# How do I calculate the concrete needed for a fence post?

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? Commented Jul 29, 2019 at 21:39
• 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. Commented Jul 29, 2019 at 22:50

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. Commented Jul 30, 2019 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. Commented Jul 30, 2019 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. Commented Jul 31, 2019 at 16:34

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". Commented Jul 31, 2019 at 16:36

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.

rule of thumb is post hole is twice the size of post being used 1 bag of concrete 60#or80# will be fine placed at the bottom of post hole allow 6or8 inches below top of ground for dirt or gravel packing i dig more than 6000 post holes a year for 34 years and this works most of the time

• Some formatting to make actual sentences would be great. Feel free to edit that in. Commented Nov 21, 2023 at 19:34