Post Hole Concrete Calculator
The post displaces more concrete than you think.
Post Hole Concrete Calculator
Per hole = (cylinder volume - post volume) / 1728; bags = ceiling(total / bag yield).
Calculated instantly
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What This Calculator Does
The Post Hole Concrete Calculator estimates the concrete needed for fence, deck, and mailbox posts by subtracting the buried post from the cylindrical hole. It reports volume per hole, total volume, bags at your yield, and the weight of the cured concrete.
How It Works
The hole is treated as a cylinder: pi times radius squared times depth. The buried post is treated as a square prism: width squared times the same depth. Subtracting the post gives the net concrete volume in cubic inches, which is divided by 1,728 to convert to cubic feet. Multiplying by the number of posts gives the total, which is divided by the bag yield and rounded up.
Example
A 10 in diameter hole 36 in deep has 2,827 cubic inches. A 4 in by 4 in post displaces 576 cubic inches, leaving 2,251 cubic inches, or 1.30 cubic feet per hole. Eight posts need 10.42 cubic feet, which is 18 bags at 0.6 cubic feet each, and about 1,511 lb of cured concrete.
Formula
pi x radius squared x depthpost width squared x depth(hole - post) / 1728 cubic feetceiling(total cubic feet / bag yield)Reading the Results
Concrete per hole
Net volume after the post is subtracted. It assumes the post runs the full depth of the hole and the hole is a clean cylinder.
Bags required
Whole bags at your yield, rounded up. Bag yield varies by mix and by how much water is added, so a little spare is sensible.
Total weight
Cured concrete at about 145 lb per cubic foot. Use it to plan transport and to check a vehicle or trailer load.
Sources and References
ACI 332 - Residential Code for Concrete Construction
Residential concrete practice
Guidance on footings and concrete placement for residential work.
International Residential Code - Footings and Foundations
Frost depth and footing requirements
Local frost depth and footing rules govern how deep a post support must be.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
How many bags of concrete do I need per fence post?
It depends on hole diameter and depth. A 10 in hole 36 in deep with a 4 in post needs about 1.3 cubic feet, which is roughly 2.2 bags of 80 lb mix. Shallow or narrow holes need less; a wide hole in frozen ground needs more.
Should the post sit on gravel at the bottom of the hole?
A few inches of compacted gravel improves drainage and gives the post a stable base, but it reduces the concrete depth. Add the gravel thickness to the hole depth so the concrete still reaches the required embedment, and check local frost depth requirements.
How deep should a post hole be?
Below the local frost line for structural posts, and deep enough for the lateral load the post carries. Fence posts often use one third of their above-ground height in the ground, but a deck or pergola needs an engineered footing. Local code is the starting point.
Does the calculator subtract the post correctly for a round post?
It subtracts a square section of the width you enter, which is conservative for a round post. For a 6 in round post, enter about 5.3 in as the equivalent square, or expect a slight overestimate. Overestimating concrete is safer than running short mid-pour.
Can I use fast-setting concrete without mixing?
Some fast-setting mixes are designed to be poured dry into the hole and wetted, but the volume needed is the same. Follow the bag instructions exactly, because the water ratio affects strength. Do not use a fast-set mix where you need to adjust the post after it starts to cure.