Calculators/Concrete

Post Hole Concrete Calculator

Estimate how much concrete you may need for setting fence posts, deck footings, and mailbox posts. Calculates volume per hole while subtracting the space taken up by the post itself.

Last reviewed: June 2026

in

Typical rule of thumb is hole diameter = 3x post width

in

Check local building codes for frost line requirements

in

Note: A standard "4x4" lumber post is actually 3.5" x 3.5".

%

Results update automatically as you enter measurements.

Your Material List

For this project, buy or order approximately

Use these quantities as a shopping list, supplier note, quote check, or quick jobsite material estimate.

Bags Required
1Bags
Concrete per Hole
0.528ft^3
Total Volume
0.58ft^3
Total Volume
0.022yd^3
Notice: Always check your local building codes for frost depth requirements. Post holes often need to be dug below the frost line to prevent heaving. For significantly load-bearing projects (like large decks or structural columns), always verify requirements with a structural engineer.
Note: Bag yields vary by manufacturer. Always verify the yield on the bag packaging.
Estimate note: Actual needs vary due to compaction, grading, cuts, bag yield, and supplier differences. Confirm requirements before purchasing.

Formula and Assumptions

Formula used

Hole volume = π × radius² × depth. Post volume = post width × post width × buried depth. Concrete per hole = hole volume minus post volume. The calculator multiplies by the number of holes, applies your waste factor, and divides by the selected bag yield.

Unit conversions

Inches are converted to feet before volume is calculated. Cubic feet are converted to cubic yards by dividing by 27. Bag counts are rounded up because stores do not sell partial bags.

Waste factor explanation

Waste factor helps account for uneven surfaces, cuts, spills, compaction, settling, and measurement differences. The right buffer depends on your project and material.

Material Assumptions and Disclaimer

Bag yield or density assumptions

  • The post is embedded all the way to the bottom of the hole.
  • The hole is a perfect cylinder (in reality, hand-dug holes frequently taper or bell out, consuming more concrete).
  • Standard industry average yields are used for bag sizing.

Estimate disclaimer

These tools are useful for DIY planning and quick jobsite estimates, but the results are still estimates. Verify quantities for structural work, code requirements, supplier material specs, product labels, or professional requirements before purchasing or quoting a job.

When to be careful

  • This calculator is for volumetric estimates only, not structural engineering.
  • Always set posts below the local frost line to prevent seasonal heaving.

Example Calculation

Setting three standard 4x4 posts (which measure 3.5" x 3.5") in 8-inch diameter holes that are 24 inches deep. Using 80-lb bags and 10% waste:

  1. Hole volume: π × 4² × 24 ≈ 1,206.37 cubic inches
  2. Post volume: 3.5 × 3.5 × 24 = 294 cubic inches
  3. Concrete per hole: 1,206.37 - 294 = 912.37 cubic inches
  4. Convert to cubic feet: 912.37 ÷ 1728 ≈ 0.528 ft^3
  5. Multiply by 3 posts: 0.528 × 3 = 1.584 ft^3
  6. Add 10% waste: 1.584 × 1.10 = 1.742 ft^3
  7. Divide by 80-lb yield: 1.742 ÷ 0.60 = 2.90 bags
  8. Round up to nearest whole bag = 3 bags.

You need 3 80-lb bags for the three posts.

Frequently Asked Questions

How wide should my post hole be?

A general rule of thumb for fence posts is that the hole diameter should be about three times the width of the post. So, a 4x4 post (which actually measures 3.5 inches wide) needs an approximately 10 to 11 inch hole. This gives 3+ inches of concrete thickness completely surrounding the timber.

How deep should my fence post holes be?

Typically, posts should be buried at a depth equal to one-third to one-half of the above-ground height. However, the absolute most critical rule is that the bottom of the footing **must sit beneath the local frost line**, otherwise winter freezing and thawing will heave the post out of the ground.

Should the post sit in the dirt or on concrete?

For wood posts, many builders recommend adding 2 to 4 inches of gravel or a small concrete footer at the very bottom of the hole before sinking the post. This keeps the wood from constantly wicked moisture directly from the earth.

Do I need to mix concrete in a wheelbarrow for posts?

Not always. If you are using "fast-setting" concrete mix (often sold in red bags), you can actually pour the dry mix directly around the braced post in the hole, and simply pour water over it according to the bag directions. Standard concrete mixes, however, should be properly pre-mixed with water.

Why subtract the post volume?

Because the post takes up space! If you only calculated the volume of the 12-inch cylinder hole, you would vastly overbuy concrete. An 8-inch hole, 24-inches deep, holds about 0.70 cubic feet. But a 4x4 post takes up roughly 0.17 cubic feet of that space, lowering the actual concrete demand.

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