Fence Post Concrete Calculator

How many bags of concrete does one fence post need?

A post hole is a cylinder with a post standing in the middle of it. So the concrete you need is the volume of the cylinder minus the volume the post takes up. In formula form, with everything in inches:

Concrete (cu ft) = [ π × (hole diameter ÷ 2)² × depth − post width² × depth ] ÷ 1728

The 1728 at the end converts cubic inches to cubic feet, which is the unit printed on every bag. A standard 4x4 in a 10.5" hole dug 24" deep works out to 1.03 cubic feet of concrete — three 60 lb bags, or two 80 lb bags, per post.

The table below is generated by this calculator using the recommended hole diameter (3× the post width) and a depth equal to one third of the fence height, with a 24" floor. It covers the three configurations that account for most residential fences in the US.

Sizing the hole: the 3× rule and the one-third rule

Two rules of thumb decide the shape of the hole, and both come straight from the manufacturers.

Diameter: dig the hole three times the width of the post. A 4x4 (3.5" actual) gets a 10.5" hole; a 6x6 (5.5" actual) gets a 16.5" hole. That gives you a collar of concrete roughly 3.5" thick on a 4x4 — thick enough to resist the leverage a fence panel applies in a crosswind, and wide enough that you can still plumb the post after it is in the hole.

Depth: bury between one third and one half of the above-ground height, and never less than 24". Quikrete states the rule slightly differently — one third of the total post length plus 6" for gravel — which lands at the deeper end of that range. A 6 ft fence built from 9 ft posts, following Quikrete, gets 36" in the ground. Deeper is stiffer; a gate post or an end post carries far more load than a line post, so many builders go one full foot deeper on those two.

One more thing about diameter: widening the hole costs you concrete fast, because volume grows with the square of the radius. Going from a 10" hole to a 12" hole is only two inches on a tape measure, but it is 44% more concrete in every single hole. Dig to the size you need, not to whatever the auger happened to make.

Frost line: the number that overrides every rule of thumb

Water expands about 9% when it freezes. When the soil around a shallow footing freezes, it swells and pushes the footing upward — frost heave. Over a few winters that motion racks a fence out of alignment, pops gates out of square, and cracks the concrete collar. IRC Section R403.1.4 requires footings to extend below the frost line for exactly this reason, and most building departments apply the same logic to fence and deck posts.

The practical rule: the bottom of the concrete must sit below your local frost depth. If the one-third rule says 24" and your frost line is 42", you dig 42" — or deeper. Frost depth is set by your jurisdiction, not by a national table, so call the building department or check the local amendment before you rent the auger. The bands below are typical published values and are meant only to show the scale of the variation.

Step by step: setting a post with fast-setting concrete

Fast-setting concrete is what changed post setting from a two-day job into an afternoon. You do not mix it in a wheelbarrow — you pour the dry mix into the hole and add water on top. Quikrete Fast-Setting Concrete sets hard in 20 to 40 minutes and holds weight in about 4 hours.

• 1. Dig the hole 3× the post width, and 6" deeper than your target depth to leave room for gravel.
• 2. Pour about 6" of all-purpose gravel in the bottom and tamp it flat with a 2x4 or the post itself. This drains water away from the end grain — the single biggest cause of posts rotting off at the ground line.
• 3. Set the post, then brace it with two 2x4s on adjacent faces. Plumb it with a post level in both directions before any concrete goes in.
• 4. Pour the dry mix in around the post, filling to 3" to 4" below grade. Keep it even on all four sides so nothing pushes the post out of plumb.
• 5. Add water — roughly a gallon per 50 lb bag — and let it soak into the mix. Poke it with a stick to release trapped air. Do not stir it into a slurry.
• 6. Wait 4 hours before pulling the braces, and give it 24 hours before hanging panels, rails or a gate.
• 7. Cap the last 3"–4" with soil mounded and sloped away from the post, or crown the concrete itself so rain runs off instead of pooling against the wood.

Wet-set, dry-set, and mixed: which method to use

There are three ways to get concrete into a post hole, and they are not interchangeable.

Wet-set (the manufacturer method) means pouring dry fast-setting mix into the hole and adding water on top, as described above. It is fast, it needs no mixing tools, and Quikrete publishes it as the intended use of the product. This is the right choice for the overwhelming majority of fence jobs.

Mixed concrete means combining standard concrete mix (#1101) with water in a wheelbarrow or mixer, then shoveling it in. It is slower and messier, but it gives you full control over consistency and it is what you want for a heavy gate post, a mailbox post that has to look finished, or anything you plan to trowel.

Dry-set means dumping the dry mix in and walking away, letting ground moisture cure it over weeks. It is popular in DIY forums and it is the one to avoid. Curing is uneven and unpredictable, strength is a fraction of the rated psi, and in dry soil parts of the bag may never hydrate at all. If you are not adding water, you are not making concrete.

A note on our numbers: this calculator fills the entire hole. Quikrete's own rule of thumb — two 50 lb bags for a 4" post in a 10" hole 2 ft deep — assumes you stop 3"–4" below grade and cap with soil, so it lands about one bag lower per hole. We round up on purpose. A spare bag costs a few dollars; a second trip to the store in the middle of a pour costs your afternoon.

Common mistakes that cost you a fence

Almost every fence that fails early fails for one of these reasons, and every one of them is free to avoid.

• Using the nominal size in the math. A 4x4 is 3.5" wide. Calculating with 4" underestimates the concrete volume and, more importantly, gives you a hole that is too small if you sized it at 3× the wrong number.
• Skipping the gravel base. Concrete holds water against the bottom of the post. Six inches of gravel gives that water somewhere to go.
• Filling flush with grade or above it, without a slope. A flat concrete collar becomes a bowl that keeps the post permanently wet at the exact spot where rot starts.
• Ignoring the frost line because the one-third rule "said 24 inches". The deeper of the two numbers always wins.
• Setting posts at final height before the concrete cures. Set them long, let them cure, then cut all the tops to a string line — much faster than fighting each post to height while it is still wet.
• Bracing on one side only. Two braces on adjacent faces, or the post walks out of plumb as the mix sets.
• Not accounting for gate and corner posts. They take the load of the whole run. Go one size up, one foot deeper, or both.

Frequently asked questions

Sources

• QUIKRETE — Setting Posts in Concrete
• QUIKRETE — Concrete Mix No. 1101 data sheet (bag yields)
• QUIKRETE — Fast-Setting Concrete Mix No. 1004-50 data sheet
• SAKRETE — High-Strength Concrete Mix technical data sheet

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