylinder, r₂ is the inside diameter of the cylinder, h₁ is the height of the cylinder and h₂ is the thickness of the bottom. If the cylinder is open on both ends, the third term "disappears" because  h₂ = 0.
Instead of burning up hundreds of words with excessive verbiage, our Dumbass of the Day could have covered both eventualities in about 100 words... but then eHow wouldn't have paid her because her post was too short. Feh.

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