Post Hole Concrete Calculator

Use this post hole concrete calculator to estimate how many concrete bags you need per fence, gate, deck, mailbox, or sign post. Hole diameter, depth, gravel base, and actual post displacement stay visible so the shopping list matches the install method.

Quick answer: a 10 in × 24 in mailbox hole often lands near 2 × 50-lb bags, many line fence posts land around 3 to 5 × 50-lb bags, and gate or deck posts usually need more. For fence work, pair the result with the fence post depth guide and the concrete-per-post guide.
By: CalcHub Editorial Operated by: Cloudtopia
Maintenance: Updated when formulas, supplier packaging, or guidance change.
Method: Research + supplier packaging + formula verification.
Units:
Line Fence Post: Line-post planning with gravel-base defaults and fence-line count support.

Uniform post-hole estimate

Use one set of hole and post dimensions for the whole mailbox, deck, sign, or one-style fence run.

holes
in
in
Use actual lumber size for displacement. A nominal 4x4 is usually 3.5 in.
in
in
Method settings
Keep the method assumptions visible so bag totals match how you actually plan to set the post.
in
SAKRETE Fence Post Mix 50 lb
0.5 cu ft per bag • 50 lb
Dedicated fence-post mix with higher yield than many generic 50 lb bags.
%
$/bag
$/yd
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Common Post-Hole Presets and Starter Bag Counts

Use case Typical starter hole Planning bag read Why it varies
Mailbox / sign post Around 10 in wide × 24 in deep Often 2 × 50-lb bags with a gravel base Small-count jobs stay bag-first, but gravel-base and product-yield choices still change the bag count.
Line fence post Around 10 to 12 in wide × 24 to 30 in deep Often 3 to 5 × 50-lb bags Spacing, frost depth, and whether you use a gravel base all change the total material list.
Gate / corner post Often wider and deeper than line posts Often 5 to 7 × 50-lb bags Heavier loads mean bigger holes, more overage, and more concrete per post.
Deck post Treat as structural footing planning Use the tool for a shopping estimate, then verify locally Footing diameter, frost depth, and code requirements can exceed simple residential rules of thumb.

Method Assumptions That Change the Bag Count

Gravel Base vs Full-Hole Concrete

The route treats gravel as a real deduction, not a hidden opinion. Turn the gravel base on when part of the hole will not be concrete, and the result will show both lower concrete volume and a separate gravel quantity.

Fill Depth Matters More Than Bag Weight

Two holes with the same depth can need different bag counts if one is only concreted partway. The calculator lets you fill the full remaining depth after gravel or use a custom concrete fill depth when your method is different.

Bag Weight Is Not Bag Yield

The bag selector uses product yield, not just weight class. That matters because a branded 50 lb fence-post mix can yield materially more volume than a different 50 lb fast-setting bag, even though both weigh the same.

Deck and other structural posts stay in the calculator because you still need a shopping estimate, but the output is planning-only. Check local frost depth, footing dimensions, permits, and engineering requirements before you build.
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Worked Examples You Can Reproduce in the Calculator

Mailbox post with a fast-setting 50 lb mix, a 10 inch hole, 24 inch depth, and a 6 inch gravel base.

  1. 1 Preset: Mailbox Post with a 10 in hole, 24 in total depth, and a 6 in gravel base.
  2. 2 Concrete fill depth is the remaining 18 in after gravel.
  3. 3 Subtract the actual 3.5 in × 3.5 in post displacement from the round hole volume.
  4. 4 That leaves about 0.69 cu ft of concrete per hole.
  5. 5 Using QUIKRETE Fast-Setting 50 lb at 0.375 cu ft per bag rounds to 2 bags.
Order 2 bags of QUIKRETE Fast-Setting 50 lb for a single mailbox post when the hole uses that gravel-base assumption.

Fence line with line posts plus a small gate group, using fence-post mix instead of a generic bag class.

  1. 1 Fence length: 72 ft at 8 ft spacing derives 10 line posts.
  2. 2 Add 2 gate / corner posts with deeper, larger holes than the line-post group.
  3. 3 Line posts use a 12 in × 30 in hole with a 4 in gravel base.
  4. 4 Gate posts use a 12 in × 36 in hole with a 6 in gravel base and a 6x6 actual 5.5 in post.
  5. 5 Select SAKRETE Fence Post Mix 50 lb to convert both groups into one shopping list with bag subtotals and a combined total.
The grouped mode keeps line-post bags and gate-post bags separate, then combines them into one purchase-ready total.

Deck-post planning where you still need a bag count even though local footing rules govern the final build.

  1. 1 Preset: Deck Post with a 12 in diameter hole and 36 in depth.
  2. 2 The calculator keeps the actual 5.5 in × 5.5 in post displacement visible instead of assuming the hole is all concrete.
  3. 3 Switching from a generic 80 lb bag to a different yield model changes bag count without changing the raw concrete volume.
  4. 4 The result card still flags the job as structural planning only so frost depth, code, and footing design are not treated as solved by the bag math.
Use the deck preset for material planning, then verify the footing dimensions and structural details with code or engineering before pouring.

What the Calculator Counts and Subtracts

Concrete volume

Each hole starts as a round cylinder. The calculator subtracts the actual post displacement from that hole volume, then applies the chosen concrete fill depth and waste allowance before converting to bag totals and cubic yards.

Grouped projects

Fence jobs often need one geometry for line posts and another for gate or corner posts. The grouped mode derives or accepts the line-post count, lets the heavier group use its own hole assumptions, then sums both into one shopping list.

Frequently Asked Questions

How wide should a post hole be? +
A common planning rule is to start around 2.5x to 3x the post width or diameter, then adjust for soil, load, and local guidance. Fence line posts often land near 10 to 12 inches, while heavier gate and deck posts usually need a wider hole and a deeper footing plan.
How deep should a post hole go? +
Many DIY planning guides start around one-third to one-half of the exposed post height, but frost depth and local code can override that quickly. For structural deck posts or cold-climate footings, use the calculator as a shopping estimate only and verify the final depth with your local requirements.
Should I use gravel at the bottom of a post hole? +
Gravel is a method choice, not a universal rule. Some installers use a gravel base for drainage or leveling, while others concrete more of the hole. The calculator exposes that choice directly so your bag count matches your install method instead of hiding a default.
Why do different 50 lb products need different bag counts? +
Bag weight is not the same thing as yield. A 50 lb fast-setting mix and a 50 lb fence-post mix can cover different cubic-foot volumes, so the same hole geometry can need a different number of bags depending on the product you buy.
When does this need engineering or local code review? +
Deck posts, gate anchors, structural footings, and any project affected by frost depth or permit rules deserve a code check before you build. This calculator is best for material planning, not final structural design.

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Disclaimer: This calculator provides estimates for planning purposes only. Actual material requirements depend on site conditions, compaction, grading, and local building codes. Always verify measurements on-site and consult with your material supplier before purchasing.