Post Hole Concrete & Bag Estimator
Estimate concrete around square posts in round holes, apply a waste allowance, and round to the actual bag yield.
Set your project details
Enter the hole and post dimensions you have chosen, and the cubic-foot yield printed on the concrete bag. The drawing shows the annular space being filled.
See how the quantities change
The labeled view updates when you edit the inputs. It prints with the result.
Swipe the diagram sideways to see every label. The calculated quantities are listed below.
Your estimate appears here
Enter your measurements and calculate to see required and purchase amounts.
The calculation
Worked example (U.S. customary)
For 12 round holes, 12 in wide and 30 in deep, around 3.5 in square posts, the geometric concrete volume is about 21.01 cu ft before allowance.
Use the hole and post as two shapes
The round hole has a cylindrical volume. A square post displaces some of that space, so the calculator subtracts the square post cross-section over the same depth. The result is concrete around the post, before mixing loss.
Read finished yield on the bag
Bag weight is not finished volume. Enter the cubic-foot yield stated for the actual product, apply any mixing or spill allowance, and round the calculated order up to whole bags. The result also shows the geometric total so you can see what the allowance added.
Confirm the footing design
Post depth and hole diameter depend on the fence design and local conditions. Some installations include gravel at the bottom, bell-shaped footings, or other details that change concrete volume. Measure those separately and confirm local requirements before purchase.
What to check before buying
Hole size and depth must come from the actual fence design, ground and local requirements. Some installations use gravel drainage or a different footing shape; this calculator does not determine structural adequacy.
Common questions
Why subtract the post volume?
Concrete fills the hole around the post, not the space already occupied by the post.
Does this calculate a safe post foundation?
No. It estimates quantity from dimensions that have already been chosen.
Read the related Estimate concrete for fence post holes, or use the full fences planner for a complete material list.