Setting a fence is mostly digging and waiting — but get the concrete wrong and you either run out with posts braced and the clock ticking, or you haul home a pile of leftover bags. Here is how to size it.
The two numbers that matter
Concrete per hole comes down to hole diameter and depth. A common guideline: make the hole about three times the post's width, and bury about one-third of the post's above-ground height. So a 4×4 post standing 6 ft tall wants roughly a 10–12 inch wide hole, 24–36 inches deep. Plug your hole size and post count into the fence post concrete calculator for the bag count.
Why the hole is a cylinder
A round hole's volume is π × radius² × depth. A 10-inch-wide, 36-inch-deep hole is about 1.6 cubic feet. An 80-lb bag yields about 0.6 cubic feet, so each hole takes roughly 2.5–3 bags. Multiply by your post count and add a bag or two of cushion.
Bagged, or mix a batch?
For a handful of posts, fast-setting bags poured dry into the hole (then wetted) are easiest — no mixing. For a long run with many posts, the per-bag cost adds up; once you pass roughly a cubic yard total, a small mixer or ready-mix can be cheaper.
Set them right
- Add 4–6 inches of gravel at the bottom of each hole for drainage, so the post doesn't sit in water.
- Brace each post plumb in two directions before the concrete sets.
- Slope the top of the concrete away from the post so water sheds off.
- Let it cure before hanging panels — usually 24–48 hours for fast-set.
Do the math once, buy a couple of spare bags, and you'll set the whole run in an afternoon. For pads and footings, see the concrete calculator.