Practical guide

Estimate concrete for fence post holes

Use hole geometry and bag yield to make an initial material list.

Short answer: Subtract the square post’s occupied volume from each round hole, multiply by the number of identical holes, apply the chosen allowance, then divide by finished yield per bag and round up.

Adjustable exampleChange the quantities that matter here
Post-hole fill section12 in diameter30 inSection is illustrative; post displacement is subtracted.Around each post1.75 cu ft12 holes · 21.01 cu ft36 bags**Example yield: 0.6 cu ft per bag

Swipe the diagram sideways to see every label. The full calculation is written below it.

A 12-in hole at 30 in depth needs 1.75 cu ft around a 3.5-in square post. 12 holes need 21.01 cu ft, or about 36 example 0.6-cu-ft bags before loss. Use the actual bag yield and site design.

Calculate the hole volume

A round hole is a cylinder: π × radius squared × depth. Convert inches to feet first. The post occupies some of the hole, so subtract an approximate post volume over the same depth.

Use the actual bag yield

Concrete bags are sold by weight, but the coverage for this calculation is their finished cubic-foot yield. Read the yield on the product you intend to buy and enter it in the post-hole estimator. The tool rounds up to whole bags.

Confirm the construction details

Hole dimensions depend on the fence, soil, frost conditions, and local requirements. The tool estimates purchase quantity from dimensions you provide; it does not determine a safe post design. Gate posts and unusual conditions deserve a separate on-site check.

Hypothetical worked example

For a 12-inch-diameter, 30-inch-deep cylindrical hole, gross volume is about 1.96 cubic feet. A 3.5-inch square post occupies about 0.21 cubic feet within that depth. The approximate concrete fill is 1.75 cubic feet per post before any loss.

Which situation matches your project?

All holes use the same chosen dimensionsEnter the hole count, diameter, depth, post width, and finished bag yield.Calculate the bags →Gate posts need different holesCalculate that hole group separately, then combine its bags with the line-post group without counting the same hole twice.Estimate a second group →Only bag weight is knownFind the finished cubic-foot yield on the actual product data sheet or bag before converting volume to bags.See an example yield sheet →

Limit to check: The formula assumes straight cylindrical holes and square posts over the entered depth. Confirm hole design, local requirements, and utility marks before digging.

Try the focused calculation

Post Hole Concrete & Bag EstimatorEstimate concrete around square posts in round holes, apply a waste allowance, and round to the actual bag yield.

Further reading

YardSetup prepared this guide from the stated geometry and the linked references where provided. The adjustable example runs in your browser. See our formulas and limits or report an error.

Put it into practice: Open the Fence Material Planner and enter your own project measurements.