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Fence Post Concrete Bag Calculator

Calculate concrete bags for fence posts from post count, hole size, post displacement, bag yield, and waste.

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Fence post concrete bag calculator

Estimate concrete volume and bag count from post holes, post displacement, bag yield, and waste.

Used for square/rectangular posts only
ft³ per bag; many 80 lb bags yield about 0.6 ft³
Bags needed
17
Rounded up with waste
Concrete per post
0.921 ft³
Hole minus post displacement
Total concrete
10.13 ft³
Includes waste allowance
Estimated cost
$102
Bags × price
Check local code, frost depth, gate loads, soil conditions, and utility locations before digging.

What this estimate does and does not do

This is a material quantity estimate for ordinary fence posts. It does not decide whether your post depth, hole diameter, concrete mix, or footing design is structurally adequate. Fence height, wind exposure, gates, corners, frost depth, drainage, soil type, and local rules all matter. Call before you dig and check local requirements.

Formula used

hole volume = π × (hole diameter / 2)² × hole depth
post displacement = buried post cross-section × hole depth
concrete per post = hole volume - post displacement
total concrete = concrete per post × post count × (1 + waste %)
bags needed = ceil(total concrete / bag yield)

Worked example

For ten 4×4 posts in 10 inch diameter holes, 24 inches deep, using 80 lb bags yielding about 0.6 ft³:

hole volume ≈ 1.09 ft³
post displacement ≈ 0.17 ft³
concrete per post ≈ 0.92 ft³
total with 10% waste ≈ 10.1 ft³
bags = ceil(10.1 / 0.6) = 17 bags

Assumptions

  • Imperial mode uses inches for hole/post dimensions and cubic feet for bag yield.
  • Metric mode uses metres for dimensions and cubic metres for bag yield.
  • Hole walls are assumed cylindrical and straight, which is rarely perfect in real soil.
  • Bag yields vary by brand, mix, water amount, and compaction.

Frequently asked questions

How do you calculate concrete for a fence post hole?

Calculate the round hole volume with π × radius² × depth, subtract the volume displaced by the buried post, multiply by the number of posts, add waste, then divide by bag yield.

What bag yield should I use?

Use the mixed yield printed on your bag. A common 80 lb premix bag is often around 0.6 cubic feet, but brands and bag sizes vary.

Why subtract the post volume?

The buried post occupies part of the hole, so that space does not need concrete. Subtracting post displacement gives a more realistic material estimate.

Is this suitable for deck footings or structural piers?

No. This is for fence post material estimating only. Structural piers, decks, retaining walls, frost-depth footings, and permitted work need code-compliant design.