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I'm trying to determine how much concrete I'll need to set fence posts for a 6ft tall privacy fence. From what I've read these are the parameters I'll be using for each post hole.

Post: 4in x 4in x 8ft

Hole Diameter: 12in

Hole depth: 30in (6in of gravel + 24in for post)

When I calculate the concrete needed, do I use the 30in depth or just the 24in for the part of the post in the ground? At the moment I plan on using 3-50lb bags of sakrete fast drying per hole. Is this too much, too little, or just enough?

  • "Is this too much, too little, or just enough?" Let me ask Goldilocks. Are you sure 30" is code for the frost line in your area? – Alaska Man Jul 29 at 21:39
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    Rough calculation: Hole cross section = (6" x 6" x 3.14) - (3.5" x 3.5") = 100.79" round to 100" x 24" = 2,400 in^3. /12/12/12 = 1.38 ft^3. Various sources for "how much per bag" but using one that has it a 0.375 ft^3 per 50 lb. bag would result in 3.68 bags per hole. So seems to be in the right ballpark. – manassehkatz Jul 29 at 22:50
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I never have used 12 inch diameter holes for fence posts, more like 6 to 8 inches diameter and 24 inches deep for a 6 ft fence. One bag of concrete per post.

  • I think that 12" is a rule on the concrete bag for "3 times the post diameter" or some such, but I've never seen anyone use a hole that big. Maybe for a deck or foundation footing that equation makes sense, but it's overkill for a fence. – JPhi1618 Jul 30 at 15:00
  • Yeah, an inch or two of concrete around a post fills the hole, but doesn't add much stability in soft soil and it's likely to crack away in short order. – isherwood Jul 30 at 15:49
  • Accepting this answer as it coincides with much of what everyone else has been saying and what appears to be the most common method of setting 6ft posts for a fence. – jwaldorf2 Jul 31 at 16:34
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Well, I always use fast setting fence post concrete and you put 2" of gravel on the bottom of the hole, then fill the hole 1/3 of the way up with water, dump one bag of fence post concrete and level the post, then hold it that way for 2 minutes and let dry overnight. The reason fence post concrete is different is that it is packaged to be one bag per post based on a 12-18" hole diameter and a 4x4 post, no pre-mixing, fast setting.

  • If I could accept 2 answers I would. I plan on using your method of placing gravel and using the fast setting concrete like you've described. The reason I didn't accept is because I plan on using a hole diameter of 8". – jwaldorf2 Jul 31 at 16:36
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I am kind of partial to J. Raefield and J Stewarts methods. Very simple and straight forward. I too think 12" is too much although some deck standards for posts are 12". I plan on using 8" on the three posts I have to dig in my deck rebuild. Hoping the code officer lets me go with 8" vs 12"!? Also I plan on using 8" Sonotubes and filling it to the top of a 48" tube height set in the ground to a depth of between 36" and 42". Which when I did the calculations it became 2 bags per tube or about 3.1 yards of concrete for the three piers I need. It is suggested you have it delivered when its over 2 yards which I feel is less work but worth the extra costs.

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