Post Hole Concrete Calculator
Estimate how much concrete a fence post hole needs from the actual hole diameter, depth, and post size. Add separate post groups when line, corner, end, or gate posts use different hole dimensions.
How to measureActive post group
Project options50 lb · 10% extra
The 10% default is a planning allowance, not a code requirement. Custom bag yield should match the mixed yield printed on the product when it differs from a preset.
How to Use
- Count identical holes - Enter how many holes share the same diameter, depth, and post size.
- Measure the actual hole - Use the round hole diameter and total excavated depth you actually plan to build.
- Choose the post size - Use the 4x4 or 6x6 wood preset, a round post, or custom actual post dimensions.
- Add another post type when dimensions change - Keep larger corner, end, or gate-post holes in separate groups instead of forcing the whole project into one repeated size.
- Review the whole-project order - The calculator subtracts post displacement and optional non-concrete portions, sums every group, adds the selected planning allowance, and rounds bags once at the project total.
How much concrete a fence post needs depends on the hole, not the post alone. Enter the actual hole diameter, hole depth, post size, and number of matching holes. When line, corner, end, or gate posts use different dimensions, add them as separate groups so the whole project lands in one total.
How to Use the Post Hole Concrete Calculator
Start with one group of identical holes. Enter the number of holes, the measured round hole diameter, the total excavated depth, and the post size. The 4x4 and 6x6 wood presets use their actual surfaced dimensions rather than treating the nominal label as the true cross-section.
When another part of the project uses different dimensions, choose Add Another Post Type instead of repeating a separate calculation. This is useful when gate, terminal, corner, or end posts use larger posts or deeper or wider holes than the line posts.
Advanced options let you deduct an actual gravel base or a section left without concrete below grade. Leave those deductions at zero when the installation does not use them.
Post Hole Concrete Formula
For each post group, the calculator finds the concrete-filled portion of a round hole, subtracts the post displacement, multiplies by the number of matching holes, then combines every group:
Round hole area = π × (hole diameter ÷ 2)²
Rectangular post area = Actual post width × actual post thickness
Round post area = π × (post diameter ÷ 2)²
Concrete per hole = (hole area - post area) × concrete fill depth
Post-group total = Concrete per hole × number of matching holes
Project raw concrete = Sum of all post-group totals
Concrete with planning allowance = Project raw concrete × (1 + waste percentage)
Bag count = Concrete with planning allowance ÷ mixed yield per bag, rounded up once
The geometry uses the dimensions entered by the user. This calculator estimates material quantity; it does not determine the required structural hole diameter or depth.
Concrete for Fence Posts: Line, Corner, and Gate Holes
Concrete for fence posts is rarely one repeated number, because a fence rarely uses one repeated hole. Line posts can use one setup while corner, end, terminal, or gate posts use different posts or hole dimensions. Treating every hole like the smallest repeated group can materially understate the project total.
In one worked example, 18 line posts use 10 in diameter holes at 30 in deep, four corner/end posts use 12 in holes at 36 in deep, and two 6x6 gate posts use 16 in holes at 42 in deep. With post displacement included, the mixed project requires about 37.42 ft³ of raw concrete. Treating all 24 holes like the line-post holes gives about 27.62 ft³, which is about 26% below the mixed-project requirement.
That 26% figure belongs only to the stated example; it is not a universal fence correction factor. The purpose of separate groups is to calculate the actual geometry of the project instead of applying somebody else's average.
Why Actual Hole Diameter Matters
Round-hole volume changes with the square of diameter. A 12 in round hole has 44% more gross cross-sectional area than a 10 in hole at the same depth. After a fixed post cross-section is deducted, the percentage difference in concrete can be even larger.
A common question is how much concrete a 4x4 post takes, but the post is the smaller half of that answer. The same 4x4 set in a 10 in hole and a 12 in hole produces two different orders, and the gap widens as the hole gets deeper.
Measure or use the project's intended hole diameter rather than rounding the job to a preset diameter from another calculator. QUIKRETE installation guidance commonly describes a hole about three times the post width, but local conditions and the selected fence or post system can require different dimensions.
