Post Hole Concrete Calculator — Bags per Post

The concrete a post hole needs is a subtraction: the volume of the hole minus the volume the post occupies, calculated over the concrete depth only. The gravel base at the bottom is excluded, because it is not concrete. So for a 12 in hole 24 in deep with a 4 in gravel base, the concrete depth is 20 in, and the hole volume is π × (0.5)² × 1.667 = 1.31 cubic feet. The post displaces some of that, and what is left is what you pour.

The actual-size point matters more than people think. A 4×4 post is not 4 in wide — it is 3.5 in, because lumber is sold by its nominal size and planed down to the actual dimension. The cross-section of a real 4×4 is 0.085 sq ft, not 0.111 sq ft. Over a 20 in pour that is roughly a third of a cubic foot of difference per post. A calculator that uses the nominal 4 in figure overstates the post's displacement and tells you to buy less concrete than the hole actually needs, which is why competing calculators come out short.

Here is the worked example in full. A 4×4 (actual 3.5 in) in a 12 in hole 24 in deep with a 4 in gravel base gives a 20 in concrete depth. The hole volume is 1.31 cubic feet. The post displacement is 0.085 × 1.667 = 0.14 cubic feet. The net is 1.17 cubic feet of concrete. An 80 lb bag yields 0.60 cubic feet, so 1.17 ÷ 0.60 = 1.95 bags, which rounds up to 2 bags per post.

Bags round up per post, not across the job. You cannot pour two thirds of a bag into one hole and carry the rest to the next, so each post gets its own whole bags and the total is the per-post count times the number of posts. Two bags per post across 13 posts is 26 bags, not 25.4 rounded at the end. This is stated on the results so you know why the total looks slightly higher than a straight division would suggest.

Two rules of thumb keep the sizing honest. First, the hole diameter should be about three times the post width — a 3.5 in 4×4 wants a 10 to 12 in hole. Second, about a third of the post's total length goes in the ground, so an 8 ft post is buried roughly 32 in. These are starting points, not code: fence height, wind load, gate posts and soil all change the answer. The sizing check under the results compares your values against these rules and says so.

Frost depth is the other number that matters. Water in the soil freezes and expands, and a post set above the frost line lifts and leans over winter. Typical frost depths run from 12 in on the Gulf Coast to 48 in or more in northern states. Your building department gives the figure that applies to your site, and the footing calculator with frost depths by region has the full table. This page checks your depth against the zone you pick and warns if the hole is too shallow.

The gravel base is 4 to 6 in of crushed stone at the bottom of the hole, below the concrete. It lets water drain away from the post rather than pooling against the wood, which is the main cause of rot at the base of a fence post. Without it, water sits in the bottom of the hole and the post rots from the bottom up. Set the gravel base to 0 if you are skipping it, but be aware the post will not last as long.

Fast-setting mix is the one product genuinely designed for posts. It is poured dry into the hole around the post, then water is added on top — no mixing in a tub or wheelbarrow, no hauling wet concrete. It sets hard enough to hold the post in under an hour, which means you can set and brace posts in a single morning. The honest caveat is that it costs more per cubic foot than standard mix, and the concrete bag calculator compares the two side by side. For a handful of posts the time saved is usually worth the extra cost; for a long fence run the price gap adds up.

Post spacing and the end-post arithmetic catch people out. A 100 ft fence at 8 ft spacing needs 13 posts, not 12, because both ends get a post. The formula is floor(100 ÷ 8) + 1 = 13. The fence run tab does this automatically and shows the bay count beside the post count, so you can check it against your panel width before you buy.

If you are pouring into a round cardboard form rather than a plain dug hole, the form tube calculator works out tube counts and cutting. A formed pier is a cleaner pour and keeps the concrete above grade, while a plain dug hole is faster and suits agricultural and residential fence.