How Much Concrete for a Wooden Fence Post?

Wood fence posts come in more sizes than the standard 4×4 that gets most of the attention — 4×6 for rail fencing, 6×6 for gates and tall privacy panels, 8×8 for structural and livestock work, and round posts for a more rustic look. Each one needs a different amount of concrete, and wood specifically comes with one quirk that doesn’t apply to metal or vinyl posts.

Quick answer

A standard 4×4 wood post needs about 3 bags of 80 lb concrete in a 10-inch hole, 30 inches deep. A 6×6 needs about 6 bags, and an 8×8 structural post can need 13 bags or more. Wood posts have one extra wrinkle non-wood posts don’t: pressure-treated lumber is sold wet and shrinks as it dries, which slightly changes the numbers depending on when you measure it.

For your exact wood post size and hole dimensions, the Fence Post Concrete Calculator gives you an instant bag count.

Every common wood post size, one chart

80 lb bags needed by wood post size, standard hole and depth.
Post size	Actual dimensions	Hole size	Depth	80 lb bags
4×4	3.5 × 3.5 in	10 in	30 in	3
4×6	3.5 × 5.5 in	14 in	30 in	5
6×6	5.5 × 5.5 in	16 in	30 in	6
8×8	7.25 × 7.25 in	22 in	36 in	13
Round wood post (5 in)	5 in diameter	12 in	30 in	3

4×4 is standard for most residential privacy and picket fences. 6×6 steps in for gates, corners, and taller or heavier panels. 8×8 is mostly reserved for structural work — pergola posts, deck supports, or livestock enclosures. 4×6 shows up in rail fencing where the extra width faces the rail direction.

The wood-specific quirk: wet lumber vs. dried lumber

Pressure-treated lumber is typically sold quite wet — sometimes close to dripping — as part of the treatment process. A freshly purchased “4×4” often measures closer to 3.75-4 inches per side at the time you buy it, not the fully dried 3.5 inches most references (including the chart above) assume. As the post dries out over its first year in place, it shrinks down toward that standard 3.5-inch dimension. Kiln-dried-after-treatment (KDAT) lumber skips this — it’s already dried to standard size before it reaches the store.

For your concrete calculation, this matters only slightly: a post measuring larger at install time displaces marginally more volume than the standard chart assumes, meaning you’d technically need a hair less concrete on installation day than the numbers above show. In practice, the difference is small enough that it rarely changes your final bag count — but it’s worth knowing about, especially since this same shrinkage is exactly what causes the loose, wobbly feeling some wood posts develop after their first year (covered in full in Why Is My Fence Post Leaning or Loose?).

Why post size actually matters, beyond concrete volume

The concrete question is really a downstream effect of a bigger decision: what size post does this fence actually need? A 4×4 is fine for a standard 6 ft privacy fence with light-to-moderate panels. Step up to 6×6 for taller fences, heavier solid panels, or anywhere wind exposure is significant — the wider post resists twisting and leaning noticeably better, independent of how much concrete surrounds it.

The formula

volume = ((3.14 × radius²) − (post width × post width)) × depth × 1.10

For a round wood post, swap the post’s square cross-section for its own circle formula instead.

What to decide before you calculate

Post sizeMatch it to your fence height and panel weight, not just what’s cheapest.
Wet or kiln-dried lumberWet PT lumber measures slightly larger at install; use 3.5 in as a safe standard estimate either way.
Hole diameterRoughly 3 times the post’s actual width.
DepthSet by fence height and local frost line — see our full depth guide.

Worked examples

Example 1 — 4×6 wood post, rail fencing, 14 in hole, 30 in deep

Hole volume3.14 × 0.583² × 2.5 = 2.67 cu ft
Post volume0.292 × 0.458 × 2.5 = 0.33 cu ft
Net, +10% waste2.57 cu ft

2.57 ÷ 0.60 = 4.28, rounded up to 5 bags.

Example 2 — 8×8 structural post, 22 in hole, 36 in deep

Hole volume3.14 × 0.917² × 3 = 7.92 cu ft
Post volume0.604 × 0.604 × 3 = 1.10 cu ft
Net, +10% waste7.50 cu ft

7.50 ÷ 0.60 = 12.5, rounded up to 13 bags — worth comparing against a ready-mix quote at this scale.