Choosing Fence Post Hole Depth
Hole depth is an input, not an output. The calculator does not decide the structurally required embedment depth, frost depth, wind design, soil capacity, gate loading, or local-code requirement for the project.
Manufacturer guidance can provide a starting point for a specific installation system, but it is not universal. QUIKRETE publishes general post-setting guidance tied to post height and a gravel layer, while fence-system manufacturers can publish their own embedment requirements. Confirm the design for the actual project and jurisdiction before ordering concrete.
Gravel Base and Concrete Below Grade
A gravel layer or an intentionally unfilled section at the top of the hole physically replaces concrete volume, so the calculator subtracts those depths when you enter them. Optional deductions default to zero so an unselected installation assumption never changes the result.
QUIKRETE's post-setting instructions use a 6 in gravel base and stop Fast-Setting Concrete about 3 to 4 in below grade before backfilling. Sakrete describes a 4 to 6 in gravel layer and a different top-finishing method. Those differences are why the calculator asks what your installation actually uses instead of enforcing one manufacturer's method.
Concrete Bag Yields Used by the Calculator
The bag presets use approximate mixed yields published for standard QUIKRETE Concrete Mix. Specialty products can differ, so use Custom Yield when the package label gives another value:
| Bag Size | Approximate Mixed Yield |
|---|---|
| 40 lb | 0.30 ft³ |
| 50 lb | 0.375 ft³ |
| 60 lb | 0.45 ft³ |
| 80 lb | 0.60 ft³ |
The geometry result does not change with bag choice. Bag yield only changes how many packages are needed to cover the calculated project volume.
Planning Allowance and Bag Rounding
The default 10% planning allowance is editable. It is a material-planning allowance for irregular excavation, soil collapse, spillage, and field variation, not a code requirement.
CalcShed sums the exact concrete volume of every post group first, applies the project allowance once, converts the final volume to bags, and rounds up once. Rounding every post group to whole bags separately can inflate the purchase quantity.
Worked Mixed-Post Example
| Post Group | Quantity | Post | Round Hole |
|---|---|---|---|
| Line posts | 18 | 4x4 nominal wood (3.5 x 3.5 in actual) | 10 in diameter x 30 in deep |
| Corner/end posts | 4 | 4x4 nominal wood (3.5 x 3.5 in actual) | 12 in diameter x 36 in deep |
| Gate posts | 2 | 6x6 nominal wood (5.5 x 5.5 in actual) | 16 in diameter x 42 in deep |
This worked example totals about 37.42 ft³ of raw concrete before waste. It demonstrates mixed-group calculation; the dimensions are not recommendations for another fence.
Related Calculators
Source Notes
- Standard QUIKRETE Concrete Mix technical data lists approximate mixed yields of 0.30 ft³ for 40 lb, 0.375 ft³ for 50 lb, 0.45 ft³ for 60 lb, and 0.60 ft³ for 80 lb bags. Product-specific yields can differ.QUIKRETE Concrete Mix No. 1101 Technical Data
- QUIKRETE's general post-setting instructions describe a hole about three times the post width, a 6 in gravel base, and concrete stopping about 3 to 4 in below grade. These are installation examples, not universal structural dimensions.QUIKRETE - Setting Posts in Concrete
- Sakrete describes a 4 to 6 in gravel layer and notes that post-hole depth depends on conditions including the regional frost line. Its finishing method differs from QUIKRETE, which is why gravel and top clearance remain user inputs.Sakrete - Setting Posts & Poles
- The PostMaster+ fence system distinguishes line, corner, and gate posts and directs installers to confirm local code and project requirements. This supports separate project groups without making CalcShed choose structural dimensions.Master Halco - PostMaster+ Installation Guidance
- Nominal lumber labels are not the same as finished dimensions. CalcShed uses the standard surfaced dimensions for the 4x4 and 6x6 wood presets and provides custom actual dimensions when the post differs.NIST - American Softwood Lumber Standard (PS 20)
Next Steps
After the post-hole concrete quantity is known, finish the surrounding fence and material plan:
Post holes connect fence layout with concrete ordering. Browse the full construction calculator collection for concrete, fencing, footing, and material tools.