Fence Post Hole Concrete: Bag Counts and Common Mistakes
Reviewed by the OnlineFree.app team · Updated
Key points
- A 9-inch by 30-inch hole with a 4x4 post needs about 0.89 ft³ of concrete, roughly two and a half 50 lb bags.
- Ten such holes need about 9.8 ft³, or 27 bags of 50 lb mix after a 10% waste margin.
- Bag size changes the number of bags you carry, not the total weight of concrete you buy.
- Subtract any gravel base from the hole depth before entering it, or you will overbuy by about 20%.
- The calculator warns below 32 inches of depth, but only you can confirm your local frost depth.
How many bags of concrete per fence post hole?
For a typical 9-inch-diameter hole dug 30 inches deep with a 4x4 post in it, budget about 0.89 cubic feet of mixed concrete per hole — roughly two and a half 50 lb bags. Ten posts at those dimensions come to about 9.8 ft³, or 27 bags of 50 lb mix once a 10% waste margin is added.
The Fence Post Hole Concrete Calculator does that arithmetic while you adjust the numbers. You enter the post count (1–500), the hole diameter (4–18 in), the hole depth (12–60 in), the post size, the bag size on the shelf, and a waste margin between 0 and 30%. The result card updates as you type and always rounds the bag count up.
The output is split three ways: a headline bag count with total volume in cubic feet and cubic yards, a per-post breakdown showing hole volume minus the submerged part of the post, and a comparison table of bag counts at 40, 50, 60 and 80 lb. That last table tends to be the one people actually use, because it answers what is on the shelf rather than what you assumed.
Does bag size change how many bags you buy?
Yes, but only the count. The tool uses the standard yields of 0.30 ft³ for a 40 lb bag, 0.375 ft³ for 50 lb, 0.45 ft³ for 60 lb and 0.60 ft³ for 80 lb. The same 9.8 ft³ pour works out to 33, 27, 22 or 17 bags respectively, and the total weight you carry barely moves — roughly 1,320 to 1,360 lb either way.
So the decision is about handling, not volume. An 80 lb bag is usually the cheapest per cubic foot and cuts the number of bags you open by more than a third, but each one is 80 lb to lift off a pallet and into a mixer. Mixing alone in a wheelbarrow, 50 lb bags are the practical middle ground.
Packaged concrete in the US is made to standards published by ASTM International, and every bag prints its own yield. If the bag in your hand says something different from the figure the tool assumed, trust the bag — a different printed yield changes the count, and mixing two brands mid-pour is an easy way to come up short.
Common mistakes that leave you short
The most frequent error is entering the full hole depth after dropping gravel in the bottom. Six inches of drainage gravel in a 30-inch hole means the concrete only fills the top 24 inches — 0.88 ft³ instead of 1.10 ft³. Enter the concrete depth, not the hole depth, or you will buy about 20% more mix than you can pour.
The second is depth itself. The calculator flags anything under 32 inches, a common minimum in cold climates, but it has no idea where you live. Frost depth is set locally, and the background on frost heaving explains why shallow holes push posts up after a hard winter. Confirm the number with your building department before you dig.
Then there is the waste margin. The 10% default covers spillage, a torn bag and the slight bell an auger leaves at the bottom of a hole. Hand-dug holes in rocky ground and sloped sites justify 15–20%. The tool always rounds up, but buying one spare bag is still cheaper than a second trip to the store.
Finally, water. Strength comes from the water-to-cement ratio, and a mix that pours like soup is a weak mix. Use the water quantity printed on the bag rather than eyeballing it, and remember that the calculator counts dry mix — it has nothing to say about how much water you add.
How the hole volume math actually works
Every number in the tool comes from one formula: π × (diameter ÷ 2)² × depth, converted from inches to feet at the end. A 9-inch hole 30 inches deep is π × 4.5² × 30 = 1,909 in³, which is 1,909 ÷ 1,728 = 1.10 ft³.
Then the post comes back out. A nominal 4x4 is actually 3.5 inches square once it is dressed, so 3.5 × 3.5 × 30 = 368 in³, about 0.21 ft³ of displaced concrete. Net per hole: roughly 0.89 ft³. Multiply by ten posts, add 10%, and you get 9.8 ft³ — 0.36 cubic yards — which divides into 26.2 bags of 50 lb mix, rounded up to 27.
You can run that by hand for one or two posts as a sanity check. The calculator earns its keep at 40 posts, or at 200, or when you are standing in the aisle deciding between two bag sizes and do not want to do long division on your phone's calculator app.
What the calculator does not cover
It counts concrete for post holes and nothing else. It does not include a slab, a footing for a gate post, rebar or the gravel base, and it does not know your local frost depth or whether your jurisdiction requires an inspection before backfilling. Treat the bag count as a purchasing estimate and verify anything structural with your building department.
It also will not tell you how to mix. Water quantity, cure time and whether you need a fast-setting product all come from the bag's own instructions, and those differ by product. If you are also pouring a gate-post footing or a small pad, estimate those separately and add the totals — the tool models one hole profile at a time.
The Fence Post Hole Concrete Calculator is one of the free browser tools at OnlineFree.app — no download, no account, and the number fields open a numeric keypad on a phone. It was built for a 30-second decision at the shelf, so if the answer looks wrong, check your hole dimensions first. That is usually where the mistake is.
Frequently asked questions
How many bags of concrete do I need for 10 fence posts?
For ten 9-inch-diameter holes dug 30 inches deep with 4x4 posts, expect about 9.8 cubic feet of concrete — roughly 27 bags of 50 lb mix once a 10% waste margin is included. Enter your own post count, hole diameter and depth in the Fence Post Hole Concrete Calculator and it rounds the bag count up for you.
Does the post itself change how much concrete I need?
Yes. The Fence Post Hole Concrete Calculator subtracts the submerged volume of the post from the hole volume, which is why the per-post breakdown shows two figures. A 4x4 post sitting 30 inches deep displaces roughly 0.21 ft³, about a fifth of a 9-inch by 30-inch hole. Ignoring that usually causes a small overbuy rather than a shortage.
Is it cheaper to buy 80 lb bags of concrete?
Bag size changes the count, not the total weight. The calculator compares the same 9.8 ft³ pour as 33 x 40 lb, 27 x 50 lb, 22 x 60 lb and 17 x 80 lb bags, all within about 40 lb of each other in total weight. Compare shelf price per cubic foot, not per bag, then pick the heaviest bag you can handle comfortably.
How deep should fence post holes be?
A common rule is one-third of the post's above-ground height, but the controlling number is your local frost depth, which the calculator cannot know — it only warns when the depth you enter falls below 32 inches. Holes that stop above the frost line can heave. Check with your building department before digging.
Can I use the calculator on my phone at the hardware store?
Yes. The Fence Post Hole Concrete Calculator runs in a mobile browser with the result card pinned above the inputs as a sticky summary bar, and number fields open a numeric keypad. There is no download, account or sign-up, so you can change hole diameter or bag size at the shelf and watch the bag count update.