Fence and Post Concrete Estimator
Concrete Post Hole Calculator
Calculate concrete for fence, deck, gate, mailbox, and sign post holes by diameter, depth, gravel allowance, hole count, and bag size. Get cubic feet, cubic yards, 40/60/80 lb bag counts, waste, and material-only cost.
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Results
Live estimateQuick answer
One 12 in diameter post hole that is 36 in deep needs about 2.59 ft3 of concrete with 10% waste. That rounds up to about 5 common 80 lb bags, 6 common 60 lb bags, or 9 common 40 lb bags.
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
52/100
| 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. |
| 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. |
Post hole formula
One round hole
Volume per hole = pi x (diameter / 2)^2 x concrete-filled depthVolume per hole = pi x (1.000 / 2)^2 x 3.000Total 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
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.
- 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
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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.