Fence length divided by post spacing, plus one.
Rule of thumb: three times the post width.
About one third of the total post length, below the frost line.
10% covers spillage and holes that come out wider than planned.
An estimate for planning, not an engineering specification. Hole depth must clear your local frost line, and some areas require a permit or inspection for fencing. Check local building codes and call your utility locating service before digging.
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How to use it
Count your posts
Divide the fence length by your post spacing, then add one for the final post.
Size the hole
Three times the post width is the standard rule. A 4x4 post wants a 10 to 12 inch hole.
Check the depth
One third of the total post length, and always below your local frost line.
How the calculation works
The calculation starts with the volume of the hole, treated as a cylinder: pi times the radius squared, times the depth. From that it subtracts the volume of the post itself, because the post occupies space the concrete does not fill. A surprising number of calculators skip this and overestimate by 15 to 20 percent.
That gives the concrete needed per post. Multiply by the number of posts, add a waste allowance, and divide by the yield of your bag size. A 60 lb bag yields about 0.45 cubic feet of mixed concrete; an 80 lb bag yields about 0.60.
The two rules of thumb come from standard fencing practice: hole diameter three times the post width, and depth one third of the post length. Both exist to resist the leverage wind puts on a fence. A tall privacy fence is effectively a sail, and the only thing holding it is the mass of concrete around each post.
What this calculator does not do
It does not know your frost line. In cold climates posts must sit below the depth at which the ground freezes, or they will be pushed upward over successive winters. Your building department publishes this figure.
It assumes a uniform cylindrical hole. A hole dug by hand ends up wider at the top, which uses more concrete than the maths suggests. The waste allowance exists for exactly that.
It does not account for gravel at the base of the hole. If you are adding drainage gravel, reduce the hole depth by that amount.
Questions
How deep should a fence post go?
About one third of the total post length, and below the frost line in cold climates. A 6 ft fence usually means a 9 ft post with 3 ft in the ground.
Do I need to mix the concrete first?
Not necessarily. Many installers pour dry mix into the hole and add water on top. It works for fencing, though a properly mixed batch is stronger.
Can I use gravel instead?
Yes, and it drains better, which extends post life. It holds less firmly though, so it suits lighter fences rather than tall privacy panels.
How long before I attach panels?
Most mixes are safe to work around in 4 to 6 hours, but give it a full 24 hours before putting real load on the posts.
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