How Much Concrete Per Post Hole?

Post holes look small until you price them. Four deck piers at 12 inches by 4 feet is about a quarter yard of concrete — but try finding a truck that will deliver a quarter yard. You'll be buying bags, so the question is exactly how many, and the answer depends on three things: hole diameter, depth, and how honestly you subtract the post itself.

The numbers by hole size

Here's what the cylinder math gives you for the common sizes, with 10% added for the holes being rougher than they look (they always are):

Two things worth noticing. First, depth does most of the work: the 12-inch hole jumps from 2 bags to 6 bags mostly because it went deeper, not wider. Second, the table above is the full hole. A Sonotube form inside the hole changes nothing — the tube is the hole. But a wood post standing in the hole takes up real space, which brings us to the fence math.

Fence posts: subtract the post, not the hole

A "4x4" post is actually 3.5 by 3.5 inches. In a 10-inch hole that's 2 feet deep, the wood displaces a bit over a quarter cubic foot, so the concrete per post is closer to half a cubic foot than the 0.9 the raw hole holds. Over 14 posts on a 100-foot fence run, that difference is about four bags — call it $30 you'd otherwise pour into the ground doing nothing.

Our fence calculator does that subtraction automatically. A 100-foot privacy fence, posts every 8 feet in 12-inch holes at 24 inches, comes to 14 posts and 53 bags of 50-lb concrete. Tighten the spacing to 6 feet with 10-inch holes and you get 18 posts but only 45 bags — more posts, smaller holes, less concrete. People intuitively assume more posts means more bags. It's usually the other way, because hole diameter drives volume faster than post count drives it.

Fast-set and the dry-pour question

Walk down the fence post aisle and you'll see fast-setting concrete, sold specifically for posts. The printed method is legitimate: set the post, dump the dry mix in the hole, pour a gallon or so of water over each bag's worth, and it cures as it sits. It's not a hack — Quikrete publishes it. It sets in 20 to 40 minutes, which matters when you're bracing posts plumb one at a time.

You'll also find forum posts swearing you can skip the water entirely and let groundwater do the work. Don't. Dry mix without enough water cures weak and patchy, and the failure shows up two winters later when the post rocks. The one-gallon-per-bag pour is the floor, not a suggestion. Also know that fast-set trades working time for convenience and costs a bit more per bag — for deck piers, where you might want to adjust a bracket, regular mix with a normal cure gives you more control.

Depth: the part inspectors actually check

Fence posts are forgiving; deck footings are not. Deck piers need to sit below your local frost line so frozen ground can't grab and heave them — anywhere from 12 inches in the coastal South to 48 inches or deeper up north. Your building department has the exact number and it's worth one phone call before you dig anything. Bell-bottoming the hole (wider at the base) adds a lot of concrete but is sometimes required for heavy loads; if your plan set calls for it, our Sonotube footing calculator gives you the per-pier bag count so you can price the upgrade honestly.

One last habit from people who set posts for a living: mix slightly wet for posts. A soupy mix flows into the voids around the post and self-levels, where a stiff mix leaves voids you'll never see. For slabs, wet mix is weaker and nobody recommends it. Posts are different — you want the concrete to find every gap.

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