Post Holes for Dummies, Part 1

Post hole

Sometimes our research here at the Antisocial Network uncovers mistakes so obvious that even the iconic fifth-grader (the one a lot of people aren't smarter than) could tumble to it. Other times, the errors are more subtle. We're not all that sure which category today's entry in the dumbass stakes should fall into - we'll let you know. So without further ado, here's eHow.com's Larry Simmons, caught holding forth (without benefit of first-hand knowledge) on the topic of "How to Calculate Concrete for Post Holes."¹

Here's our suggestion: if we were assigned this particular problem, we'd probably start with the dimensions of a standard post hole, calculate the cubic footage of concrete needed for each hole, and multiply by the number of holes we'd need to fill... wait... we seem to be forgetting something, though...

Well, Larry wrote his assignment up the same way: according to Larry, a standard post hole

"...has a radius of four inches and a depth of 30 inches [for] a volume of 4^2x30x3.14 = 1507.2 cubic inches."

He's using the formula for the volume of a cylinder, V = h * π * r² (where V is volume, h is the height of the cylinder, r is the radius, and π is 3.14). Simmons got that part of the task right, all the way to the first decimal place.  Next, Larry converted his measurements to cubic feet: "Divide the volume in cubic inches by 1,728... 1507.2 divided by 1728=.87 cubic foot of volume per hole." Again, the work is mathematically correct. And then Simmons calculates the total needed by multiplying the per-hole volume by the number of holes, then instructs us to calculate the number of bags necessary:

"For example, a 100 hole project using the example hole would require 87 cubic feet of concrete."

At 2/3 ft³ per 60-pound bag for ready-mix, that works out to 131 bags (a bit under four tons). You should know that with posts on 8-foot spacing that represents 800 or so linear feet of fence, which is a pretty big back yard; or even more, since it's not generally necessary to cement in every post. Every second (or third, depending on the wind load on the fence) post is usually sufficient, as long as you reinforce the posts at the corners and on both sides of any gates.

Whoa...wait... now we remember what was missing, and it's something Simmons didn't bother to include in his calculations, either:

The post takes up a portion of the space in every hole, so Larry's 0.87 ft³ volume per hole is too big! 

If you use 4 x 4 posts inserted 30" into each hole, then the wood takes up 367.5 in³ of each hole (3½" x 3½" x 30"), reducing the volume of concrete needed to 1139.7 in³ or 0.66 ft³ per hole, an overcalculation of more than 30%! The best Larry could do was offer his readers a tip (at the very bottom of the page below all of eHow's advertising links) that there might be "some overage." No duh, Mr. Simmons!

Larry started all this out by informing his readers that "...Too much concrete on hand results in wasted spending." It appears to us that the wasted spending was what Demand Media paid our Dumbass of the Day for this content. 

¹ The original has been deleted by Leaf Group, but can still be accessed using the Wayback machine at archive.org. Its URL was    ehow.com/how_6022745_calculate-concrete-post-holes.html

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