Post Hole Concrete Calculator

What this post hole concrete calculator is estimating

A post hole is not a simple full cylinder of concrete because the post itself displaces some of that volume. A useful fence post concrete calculator therefore has to estimate the volume of the hole, subtract the post cross-section, and then multiply the net fill volume by the number of posts you plan to set.

That makes this tool useful for fencing, small deck supports, gate posts, and other projects where you want a practical bag count before you buy concrete. It gives you the total fill volume, shows the amount per hole, and converts the result into 50 lb, 60 lb, or 80 lb bag equivalents so the estimate is easier to use in the store yard.

Core post-hole concrete formulas

The estimate treats the hole as a round cylinder and the post as a square section. It calculates the circular hole area, subtracts the post area, multiplies by depth, then converts the result into cubic feet. The final bag count is the total volume divided by the yield of the selected bag size and rounded up to whole bags.

How to use the bag count

Use the bag count as a purchase baseline, then keep a little margin for real-world variation. Hole walls are rarely perfect cylinders, and posts do not always stay perfectly centered, so a practical site estimate should not rely on an exact decimal volume alone.

For example, a 10 inch diameter hole that is 24 inches deep around a 4 inch square post needs about 0.87 cubic feet of concrete, which is roughly 2 standard 80 lb bags for one post. Multiply that across a fence line and the value becomes a quick way to budget materials before you start digging.

What this result does not cover

This calculator does not decide the correct hole depth, footing diameter, frost-depth requirement, or structural design for your post. It also does not account for bell-shaped footings, sloped ground, or local code rules about post embedment and concrete placement.

Use it as a concrete-estimating tool only, then confirm footing design, hole geometry, and code requirements before you dig or pour.