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We're going to build up a 4' fence around our property sometime in here in the spring or summer with 4x4 posts.

I've seen that you should bury 1/3 to 1/2 of the above ground post height; so this would make our hole 1.5-2' deep with gravel below. I've also seen that the diameter of the hole for a 4x4 post should be 12".. which would make the radius 6". After going through the math for the hole ( pi x r^2 x h ) it comes out that the hole will have a volume of 1.57 cu ft, but the post itself will take up 0.17 cu ft, so the volume to be filled with concrete will become 1.4 cu ft per post. Right?

The question I have now is that a 50lb bag of fast-setting Quickrete apparently fills 0.375 cu ft after setting. This would mean that we'd need almost 4 bags per post (3.75) - this is also the number that comes up from Quickrete's online calculator. I'm not against this exactly, except that I've seen people saying they've used half a bag per post or a bag a post. How accurate is this and how much should we need? At that rate, we're almost looking at 100+ bags. The 80lb bags fill about 0.6 cu ft and are about a dollar cheaper, is there anything against going with it?

What type of concrete do I need to use, and how much?

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A 4' fence is a small, lightweight fence. You're not going to need 12" holes, and you shouldn't need to bury the posts 1/2 of the height... 18" deep should be plenty. I'm interested in what some people with more fence experience say. More than 2 bags per post seems like a huge amount. – JPhi1618 Mar 22 at 16:06
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yea, 12" is a giant hole. That's way overkill. In fact, a lot of people say that any concrete is overkill, and you should simply be using compactable crushed rock. – DA01 Mar 22 at 16:39
    
@JPhi1618 A third of the height would put it at 16" deep, but I'm planning on buying 12' long posts (because 10 was too short) and cutting them in half (so to get to 4ft above grade would be 2ft below). I could just make additional cuts to clear off that extra half a foot. – TFK Mar 22 at 17:27
    
Smells like overkill. You'd want some decent footings around the end posts, and any corner posts, and any posts that are likely to suffer from car intrusion. And gateposts as well. But for "stringers" definitely less than that. Does your local council/county/authority have any requirements? – Criggie Mar 22 at 19:48
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@TFK, ends only get support on one side, while corners get loaded from two different directions. – Mark Mar 22 at 23:55
up vote 6 down vote accepted

Wow! I normally use post hole diggers (6" diameter?) 3 feet deep and ½ a bag of redi-mix per post. I did a free standing section of 6 foot privacy fencing this last summer and we just had a storm of >50mph winds and it is still standing and solid.

So, some of it depends on the quality of your soil but 4 bags per post would be major overbuilding in my opinion.

If you had access to an 8" auger and a full bag per post would still be overbuilding but if you are OCD then that would probably satisfy your worries.

Good luck!

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I'm not worried nor OCD. I'm just cheap and don't know much about concrete. I was hoping that everyone would prove me, the online salesmen, and Quickcrete's 'calculator' wrong. – TFK Mar 22 at 17:35
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Understood, I am pretty cheap myself. I believe the purpose of the concrete is to re-solidify the ground after disturbing the compacted soil. So, I figure make the hole as close to the size of the post as possible. Then the quick set concrete will act like 20 year compacted soil. – ArchonOSX Mar 22 at 22:12
    
Wonderful, thanks for the answer. Also, if it matters, it's going to be a wire fence - so wind shouldn't be any form of issue – TFK Mar 24 at 14:41

Just to "sort of" sidestep the question. You don't need any concrete or gravel whatsoever & all of that nonsense is a complete lie & does nothing. Just dig 6" holes, drop in the posts, double plumb them & backfill & tamp with the dirt you dug or drilled out. I have 3 fences of 6' high that I did 15-years ago & none are failing, falling nor rotting & are unchanged from the day they went in.

Now, concrete used for the purpose of having replaceable posts is different & in that case you do want a 12" hole & likely 2 bags. This I have no problem with, but most no-one does it. Monkey-see-monkey-do says to "just pop-out that 200-pounds" & do the whole absurdity all over again from scratch.

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this can be true, but does depend on the type of soil one has in their area. There's a big difference between sand and hard clay, for example. – DA01 Mar 22 at 16:40
    
Yep, quite true. But, with or without concrete you must go deeper for the lighter soils. I have quite a light clay content & I'm just 3' down. Hurricane's & Mega Noreaster's haven't affected anything. – Iggy Mar 22 at 16:47
    
Certainly make sure you ask around... The builders in our area must have felt like they could get away with this method, but all of the fences in the development are falling down. I'd never even think of doing that anywhere around here (North Texas). – JPhi1618 Mar 22 at 17:44
    
You won't know until you try. Not a speck of concrete was used for any fence for decades & centuries before. It really only became "trendy" 20-years ago. If anything you're providing a bigger footprint for freeze-thaw to get a grip on. Soil drains, concrete doesn't. – Iggy Mar 22 at 17:49
    
@Iggy, sounds like all you're getting is wind loads. I recently came across a stretch of supposedly well-built fence that fell over after a moose leaned on it. – Mark Mar 22 at 23:59

I am an oklahoma farmer, have built miles of durable barb wire fence. The rule of thumb out here is a fence post hole should bottom out below the frost line, or else the posts will heave out. At my latitude of 36 deg, that is 2 to 3 feet deep. The hole should be wide enough that a tamper-bar will fit between the post and the edge of the hole. That is a long heavy rod of steel with a 2 inch disk on the end. You tamp the hole as you fill it, because the barb wire puts so much tension on the post. Corners and gate entries are fortified with H-braces, an extremely rigid structure made of 9 gauge wire, two posts 8 feet apart, and an 8 foot pipe (the center of the H). Concrete would probably help act as a "dead-man"

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here in ontario, the code requirement for any fence adjoining a property line in any incorporated municipality is an 8" dia hole with either a 2" steel or 4" pressure treated spruce post set to a minimum depth of 48" below grade. the post must be set into either compacted sand or mpa20 or higher concrete, and the post must be embedded full depth minus the diameter of the excavated hole. this is for any fence up to a maximum of 6', which in most towns is the maximum allowed by municipal bylaws. anything over that height requires an engineer. if its solely on your land (more than 6" within the surveyed lot line) then there are absolutely no requirements or stipulations.

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