How Many Bags of Concrete Per Fence Post? (4x4 Chart)

How many bags of concrete per fence post? Plan on 1 to 2 80lb bags per post for a typical 4x4 set in a 10in wide, 24in deep hole, which works out to about 2 bags. Step up to 3 to 4 bags for a deeper 12in x 30in to 36in hole, the kind you dig for gate posts and corner posts. Everything below assumes a 4x4 post, which is actually 3.5in wide, and the concrete volume is the hole minus the post itself.

How many bags for a 4x4 post?

Here is the whole answer in one list. Find your hole size and read across:

• 8in diameter x 24in deep: 0.53 cu ft of concrete. That is 1 x 80lb bag, or 2 x 60lb bags.
• 10in diameter x 24in deep: 0.92 cu ft. That is 2 x 80lb bags, or 3 x 60lb bags.
• 10in diameter x 30in deep: 1.15 cu ft. That is 2 x 80lb bags, or 3 x 60lb bags.
• 12in diameter x 30in deep: 1.75 cu ft. That is 3 x 80lb bags, or 4 x 60lb bags.
• 12in diameter x 36in deep: 2.10 cu ft. That is 4 x 80lb bags, or 5 x 60lb bags.

The bag math behind those numbers is simple. An 80lb bag yields 0.60 cu ft of mixed concrete. A 60lb bag yields 0.45 cu ft. So a 10in x 24in hole needing 0.92 cu ft takes two 80lb bags with a little left over. Bags always round up. You cannot pour 1.53 bags. If your hole size is not on the list, run it through the Concrete Calculator instead of guessing.

Notice that most line items land on 2 bags. That is why the shorthand answer for a standard residential fence post is two 80lb bags. It holds for the vast majority of 4ft to 6ft privacy and picket fences.

How deep should a fence post hole be?

Two rules of thumb set your hole, and they are the reason the bag count moves around so much.

Diameter: about 3x the post width. A 4x4 post is 3.5in actual, so 3x lands you near 10in. That is the standard. An 8in hole is the minimum you should consider, and 12in is the beefy option for posts under real load.

Depth: about 1/3 of the post height above ground. A 6ft fence means 6ft of post standing up, so 1/3 of that is 24in in the ground. An 8ft post above grade would want roughly 32in. This is why tall fences eat more concrete than short ones.

Then the override: go below the frost line in cold climates. If the ground freezes and thaws where you live, water in the soil expands and shoves shallow posts upward. That is frost heave, and it will tilt a fence over a couple of winters no matter how much concrete you used. Check your local frost depth and dig past it. In a lot of northern areas that pushes you from 24in to 36in or more, which is exactly how a 2 bag post becomes a 4 bag post.

Dig with a clamshell digger or an auger, and keep the sides of the hole straight. Do not let it belly out. A wide bottom and narrow top is actually the shape you want if you can manage it, since a bell shape resists being pulled up.

Do gate and corner posts need more concrete?

Yes. Gate posts and corner posts carry loads that line posts never see, so they get bigger and deeper holes.

A line post in the middle of a run is mostly holding up fence panels. A corner post is fighting the pull of two runs at 90 degrees. A gate post is holding a swinging weight that yanks on it every single day, hundreds of times a year. That leverage is brutal.

So for gate and corner posts, jump to a 12in diameter hole at 30in to 36in deep. That is 3 to 4 x 80lb bags each. On a lot of fences that means you buy 2 bags for most posts and 4 bags for the handful that matter. Budget for it up front. Running out of concrete halfway through setting a gate post is a miserable afternoon.

End posts, where a run terminates against nothing, get the same treatment as corners.

How many bags of concrete for a whole fence?

Multiply per post bags by post count, then add the extra for gates and corners.

Count your posts first. Posts are usually spaced 6ft to 8ft apart, so measure your total fence run, divide by your spacing, and add 1 for the last post. Then do the math:

• 20 line posts x 2 x 80lb bags = 40 bags.
• Add 4 corner posts x 4 bags = 16 bags.
• Add 2 gate posts x 4 bags = 8 bags.
• Total: 64 x 80lb bags.

At that scale it is worth knowing the conversion. 1 cubic yard equals 27 cu ft, which is 45 x 80lb bags. So the 64 bag fence above is roughly 1.4 cubic yards. That is the point where some people call a ready mix truck instead of hauling pallets of bags, though most fence jobs stay in bag territory because you are pouring into dozens of small separate holes, not one slab. For the full totals on your actual layout, the Concrete Calculator handles the per post volume and multiplies it out for you. If you want the deeper background on bag yields, see how many cubic feet in a bag of concrete. And if you have other pours on the same project, the pillar guide on how much concrete do I need covers slabs, footings and steps.

Buy 10 percent extra. Holes are never as neat as the chart, and returning unopened bags is easier than a second trip.

Can you pour dry concrete in a post hole?

Yes, with fast setting mix, and it is the standard way most fence crews work.

The dry set method goes like this. Set the post, plumb it, brace it. Pour the dry mix straight into the hole around the post, filling to about 3in below grade or up to grade depending on your finish. Then add water on top per the bag directions, usually about a gallon per 50lb bag. The water soaks down, the mix sets, and you are done in 20 to 40 minutes. No mixing tub, no wheelbarrow, no mixing 64 bags by hand.

The catch: it only works with fast setting mix formulated for it. Standard concrete mix poured dry does not reliably hydrate all the way through and you can end up with dry pockets. Read the bag. If it does not say it can be set dry, mix it first in a tub or barrow to a stiff oatmeal consistency and shovel it in.

Mixing first gives you more control and a stronger, more consistent set. Dry setting gives you speed across a lot of holes. For a 60 post fence line, speed usually wins.

Should you put gravel at the bottom of a post hole?

Put 4in to 6in of gravel in the bottom of the hole before the post goes in. It gives water somewhere to go instead of sitting against the end grain of the post.

Wood posts rot from the bottom up. A concrete collar with a sealed base turns the hole into a cup that holds water right where the post is most vulnerable. Gravel breaks that. Tamp it down, set the post on it, then pour your concrete around the post above the gravel layer. The post sits on drainage, the concrete grips the sides.

Note the gravel does not change your bag count much either way. The chart numbers above are net concrete volume for the hole minus the post. If you fill 5in of a 30in hole with gravel you are pouring 25in of concrete, so you might shave a bag on the deepest sizes. Round up anyway.

Should you crown the concrete at the top?

Crown it. Slope the top of the concrete collar away from the post so water runs off instead of pooling against the wood.

Take a trowel or the edge of your shovel and shape the last inch or two into a slight dome, highest at the post, tapering down to grade a couple inches out. It takes 15 seconds per post and it is the single cheapest thing you can do to add years to the fence.

The alternative some people prefer is to stop the concrete 3in to 4in below grade and backfill with soil on top. That works too, and it looks tidier because you see dirt and not a concrete donut. Either is fine. What is not fine is a flat concrete top that is level with the post and holds a puddle after every rain.

Bottom line

For a standard 4x4 fence post in a 10in x 24in hole, you need 2 x 80lb bags of concrete, or 3 x 60lb bags. Drop to 1 x 80lb bag if you are in an 8in x 24in hole. Go up to 3 or 4 x 80lb bags for the 12in x 30in to 36in holes you dig for gate posts, corner posts, and anywhere the ground freezes deep. Multiply by your post count, add 10 percent, and buy it in one trip. Run your exact hole size and post count through the Concrete Calculator to get the total bag count before you head to the store.