Fence Post Concrete Estimator
Concrete Post Hole & Fence Post Calculator
Calculate how much concrete you need per fence post, deck post, gate post, mailbox, or sign post hole. Enter diameter, depth, gravel allowance, hole count, and bag size to get cubic feet, cubic yards, 40/60/80 lb bag counts, waste, and material-only cost.
Enter Post Hole Details
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The PDF report uses the current calculated result. Save the estimate first when you want to reuse the inputs later.
Results
Live estimateQuick answer
If the search is how much concrete for a fence post, start with one approved post hole size. A common 4x4 fence post in a 10 in diameter by 30 in concrete-filled hole needs about 1.50 ft3 with 10% waste before post displacement, or 3 common 80 lb bags. If you subtract a 3.5 in square post through the same fill depth, the same hole is about 1.27 ft3 with waste.
Post-hole scope plan
Turn post-hole bags into an install-ready scope check
Most post-hole calculators stop at cubic feet and bag count. This plan keeps that takeoff, then checks the scope items that change a fence, deck, gate, mailbox, or sign-post job: actual dug diameter, gravel deduction, embed depth, utility marking, frost or permit boundary, bag logistics, and bracing.
1 at 12 in
3 ft
None
46/100
| Scope check | Why it matters |
|---|---|
| Actual dug diameter is measured after augering, not assumed from the bit size. | Over-dug holes can change bag count faster than the visible estimate suggests. |
| Concrete-filled depth is separated from gravel or drainage stone at the bottom. | Gravel at the bottom should not be counted as concrete volume. |
| Utility marking, property line, frost depth, and permit responsibility are confirmed before digging. | Digging and inspection responsibility must be clear before quote approval. |
| Posts are braced plumb and hole backfill sequence is defined before concrete sets. | Fast-setting products and regular mixes leave different working windows. |
| Gravel deduction is intentionally zero or drainage base is not required for this scope. | This check helps turn the calculator result into a comparable quote scope. |
| Post displacement is intentionally not deducted; use the optional deduction for concrete around a 4x4 or 6x6 post. | A post inside the hole changes net concrete only when the detail does not need a concrete plug below it. |
| Gate, deck, corner, or sign-post loads are reviewed separately from regular fence line posts. | Load-bearing or high-stress posts may need a different approved hole detail. |
How Much Concrete For a Fence Post?
Use this calculator for one post or a whole fence run when you already know the required hole diameter and depth. Enter the line-post holes first, then run gate, corner, mailbox, sign, or deck posts separately when their hole sizes differ.
| Fence post case | Hole size | Post displacement | Concrete with 10% waste | Common 80 lb bags |
|---|---|---|---|---|
| 4x4 line fence post | 10 in diameter x 30 in deep | No deduction | 1.50 ft3 | 3 bags |
| 4x4 post set in concrete | 10 in diameter x 30 in deep | Subtract 3.5 in square post | 1.27 ft3 | 3 bags |
| 4x4 corner or taller fence post | 12 in diameter x 36 in deep | Subtract 3.5 in square post | 2.31 ft3 | 4 bags |
| 6x6 gate post check | 12 in diameter x 36 in deep | Subtract 5.5 in square post | 1.90 ft3 | 4 bags |
The table is a material screen, not a structural rule. Frost depth, wind load, soil, fence height, gate hardware, and local code can require larger or deeper holes before you calculate concrete.
Post hole formula
One round hole
Gross hole = pi x (diameter / 2)^2 x concrete-filled depthGross hole = pi x (1.000 / 2)^2 x 3.000 = 2.3562Net per hole = max(gross hole - post deduction, 0) = 2.3562Total holes
Total before waste = 2.3562 x 1 holesFinal volume = total x (1 + 10%/100)Bags
Bags = ceil(final volume / 0.6)Quick Post Hole Bag Counts
| Project | Hole size | Concrete with 10% waste | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 1 hole | 8 in x 24 in | 0.77 ft3 | 2 | 2 | 3 |
| 1 hole | 10 in x 30 in | 1.50 ft3 | 3 | 4 | 5 |
| 1 hole | 12 in x 36 in | 2.59 ft3 | 5 | 6 | 9 |
| 8 holes | 10 in x 30 in | 11.97 ft3 | 20 | 27 | 40 |
| 12 holes | 12 in x 36 in | 31.15 ft3 | 52 | 70 | 104 |
These examples use common bag yields and no gravel deduction. Use the calculator above when holes are over-dug, gravel replaces concrete, or the job has mixed post sizes.
Example Post Hole Estimates
4x4 and 6x6 Post Deduction Examples
| Scenario | Net concrete with 10% waste | 80 lb bags |
|---|---|---|
| 12 in x 36 in hole, no post deduction | 2.59 ft3 | 5 |
| 12 in x 36 in hole, subtract 4x4 post | 2.31 ft3 | 4 |
| 12 in x 36 in hole, subtract 6x6 post | 1.90 ft3 | 4 |
| 10 in x 30 in hole, subtract 4x4 post | 1.27 ft3 | 3 |
These examples subtract a square post for the full concrete-filled depth. Leave the deduction off when the post sits above a concrete plug or when the approved detail asks for a full empty-hole fill.
Before You Buy Bags
- Measure the actual dug diameter, not only the planned auger size.
- Subtract gravel depth if the bottom of the hole is not filled with concrete.
- Subtract 4x4 or 6x6 post displacement only when concrete fills around the post.
- Separate regular line posts from larger gate, corner, deck, or sign posts.
- Check utility marking, frost depth, permit, and structural requirements before digging.
Post Hole Planning Notes
This calculator estimates material quantity only. It does not decide post size, embedment depth, frost depth, wind load, deck code, utility location, or permit requirements.
For the full written guide, see the concrete post hole calculator guide. For fence-specific 4x4, 6x6, corner, and gate-post bag checks, see the fence post concrete calculator guide. If the project is a continuous wall or beam footing, use the concrete footing calculator.
Frequently Asked Questions
Disclaimer: This calculator is for concrete material planning only. Site conditions, base preparation, reinforcement, drainage, joints, form accuracy, and local building rules can change actual requirements. Consult a qualified contractor or engineer for structural work.