I need to dig a 12 inch diameter hole for some 4x4 fence posts that I'm digging but I only have access to an 8-inch power auger. My experience drilling holes in wood tells me that if I try to drill a hole next to an existing hole my bit will slip and go all over the place in an unsafe way. My question here is if I only have access to an 8-inch auger whats my safest bet at getting a 12 inch wide hole?

What I was thinking was to go slow and dig three holes where the center of each hole is 4 inches apart from each other until I get to the depth that I need. Something like a venn diagram with 3 circles. Working my way down into the ground maybe 6 inches at a time. This sounds pretty tedious since I have many holes to dig so Anyone have any better ideas?

My question stems from this video: https://youtu.be/aQ24ykIBVOM?t=2m54s

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    An 8" hole is fine for a 4" post, just get it accurate.
    – Jack
    Apr 26, 2017 at 14:30
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    @Jack From what I've read/viewed you need a 12 inch diameter hole if you're securing the posts with concrete. Apr 26, 2017 at 14:37
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    wouldn't most of the soil you're trying to remove just fall off the side of the auger back into the hole you've already dug?
    – brhans
    Apr 26, 2017 at 15:08
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    Dig an 8" hole and then shave 2" off all around by hand? After the auger, the sides should be fairly loose.
    – bib
    Apr 26, 2017 at 16:59
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    So what if it cracks; it will still support the pole. When I replaced original 4x4 cedar posts after termites and rot consumed them, I put in 4 x 4 pressure treated and tamped them in with dirt with a heavy steel tamping bar, i.e., no concrete. Did fine. The old cedar posts were set in concrete and the tamping bar had a chisel on the other end. This broke the concrete free of the old rotted posts. Apr 27, 2017 at 2:18

4 Answers 4


I am a fan of 4+" gap from post to soil, plain 21AA ( or any good packing ) gravel. drop 6" on bottom add some slightly downward angled screws to prevent twisting and lifting and fill/tamp/fill ... Stays dry and away from soil (rot) easy, cheap, longer lasting than just about anything that can trap moisture next to the wood. (Like concrete) and use real high treated density wood UC4A or better ... NOT the home depot gardening posts :) So much time, effort, and money can be spent that does not out perform ....


You have the general idea, but whether this is successful depends on several things, including soil type, the presence of rocks, and the weight and rigidity of the boring equipment. In any event, expect to get a good upper-body workout.

I'd bore a hole with a 2" offset, perpendicular to the fence run, then shift the equipment 4" and bore again, creating an oblong bore. You'll still have to clear it by hand as some material will fall back into the original bore cavity. You could try a third bore, but I doubt the equipment will be rigid enough to accomplish much.

You'll end up with a hole that's tapered to the bottom, but you should be able to get enough concrete around the top to secure the posts.


As @bib noted in the comments I can just dig the 8" hole with the auger and then shave 2" off the sides by hand since the soil is already loosened up from the auger. I did the shaving using a 6' Post Hole digger tamper bar

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    This solution was pre-supposed when you asked the question. It isn't really an answer.
    – isherwood
    Apr 26, 2017 at 19:54

It is just fence posts, it it was supporting a deck or something more substantial, yes 12" or larger. I don't even add water to my concrete mix when setting fence posts. The moisture in the ground will set the concrete.

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