Geometry-Based
Calculates measured cylindrical or rectangular volume instead of relying on a fixed “bags per post” guess.
Post Displacement
Round and square modes can subtract the space occupied by an embedded post.
Bag Estimates
Converts concrete volume into approximate 40 lb, 60 lb and 80 lb bag quantities.
Optional Cost
Enter your supplier’s price per cubic yard for a simple ready-mix material estimate.
What Is a Concrete Post Calculator?
A Concrete Post Calculator estimates how much concrete is required to fill the space in a post hole, around an embedded post, or in a separate concrete pier that supports a post above grade. It is useful when planning fence posts, gate posts, deck posts, pergola supports, sign posts, mailbox posts and many other projects that use concrete for below-grade support.
The important idea is that the concrete quantity depends on geometry. A wider hole, deeper hole or larger number of posts can change the volume substantially. If the post itself is embedded into the concrete, the post occupies some of the hole volume, so this calculator can subtract that displacement instead of treating the entire hole as solid concrete.
For a broader material estimate covering slabs, patios, walls, beams and other shapes, use the site’s Concrete Calculator. On this page, the calculation is focused specifically on concrete for post holes and post footings.
How to Use the Concrete Post Calculator
Good results start with good measurements. Measure the hole and post dimensions that will actually be filled with concrete rather than using a rough visual estimate. Then choose the calculator mode that best matches the shape of the project.
Choose Hole Type
Select round, square, fence run or full concrete pier.
Measure The Hole
Enter finished diameter or width, length and concrete depth.
Enter Post Size
If the post is embedded, enter its cross-sectional size for displacement.
Add Post Count
Enter the number of identical holes or let fence mode estimate it.
Review The Result
Compare cubic volume, bag counts, allowance and optional ready-mix cost.
Measure the actual concrete-filled depth
Enter only the portion of the excavation that will contain concrete. If the bottom contains a separate drainage layer or another material that is not concrete, do not count that thickness as concrete. If the concrete extends above grade in a formed collar, pier or tube, include that added concrete volume where applicable.
Use the correct post cross-section
For a round post in a round hole, use the round-hole calculator. For a square or rectangular timber post in a square excavation, use the square-hole calculator. If a post is mounted to a metal bracket on top of a cured concrete pier and does not occupy the pier itself, use the post footing / pier calculator so the full cylinder is counted.
Concrete Post Hole Formula
The calculator converts all dimensions to consistent units, calculates the geometric volume and then applies the selected ordering allowance.
All dimensions must be converted to compatible units before the formula is applied.
If no post is embedded in the concrete, use zero for post dimensions or use full footing mode.
Why subtract the post volume?
An embedded post displaces concrete. For a small post in a large hole, the difference may be modest, but across many posts it can become noticeable. Subtracting post displacement makes the geometric estimate more specific to the actual space that concrete fills.
How Much Concrete Do I Need Per Fence Post?
There is no single correct number of concrete bags per fence post because the required volume changes with hole diameter, hole depth and post size. Two fence posts can use very different quantities even if both are described as “standard posts.” The most reliable planning method is to calculate the actual annular volume around the post.
For example, increasing a round hole from 10 inches to 12 inches in diameter increases the cross-sectional area significantly because area grows with the square of the radius. Increasing depth also increases volume directly. That is why a fence post concrete calculator is more useful than a generic one-bag or two-bag rule.
Concrete Bag Calculator for Posts
After calculating cubic feet, this page converts the volume into approximate bag counts. Actual yield varies by product, mix design, water content and manufacturer, so always verify the yield printed on the bag you plan to buy.
| Bag Size | Approx. Yield Used by This Tool | Approx. Bags per 1 ft³ | Best Use |
|---|---|---|---|
| 40 lb | 0.30 ft³ per bag | 3.34 bags | Smaller batches and easier individual lifting |
| 60 lb | 0.45 ft³ per bag | 2.23 bags | Common medium-size bagged-concrete projects |
| 80 lb | 0.60 ft³ per bag | 1.67 bags | Larger hand-mixed batches where handling weight is suitable |
Bag yields above are planning assumptions used only for conversion. Product labels are the controlling source for the actual yield of the mix you purchase.
Why bag count is rounded up
You cannot normally purchase a fraction of a bag, so the calculator rounds bag quantities upward to the next whole bag. This is separate from the selected ordering allowance. For example, a 10% allowance first increases the calculated concrete volume, and then the bag conversion rounds the resulting requirement up.
Fence Run Post Calculator: Length, Spacing and Concrete
The fence-run mode combines two planning steps. First, it estimates how many posts are needed across one continuous run using the entered run length and target maximum spacing. It uses the ceiling of run length divided by target spacing, then adds one post so both ends of the run are represented.
Second, it calculates the concrete for those post holes using the round-hole formula. The result also displays the approximate actual spacing after the run has been divided into equal sections.
Gates, corners and changes in direction
A real fence layout may need extra posts at gates, corners, ends, transitions, changes in grade or structural bracing points. A single straight-run spacing calculation does not automatically understand those layout conditions. Calculate separate runs where necessary and add project-specific posts manually.