Setting fence posts is one of those jobs where a small sizing mistake multiplies across 20 or 30 holes. Too little concrete and posts lean after the first wet winter; too much and you have bought a dozen extra bags. Here is a quick way to size each hole and estimate the concrete.
Hole diameter: about 3x the post width
A good rule is a hole roughly three times the width of the post. A 4x4 post (actual 3.5 in) needs a hole about 10 to 12 in across. A 6x6 (5.5 in) needs about 16 in. That leaves a collar of concrete around the post thick enough to resist lateral load from wind and gates.
Hole depth: one third of post height, plus gravel
Bury about one third of the post's total length. For a 6 ft fence on an 8 ft post, that means roughly 2 to 2.5 ft in the ground. In frost-prone areas, go below the local frost line. Then dig an extra 6 in and fill it with gravel. The gravel lets water drain away from the end grain of the post and gives you a firm, level footing.
Estimating concrete volume
Concrete volume is the hole volume minus the post volume, measured above the gravel layer:
- Hole volume = pi x (radius squared) x depth
- Subtract the post cross-section x buried depth
- Add about 10% for irregular hole walls
As an example, a 10 in hole, 24 in of concrete depth, and a 4x4 post comes to roughly 0.95 cubic feet of concrete per hole.
Bag yields
Pre-mixed bags yield approximately:
- 50 lb bag: about 0.375 cubic feet
- 60 lb bag: about 0.45 cubic feet
- 80 lb bag: about 0.60 cubic feet
So that example hole takes about two 60 lb bags or just under two 80 lb bags. Multiply by the number of posts and round up.
Skip the arithmetic
If you are pricing a whole fence line with different post sizes, a post hole concrete calculator will give bags per hole and the total for the job in a few seconds.
Final tips
Crown the top of the concrete so water runs away from the post, check plumb on two faces before it sets, and wait at least 24 to 48 hours before hanging rails or gates.
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