Easy Garden Calc

How many bags of concrete per 4×4 fence post?

Short answer

A 4×4 fence post needs about 2.7 bags of 50 lb concrete (1.01 cubic feet) in a 10 in hole 30 inches deep with 6 inches of gravel in the bottom, including 10% for an uneven hole. That is about 1.7 bags of 80 lb, so buy 3 bags of 50 lb for one post.

How to work it out

  1. Hole: 3.1416 × (10 in ÷ 2)² × 24 in of concrete, minus the post
  2. + 10% = 1.01 cu ft per post ÷ 0.375 cu ft per bag = 2.7 bags per post
  3. One post: buy 3 bags

This is the most common fence post there is, and the one quick charts most often understate: "one bag per post" only holds for a narrow or shallow hole. Two feet of concrete around a 4×4 in a 10 in hole is close to a cubic foot. For a whole fence line, set the number of posts in the calculator — total bags round up once, not per post, if you mix in batches.

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 bags50 lb, incl. fast-setting (≈ 0.375 cu ft)
Bags per post
2.7 bags
Concrete volume
1.01 cu ft (0.0375 cu yd)
Gravel for the hole bottoms
0.27 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 per 4×4 post (10 in hole, 30 in deep) by bag size

Bags per 4×4 post (10 in hole, 30 in deep) by bag size
Bag sizeBags per postBags of concreteConcrete volume
0.3 cu ft3.4 bags4 bags1.01 cu ft
0.375 cu ft2.7 bags3 bags1.01 cu ft
0.45 cu ft2.3 bags3 bags1.01 cu ft
0.6 cu ft1.7 bags2 bags1.01 cu ft

What to know about setting a 4×4 fence post

A 4×4 is really 3½ in square, and a 10 in hole is about three times that — the width Quikrete’s instructions call for. A 6 ft privacy fence usually uses 8 ft posts; by the one-third rule that is about 32 in in the ground, so this 30 in hole is at the shallow end. Go deeper, not wider, if you need more hold.

The bag count per post is where people get caught. At 2.7 bags a post, a 20-post fence line needs about 55 bags if you mix in a wheelbarrow and share batches, but 60 if you pour each hole from whole bags. Buy the higher number if you are dry-pouring fast-setting mix.

Quikrete’s fast-setting data sheet says two 50 lb bags set a 3 in square post in a 9 in hole 30 in deep — but its method stops the concrete 3–4 in below the surface and fills the top with soil, and a 3 in post is smaller than a 4×4. If you fill to grade with a crown, expect closer to three.

Frost matters more than the bag count in cold climates: a post whose bottom sits above the frost line can heave. Ask your building department how deep posts go locally. Pouring a pad or footing instead of posts? The concrete bag calculator handles slabs and round footings, and how many bags of concrete for a 6×6 post covers heavier posts.

Different size? Use the full Fence Post Concrete Calculator.

What you’ll need

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Frequently asked questions

How many 80 lb bags of concrete per 4×4 fence post?

About 1.7 bags of 80 lb per post in a 10 in hole 30 inches deep over 6 in of gravel, so 2 bags for a single post.

How much concrete for a 4×4 post in a 12 inch hole?

About 4.1 bags of 50 lb per post (1.54 cubic feet) at 30 inches deep with 6 in of gravel. A wider hole uses concrete fast; dig deeper instead if you need more hold.

How many bags of concrete for 20 4×4 fence posts?

About 55 bags of 50 lb for 20 posts in 10 in holes 30 inches deep, if you mix in batches. Pouring each hole from whole bags takes 3 a post, 60 in all.

Other common sizes

Sources