Easy Garden Calc

How many bags of concrete for a 6×6 post?

Short answer

A 6×6 post needs about 2.6 bags of 80 lb concrete (1.58 cubic feet) in a 12 in hole 36 inches deep over 6 inches of gravel, with 10% extra. That is about 4.2 bags of 50 lb; buy 3 bags of 80 lb for one post.

How to work it out

  1. Hole: 3.1416 × (12 in ÷ 2)² × 30 in of concrete, minus the post
  2. + 10% = 1.58 cu ft per post ÷ 0.6 cu ft per bag = 2.6 bags per post
  3. One post: buy 3 bags

Six-by-six posts carry pergolas, arbors, gates and deck corners, so the holes go wider and deeper than for a fence. A 12 in hole three feet deep is a typical starting point, but a pergola or deck post often has a specified footing size, and in cold climates it has to reach below the frost line. Check the plans and your local code before you dig.

Adjust for your project

in
Often about 3× the post width
in
Often ⅓ of the post’s total length, below the frost line where required
in
For drainage · 0 if none
Yield is printed on the bag
More options: Extra · Price
%
Hand-dug holes are rarely neat cylinders
$
Blank to skip
Bags of concrete3 bags80 lb (≈ 0.60 cu ft)
Bags per post
2.6 bags
Concrete volume
1.58 cu ft (0.0586 cu yd)
Gravel for the hole bottoms
0.39 cu ft
  • Call 811 (in the US) or your local utility locate service a few days before you dig.
Check your codeCode limits vary by jurisdiction — confirm with your building department before you cut or build. Fence height, setbacks and how deep posts go (often below the local frost line) are set by local rules, not a national code.

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Post levelA post level straps to the post and reads both directions at once, so you can plumb it while the concrete is still workable.
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Bags of 80 lb concrete per 6×6 post (12 in hole) by hole depth

Bags of 80 lb concrete per 6×6 post (12 in hole) by hole depth
Hole depthBags per postBags of concreteConcrete volume
30 in2.1 bags3 bags1.27 cu ft
36 in2.6 bags3 bags1.58 cu ft
42 in3.2 bags4 bags1.9 cu ft
48 in3.7 bags4 bags2.22 cu ft

What to know about setting a 6×6 post

A 6×6 is 5½ in square, so the three-times rule would call for a hole closer to 16 in across. At 12 in, the concrete collar around the post is only about 3 in thick on each side — fine for an arbor or a light gate, thin for a heavy gate or a tall pergola. The table below shows what deeper holes cost in bags.

At this size, 80 lb bags make sense: three of them per hole instead of five 50 lb bags. They are heavy, though, so mix each one in a tub or wheelbarrow beside the hole rather than trying to lift a full bag over the edge.

Posts that hold up a roof, a deck or a porch are not fence posts. Those usually need a footing sized by the plans or the building code — often a wider pad or a round form at the bottom, with the post on a metal base above the concrete rather than buried. For those, work out the concrete with the concrete bag calculator and check the footing size with your building department.

For a regular fence line, see how many bags of concrete per 4×4 fence post.

Different size? Use the full Fence Post Concrete Calculator.

What you’ll need

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Frequently asked questions

How many bags of concrete for a 6×6 post 4 feet deep?

About 3.7 bags of 80 lb (2.22 cubic feet) in a 12 in hole 48 inches deep over 6 in of gravel, with 10% extra.

How much concrete for a 6×6 post in a 16 inch hole?

About 5.4 bags of 80 lb per post (3.26 cubic feet) at 36 inches deep over 6 in of gravel. That is the three-times-the-post-width hole, and it roughly doubles the concrete.

How many bags of concrete for four 6×6 pergola posts?

About 11 bags of 80 lb for four 12 in holes 36 inches deep, if you mix in batches. Check the pergola plans: many specify a bigger footing.

Other common sizes

Sources