Easy Masonry Calc

How much concrete for a 6×6 post?

Short answer

A 6×6 post in a 12 inch hole 3 feet deep takes about 1.9 cubic feet of concrete with 10% extra: 4 bags of 80 lb mix (3.2 bags exactly) or 6 of 50 lb fast-setting.

How to work it out

  1. Hole 3.1416 × (12 in ÷ 2)² minus the post, × 36 in of concrete
  2. + 10% = 1.9 cu ft per post ÷ 0.6 cu ft per 80 lb bag = 3.2 bags per post
  3. One post: buy 4 bags

A 6×6 measures 5½ × 5½ in, so it takes up a big share of a 12 in hole — about 27% of it. That makes the bag count lower than people expect, and it is also why the hole size matters so much for this post. Crown the top of the concrete away from the post so water can’t pool against the wood.

Adjust for your project

in
Quikrete’s rule of thumb: about 3 times the post width (10½ in for a 4×4, so a 10 or 12 in auger)
in
Quikrete: about ⅓ of the post’s total length. Local frost depth and fence rules come first
50 lb fast-setting ≈ 0.375 cu ft · 80 lb standard ≈ 0.60 cu ft
More options: Gravel · Top · Extra …
in
Many builders use 4–6 in of gravel for drainage; Quikrete’s own method uses none. 0 if none
in
Quikrete’s fast-setting method stops 3–4 in below the surface and finishes with soil or sod
%
Hand-dug holes are rarely neat cylinders — 10–15%
ft
$
Optional — an example figure; use your store’s price
Bags to buy4 bags80 lb bag
Bags per post
3.2 bags
If you share batches between holes
4 bags · 80 lb bag
Concrete for all posts
1.9 cu ft (0.0703 cu yd)
Quikrete’s rule of thumb for this post
Hole about 16-1/2 in wide and 36 in deep for a 9 ft post
  • Call 811 (in the US) or your local utility locate service a few days before you dig.
Check local rulesPost depth (often below the local frost line), fence height and setbacks are set by local rules — check with your building department or HOA before you dig.

For this job · affiliate link
Post levelA strap-on post level reads plumb on two faces at once, so you can set the post and walk away while fast-setting mix hardens.
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6×6 post 3 ft deep, by hole diameter

6×6 post 3 ft deep, by hole diameter
Hole diameterBags per postBags to buyConcrete for all posts
10 in1.8 bags2 bags1.11 cu ft
12 in3.2 bags4 bags1.9 cu ft
14 in4.7 bags5 bags2.83 cu ft
16 in6.5 bags7 bags3.91 cu ft
18 in8.6 bags9 bags5.14 cu ft

12 inch hole or 16 inch?

Quikrete’s rule of thumb is a hole about three times the post width, which for a 6×6 would be 16–18 in. A 12 in auger is far more common for gate and pergola posts, and the difference is large: at 3 ft deep, a 16 in hole takes about 3.9 cu ft with the extra (7 bags of 80 lb) against 1.9 cu ft in a 12 in hole. Which one you need depends on the load and soil — a deck post on a footing is a code question, so follow your plans.

Gate posts, pergolas and decks

A gate post carries a twisting load every time the gate swings, so many builders go deeper and wider on the hinge side. A pergola post often sits on a pier or bracket instead of being buried, which keeps the end grain out of the soil. For a deck, posts usually stand on a footing or pier — see how many bags of concrete for a 12 inch Sonotube — rather than being set in concrete, and your local code decides which is allowed.

Brace the post on two faces before pouring; a 6×6 is heavy enough to lean as the concrete goes in. The lighter fence post is on how much concrete for a 4×4 post.

Different size? Use the full Fence Post Concrete Calculator.

What you’ll need

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Frequently asked questions

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

About 3.91 cubic feet with 10% extra: 7 bags of 80 lb at 3 feet deep.

How much concrete for a 6×6 post 4 feet deep?

2.53 cubic feet in a 12 inch hole: 5 bags of 80 lb.

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

16 bags of 80 lb mixing each hole on its own, or 13 bags if you share batches.

Other common sizes

Sources