Describe the hole
Choose round or rectangular geometry and enter the concrete fill dimensions rather than assuming the entire excavation is concrete.
Concrete & masonry
Post Hole Concrete Calculator
Estimate concrete for round or rectangular post holes while accounting for the space occupied by round, square, or rectangular posts. Add multiple hole groups, an optional planning allowance, bag yield, ready-mix ordering increments, and material cost.
Quantity estimator only.Enter hole depth, diameter, post size, embedment, and any other dimensions from your plans, product instructions, contractor, engineer, or applicable requirements. This tool does not choose a safe hole diameter, embedment depth, footing size, frost depth, reinforcement, or structural design.
Calculation method
How the post hole concrete calculator works
The calculator finds the gross volume of each hole, subtracts only the volume actually occupied by an embedded post, multiplies by the number of identical holes, sums all groups, applies your chosen allowance, and only then performs bag or supplier-order rounding.
Account for the post
Choose no post, round, square, or rectangular. The calculator checks that the entered post geometry can physically fit the selected hole.
Apply your allowance
Allowance is applied to the net project concrete after post displacement, not separately to every hole.
Convert to a purchase
Bag requirements round once at whole-project level. Ready-mix rounds only when you enter a real supplier increment.
Post hole concrete formulas
Round holeGross volume = π × (diameter ÷ 2)² × concrete fill depth
Rectangular holeGross volume = length × width × concrete fill depth
Round post displacementPost volume = π × (post diameter ÷ 2)² × post length inside concrete
Square or rectangular post displacementPost volume = post length × post width × post length inside concrete
Net concrete for one holeNet concrete = gross hole volume − post displacement
Repeated identical holesGroup total = net concrete per hole × quantity
Planning volumeRequired = project base × (1 + allowance ÷ 100)
Bagged concreteBags = round up(total required volume ÷ manufacturer yield per bag)
Worked example: eight fence-post holes
Suppose there are 8 identical round holes, each10 in in diameter with 24 in of concrete fill. Each contains a 3.5 × 3.5 in square post.
Gross hole volume each =1.09083 ft³
Square-post displacement each =0.17014 ft³
Net concrete each =0.92069 ft³
Across all eight holes, the base concrete volume is about7.36554 ft³. With a user-selected10% allowance, the planning volume becomes8.10209 ft³.
If the chosen concrete product yields 0.375 ft³ per bag, the exact requirement is about 21.606 bags, so the project purchase rounds once to 22 whole bags. The 0.375 ft³ value here is an example input only; always use the yield for the exact product you buy.
Use concrete fill depth, not automatically total hole depth
The excavation may include gravel, unfilled space, or other layers that are not concrete. Enter only the vertical depth actually occupied by concrete. This avoids automatically counting non-concrete layers as concrete volume.
If the embedded post ends above the bottom of the concrete zone, use the optional post length inside concrete field. When that field is left blank, the calculator assumes the post occupies the full concrete fill depth.
How post-fit validation works
For a round post in a round hole, the post diameter cannot exceed the hole diameter. For a square or rectangular post in a round hole, the post's corner-to-corner diagonal must fit inside the hole diameter.
For rectangular holes, round-post diameter must fit within the smaller hole dimension. Square and rectangular posts are checked in either side-aligned 90-degree orientation. The calculator does not pretend to solve arbitrary diagonal placement inside a rectangular excavation.
Bagged concrete and ready-mix ordering
Bag yield is a product property, not a universal constant. Enter the volume yield stated by the manufacturer for the exact bag and mix you intend to use. The calculator sums the entire project's required volume before rounding up to whole bags.
For ready-mix, the exact planning volume is preserved unless you enter a supplier ordering increment. Minimum orders, short-load charges, delivery fees, pumping, taxes, and site losses are not invented by this tool.
Post-hole dimensions can be structural decisions
This calculator deliberately estimates quantity rather than designing the post foundation. Hole diameter, depth, post embedment, bearing area, frost protection, reinforcement, and construction requirements can depend on the supported structure, loads, soil, climate, product instructions, and applicable code.
Use dimensions from suitable project guidance, then use this calculator to estimate concrete from those dimensions.
Post hole concrete calculator FAQ
How do I calculate concrete for a round post hole?
Calculate the cylindrical hole volume with π × radius² × concrete fill depth. If a post occupies the concrete, subtract its embedded volume before multiplying by the number of identical holes.
Does the calculator subtract the post itself?
Yes, when you choose round, square, or rectangular post displacement. Choose no post displacement when the concrete zone does not contain a post or when you intentionally want gross hole volume.
What if the post does not extend through the full concrete depth?
Enter the optional post length inside concrete. The calculator uses that shorter length for displacement while preserving the full concrete fill depth for the hole.
Should I enter total excavation depth?
Only when the full excavation depth will actually contain concrete. Otherwise enter the concrete fill depth so gravel or other non-concrete layers are not counted as concrete.
Does this calculator tell me how deep or wide the hole should be?
No. It estimates quantity from dimensions you already have. Structural sizing, embedment requirements, frost depth, and reinforcement are outside this calculator's scope.
Why do square posts in round holes use the diagonal?
A square or rectangular post has to fit corner-to-corner inside a circular hole. Checking only its width could accept geometry whose corners physically extend outside the circle.
Why does bag yield have to be entered?
Concrete yield varies by product and bag size. Using the stated manufacturer yield is more reliable than assuming a universal volume based only on bag weight.
Why are bags rounded only after all holes are added?
Rounding every hole separately can overstate the purchase. The calculator first adds the exact project volume and then rounds the final bag requirement to a whole number.
Can I calculate different hole sizes in one project?
Yes. Add a separate group for each combination of hole and post dimensions, then use quantity for identical repetitions.
Can I use Metric units?
Yes. Dimension fields switch between inches and millimeters. Results are reported in cubic feet, cubic yards, and cubic meters, while purchase fields convert to the appropriate volume unit.
Can I save or share the calculation?
Yes. After a successful calculation, the current project state is stored in the URL when it fits within a safe URL-size limit. Very large projects can still be calculated and printed even when the share URL is intentionally disabled.
Reference methodology
Unit conversions use the international foot relationship of exactly 0.3048 meter. Bag calculations require user-entered product yield and round the final project requirement to whole bags. Structural dimensions are intentionally left to project-specific guidance.
Assumptions and limitations
- Round holes are modeled as vertical cylinders; rectangular holes are modeled as rectangular prisms.
- Entered dimensions represent material geometry, not structural recommendations.
- Post displacement is subtracted only for the selected post geometry and only over the entered or defaulted length inside concrete.
- Square and rectangular posts in round holes must fit by their corner-to-corner diagonal.
- Rectangular posts in rectangular holes are checked in either side-aligned 90-degree orientation, not arbitrary rotation.
- The allowance percentage is supplied by the user and is applied after post displacement.
- Bag quantities depend on the manufacturer yield entered and are rounded once at whole-project level.
- Ready-mix rounding occurs only when a supplier increment is entered.
- Cost includes only entered material price, not labor, delivery, pumping, excavation, gravel, reinforcement, forms, taxes, or fees.
- Irregular, tapered, bell-shaped, sloped, or otherwise nonstandard excavations are not automatically modeled.