How Much Concrete for a 6x6 Post? (2026 Guide)
How much concrete for a 6×6 post? A 12-in-diameter hole 36 in deep needs about 2.4 cu ft — roughly 4 bags of 80-lb mix; most fence and deck posts take 3–4 bags.
Volume of a round post hole
A post hole is a cylinder: cu ft = π × r² × depth, with the radius and depth both in feet.
For a Ø12 in hole the radius is 6 in = 0.5 ft, so the cross-section is π × 0.5² ≈ 0.785 sq ft — about 0.785 cu ft per foot of depth.
How wide should the hole be? At least twice the post's width across, so the concrete grips the post and a tamper fits around it. A nominal 6×6 is about 5.5 in wide, so twice is 11 in — Ø10–12 in is the common range, and the table below uses a 12-in hole.
Concrete per depth for a 6×6 post
Numbers assume a Ø12 in hole (cross-section ≈ 0.785 sq ft) and 80-lb bags (each places ≈ 0.60 cu ft):
| Hole depth | Concrete needed (cu ft) | 80-lb bags (theoretical) | Bags to buy |
|---|---|---|---|
| 24 in (2 ft) | 1.57 | 2.6 | 3 |
| 30 in (2.5 ft) | 1.96 | 3.3 | 3–4 |
| 36 in (3 ft) | 2.36 | 3.9 | 4 |
| 42 in (3.5 ft) | 2.75 | 4.6 | 5 |
The last column rounds up to a whole bag. At 30 in of depth, three 80-lb bags (1.80 cu ft) fall about 2 in short of a full hole — hence 3–4.
Worked example: a 6×6 set 36 in deep
Let's walk through the most common setup. The hole's cross-section is 0.785 sq ft; 36 in ÷ 12 = 3 ft of depth, so 0.785 × 3 = 2.36 cu ft of concrete.
Divide by what one 80-lb bag places: 2.36 ÷ 0.60 ≈ 3.9, so buy 4 bags. Running 60-lb bags? Each places about 0.45 cu ft, so 2.36 ÷ 0.45 ≈ 5.2 — plan on 6 bags. For other hole sizes, use the concrete calculator.
Tips for setting a 6×6 post in concrete
- Deck posts carry real weight — plan for the frost line too. A fence post mostly resists wind; a deck pier supports the deck and everyone on it. In cold climates, concrete under a structural post should reach below the local frost depth so heaving can't lift it — check your area's depth and code before pouring.
- Drop a few inches of gravel in first. About 4–6 in of crushed gravel under the post drains water from the end grain, gives the concrete a clean base, and keeps the wood out of standing water.
- Fast-setting vs. standard mix. Fast-setting concrete hardens in 20–40 minutes, handy when you need the post braced quickly; standard mix is cheaper and allows more time to plumb the post. Either works for a single 6×6 — respect the cure time before loading it.
- Follow the engineering drawings on commercial work. For an engineered deck, carport or building, pour to the plan — not a rule of thumb. Loads, hole sizes and concrete strength are specified for a reason; ask the engineer when unsure.
Pouring a whole row of posts? The same depth-by-bag math covers concrete for fence posts and single sets like a mailbox post.
FAQ
How many bags of concrete do I need for one 6×6 post?
About 4 bags of 80-lb mix for the standard pour — a 12-in-diameter hole 36 in deep. The hole holds about 2.36 cu ft and each 80-lb bag places roughly 0.60 cu ft, so 2.36 ÷ 0.60 ≈ 3.9 → 4 bags. Shallower holes take fewer: 24 in deep needs 3 bags and 30 in deep needs 3–4.
How wide should the post hole be for a 6×6?
About 12 inches — at least twice the post's width. A nominal 6×6 is about 5.5 in wide, so 10–12 in diameter is the common range. Narrower and the concrete can't grip and tamp around the post; wider and you just pour extra bags.
Do deck piers need a bigger hole than fence posts?
It depends on the load — and a deck pier usually needs more. Fence posts mostly resist wind; a deck pier carries the deck, furniture and people, so it often gets a larger or belled footing plus concrete below the frost line. When a post supports structure, err on a larger hole or have an engineer size it.
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