Post Hole Concrete Calculator
Reviewed by Engr. Talha Tariq | PEC License #: PEC-CIVIL-37815 | Last updated: 2025-10-24
Calculate how much concrete you need for post holes quickly and accurately. This free post hole concrete estimator calculator supports fence posts, gate posts, deck posts, mailbox posts, and more. Estimate concrete volume, bag counts, gravel base, and project cost — with optional smart recommendations for hole size and embedment depth.
Estimate concrete, gravel, and bag counts for post holes. Results appear after you press Calculate.
Keep all inputs in the same unit. The calculator handles cylindrical (π r² h) and rectangular volume formulas internally.
Calculator Mode
Step 1 — Units
All dimensions below will be interpreted in in.
Step 2 — Project Details
Must be at least 1
Typical 5–15%
Accounts for spillage and uneven holes
Step 3 — Hole Dimensions
Common: 8–12 in
Common: 24–48 in
Step 4 — Gravel Base
Include Gravel Base
A gravel base improves drainage below the post. Default: 4–6 inches.
Recommended: 4–6 in
Step 5 — Material / Bag Size
Select product or choose custom yield
Step 6 — Actions
Enter values above and press Calculate to reveal results.
Formulas Used
- Round hole (cylinder): V = π r² h
- Square hole (prism): V = W × L × h
- Net concrete: Vnet = Vhole − Vpost
- Bags: ceil(Vadjusted / yield_per_bag)
How to Calculate Concrete for Post Holes
Whether setting fence posts, deck footings, or a mailbox, knowing the exact amount of concrete per hole prevents costly material runs and guarantees structural stability. Our Post Hole Calculator provides fast, accurate volume sizing alongside bag estimates for DIY projects and contractors alike.
Key Features of Our Post Hole Estimator
Accurate Concrete Volume
Calculates concrete volume for round and square post holes using precise cylindrical and rectangular geometry.
Net Concrete Estimate
Subtracts post displacement from the hole volume for a more accurate material estimate, reducing waste.
Bag Count Calculator
Converts total concrete volume into estimated 40 lb, 60 lb, and 80 lb concrete bag counts.
Gravel Base Estimator
Estimates how much gravel is needed for a drainage base below each post hole, improving stability.
Smart Hole Size Recommendations
Suggests hole diameter and embedment depth based on post size, post type, and frost depth rules.
Waste & Cost Adjustment
Adds customizable waste allowance to avoid running short and estimates total project cost.
How to Use the Post Hole Calculator
- 1Choose your Calculator Mode — Quick Estimate, Advanced, or Recommendation.
- 2Select the hole shape (round or square) and enter hole diameter/width and hole depth.
- 3Enter the number of post holes for your project.
- 4Toggle the gravel base option and set gravel depth (typically 4–6 inches).
- 5(Advanced) Enter post dimensions for more accurate results with post displacement subtraction.
- 6Select bag size or enter a custom yield per bag.
- 7Click Calculate to view volume, bag counts, gravel, and cost estimates.
Post Hole Formulas
- 1) Round Hole Volume (Cylinder)Volume = π × (Diameter / 2)² × Concrete Fill Depth
Concrete Fill Depth = Hole Depth − Gravel Depth - 2) Square / Rectangular Hole VolumeVolume = Width × Length × Concrete Fill Depth
- 3) Post Displacement (Optional)Round post: V = π × (Post Diameter / 2)² × Embedded Depth
Square post: V = Post Width × Post Length × Embedded Depth - 4) Waste & BagsAdjusted Volume = Total Volume × (1 + Waste% / 100)
Bags = ⌈Adjusted Volume / Yield Per Bag⌉ - 5) Recommendation EngineRecommended Hole Diameter = 3 × Post Width
Recommended Embed Depth = max(Post Length / 3, Frost Depth)
Frequently Asked Questions
How many bags of concrete do I need per fence post?
The number of bags depends on the size of the hole and the yield of the selected concrete mix. For a typical 4×4 fence post in a 10-inch diameter hole 36 inches deep, you'll need about 2–3 bags of 80 lb concrete.
Should I put gravel at the bottom of a post hole?
A gravel base of 4 to 6 inches is commonly used to improve drainage and provide a stable foundation at the bottom of the hole. Our calculator includes an optional gravel base estimator.
How deep should a post hole be?
A common starting rule is to bury about one-third of the total post length below grade, but local frost depth, soil conditions, and post type can affect the final depth. Use our Recommendation mode for guidance.
How wide should a post hole be?
A common rule of thumb is to make the hole about three times the width or diameter of the post. For a 4×4 post (3.5 inches actual), that means roughly a 10-inch diameter hole.
Should gate posts use more concrete than line posts?
Yes. Gate posts typically carry more load and swing forces, so they often require larger holes, deeper embedment, and more concrete than standard line posts.
Related Calculators
Slab Concrete Calculator
Estimate concrete for floors, patios & driveways.
Beam Concrete Calculator
Volume for rectangular or multi-span beams.
Column Concrete Calculator
Round or rectangular column volumes in seconds.
Footing Concrete Calculator
Strip & isolated footing volume estimates.
Wall Concrete Calculator
Compute concrete for straight or segmented walls.
Pier / Caisson Calculator
Drilled shaft & caisson concrete volumes.
Tank / Trench Calculator
Rectangular & circular tank/trench volumes.
Staircase Concrete Calculator
Total volume for flights, landings & risers.
Nominal Mix (M5–M25)
Material splits for standard nominal mixes.
Concrete Yards Calculator
Convert area/thickness into cubic yards & m³.
Concrete Bag Calculator
Calculate how many concrete bags you need with unit conversions, bag sizes (40/50/60/80 lb & 20 kg), and waste buffers. Covers slabs, footings, posts, and sonotubes.