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October 13, 2026

Post Hole Concrete Calculator

Post Hole Concrete Calculator

Calculate the concrete needed for fence posts, deck posts, mailbox posts, and other post installations. Estimate total concrete volume and the number of ready-mix bags to buy.

Enter Your Post Hole Measurements

Measure across the circular hole.
Enter the full depth to be filled with concrete.
Allows for spillage and uneven holes.
Enter a price in your local currency.
Your Estimate

Concrete Requirements

Total concrete volume
—
cubic feet
Concrete bags to buy
—
including allowance
Estimated bag cost
—
Enter a price per bag for an estimate.
MeasurementPer holeAll holes

How to Calculate Concrete for Post Holes

Post hole concrete is calculated by treating each hole as a cylinder. The required amount depends on the hole’s diameter, depth, number of holes, and the concrete yield per bag.

Post hole concrete formula

Volume = π × radius² × depth

Convert the diameter to a radius by dividing it by two. Keep the radius and depth in the same unit, then multiply the volume of one hole by the number of holes. Finally, add your chosen extra allowance.

How many bags of concrete do you need?

Divide the total volume, including your allowance, by the yield of one bag. Always round up to the next whole bag. Actual bag yields vary by product, so check the packaging and use the custom yield option when necessary.

Tips for a more accurate estimate

  • Measure the actual hole diameter, not just the post width.
  • Use the depth that will actually be filled with concrete.
  • Account for bell-shaped, irregular, or oversized holes separately.
  • Check local requirements for post embedment, drainage, frost depth, and structural support.

Frequently Asked Questions

How much concrete does a 12-inch diameter by 30-inch deep hole need?

A cylindrical hole of these dimensions holds approximately 1.64 cubic feet of concrete, before adding any waste allowance.

How many 80 lb bags are needed for one post hole?

For a 12-inch diameter, 30-inch deep hole, the estimate is 3 bags at an approximate yield of 0.60 cubic feet per bag, before adding extra allowance. The calculator rounds the total order up to whole bags.

Should I include a waste allowance?

Usually, yes. A 5–10% allowance can help account for small spills and variations in hole size, but irregular holes may require more.

Disclaimer: This tool estimates concrete quantities and material costs only. It does not determine safe post dimensions, footing design, or structural requirements.