How to calculate concrete for a post hole
A round post hole is treated as a cylinder. Convert the hole diameter to a radius, square the radius, multiply by pi and then multiply by the concrete fill depth.
If a post occupies part of the hole, its volume can be subtracted before multiplying by the number of holes.
Why subtract the post volume?
Concrete does not occupy the same space as the portion of the post inside the concrete-filled section of the hole. Subtracting that volume gives a more precise material estimate than treating the entire cylindrical hole as concrete.
If you prefer a conservative estimate or do not know the post dimensions yet, select Ignore post displacement.
Post hole concrete example
Suppose you have four circular holes that are 12 inches in diameter and will contain 36 inches of concrete. Each hole contains a 4 × 4 inch square post.
Each full hole has a volume of about 2.356 cubic feet. The portion occupied by the square post is about 0.333 cubic feet, leaving approximately 2.023 cubic feet of concrete per hole.
Four holes therefore require about 8.091 cubic feet before waste. Adding 10% gives about 8.901 cubic feet. Using an estimated yield of 0.60 cubic feet per 80 lb bag gives an estimate of 15 bags.
Concrete fill depth vs. hole depth
The calculator asks for concrete fill depth rather than assuming the entire drilled hole is filled with concrete. If gravel occupies part of the bottom of the hole, exclude that gravel depth from the concrete depth entered.
Estimating concrete bags
After calculating concrete volume and adding the selected waste allowance, the calculator divides the adjusted volume by the estimated yield of the selected premix bag.
The final number of bags is rounded upward because partial bags are generally purchased as whole bags.
Premixed concrete yield can vary by manufacturer and product. The built-in bag yields are planning estimates. Enter the yield printed on the product bag or technical data sheet when available.
Waste allowance
A waste allowance can account for minor over-excavation, irregular hole walls, spillage and measurement differences. The calculator applies the selected percentage after subtracting the post volume.
Concrete volume units
Results are shown in cubic feet, cubic yards, liters and cubic meters.
For rectangular slabs and pads, use the Concrete Calculator. For a rectangular pour where you primarily need bag count, use the Concrete Bag Calculator.
Hole size and structural requirements
This calculator estimates material quantity only. It does not determine the required hole diameter, embedment depth, footing design or post size for a structure.
Required post-hole dimensions can depend on soil, frost depth, wind loads, structure height, local building rules and manufacturer instructions. Determine the required dimensions first, then use this calculator to estimate concrete quantity.
Frequently asked questions
How much concrete fits in a 12-inch post hole?
A 12-inch-diameter cylindrical hole that is 36 inches deep has a full volume of about 2.356 cubic feet before subtracting the post.
Does the calculator subtract the post?
Yes. You can choose a square or rectangular post, a round post, or ignore post displacement.
Can I account for gravel at the bottom of the hole?
Yes. Enter only the depth that will actually be filled with concrete.
Why does the calculator round the number of bags upward?
The calculated material requirement may include a fraction of a bag, while concrete bags are normally purchased as whole units.
Can I use the calculator for multiple posts?
Yes. Enter the number of equal-size holes. For holes with different dimensions, calculate each group separately.
Using this calculator effectively
The Post Hole Concrete Calculator estimates concrete volume for circular post holes while optionally subtracting the space occupied by square, rectangular or round posts. It also converts the result to cubic yards, liters and cubic meters and estimates premix bags.
Common uses
- Estimating concrete for fence posts, deck posts, sign posts and similar circular post holes.
- Accounting for the volume displaced by a post instead of treating the entire hole as concrete.
- Estimating premix bag quantity, waste allowance and optional material cost before a post-setting project.
Tips for a better estimate
- Measure the actual hole diameter and enter only the depth that will contain concrete if gravel or another base material occupies part of the hole.
- Use the actual external post dimensions when subtracting post displacement.
- Check the concrete yield printed on the selected premix product and use the custom-yield field when it differs from the built-in planning estimate.
Related calculations
Concrete Footing Calculator · Concrete Bag Calculator · Concrete Calculator