Example 3 — round wood post, 5 in diameter, 12 in hole, 30 in deep

Hole volume3.14 × 0.5² × 2.5 = 1.96 cu ft
Post volume3.14 × 0.208² × 2.5 = 0.34 cu ft
Net, +10% waste1.79 cu ft

1.79 ÷ 0.60 = 2.98, rounded up to 3 bags.

Get your exact bag count.

Enter your wood post’s size, hole dimensions, and depth for an instant, accurate bag count.

Use the free Fence Post Concrete Calculator

Treatment grade affects lifespan more than post size does

A bigger post isn’t automatically a longer-lasting one — treatment grade matters just as much. Ground Contact (GC) rated pressure-treated wood is built specifically for constant soil and concrete contact and typically lasts 15-25 years in that environment. Naturally rot-resistant species like cedar or redwood offer a different trade-off: attractive, moderately durable without chemical treatment, but usually not as long-lived in direct ground contact as properly rated pressure-treated lumber. Whatever size post you choose, matching the treatment grade to the job matters as much as getting the concrete volume right.

Mistakes specific to wood posts

Using a 4×4 where a 6×6 is really called forTall or heavy solid panels put real stress on an undersized post, regardless of how much concrete surrounds it.
Assuming all wood posts measure exactly to their nominal sizeWet pressure-treated lumber runs larger than dried, kiln-dried lumber at the same nominal size.
Skipping Ground Contact rating to save a little moneyStandard above-ground treated lumber isn’t built for the constant contact a buried post experiences.
Choosing an untreated, non-durable species for a buried postSome woods handle ground contact far worse than others — check before committing to a look you like.
Not accounting for round posts’ different volume formulaUsing the square formula on a round post overstates how much space it actually takes up.

Quick reference: which wood post for which job

Project	Recommended post size
Standard 6 ft privacy or picket fence	4×4
Rail fencing	4×6
Gates, corners, 8 ft privacy fences	6×6
Pergolas, deck supports, livestock fencing	8×8
Rustic or farm-style fencing	Round wood post

What changes the number most

Post sizeThe single biggest factor — an 8×8 needs over four times the concrete of a 4×4 at proportional hole sizes.
Post roleGate and corner posts warrant sizing up, regardless of the standard line-post choice.
Wet vs. dried lumberA minor factor for concrete volume, but relevant to the post’s long-term fit in the hole.
Local frost depth and fence heightSet the depth, which scales the volume for every size on this page.

Calculate your wood post’s concrete in five steps

Choose the right post size for your fence height and panel weight.
Confirm Ground Contact treatment rating before buying.
Size the hole to roughly 3 times the post’s actual width.
Set the depth using fence height and local frost line.
Calculate net volume, add waste, and round up to bags.

Frequently asked questions

How much concrete does a standard 4×4 wood post need?

About 3 bags of 80 lb concrete for a 10-inch hole, 30 inches deep.

Does wet pressure-treated lumber need less concrete than dried lumber?

Marginally, yes — since it measures slightly larger at install time. The difference is usually too small to change your final rounded-up bag count.

Should I use 4×4 or 6×6 wood posts for my fence?

4×4 works well for standard 6 ft privacy or picket fences. Move up to 6×6 for gates, corners, taller fences, or heavier solid panels.

How much concrete does an 8×8 post need?

Around 13 bags of 80 lb concrete for a 22-inch hole, 36 inches deep — a size mostly used for structural applications rather than standard fencing.

Does round wood post volume calculate differently from square posts?

Yes — use the circle formula for the post’s own volume instead of a squared width, since it displaces a different amount than a square post of similar nominal size.

How long do wood fence posts typically last?

Properly Ground Contact-rated pressure-treated posts typically last 15-25 years. Naturally rot-resistant species like cedar can last a while without chemical treatment, though usually not as long in direct ground contact.

What’s the difference between kiln-dried and standard pressure-treated wood?

Kiln-dried-after-treatment (KDAT) lumber is dried to standard dimensions before it reaches the store. Standard pressure-treated lumber is often sold quite wet and shrinks toward standard size over its first year in place.

Is a 4×6 post the same as a 4×4 for concrete purposes?

No — a 4×6 has one side wider (5.5 in vs 3.5 in), which changes both its own displaced volume and the recommended hole size compared to a standard 4×4.

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