Post Hole Concrete Calculator
Setting fence posts, deck posts, or mailbox posts in concrete requires knowing how much premix concrete to buy. Each post hole is a cylinder, and the concrete volume is calculated using the formula for the volume of a cylinder: pi multiplied by the radius squared multiplied by the depth. Results are given in cubic feet for each hole and in total, then converted to the number of 50-lb and 80-lb bags of premixed concrete. A 50-lb bag of premixed concrete (such as Quikrete or Sakrete) yields approximately 0.375 cubic feet. An 80-lb bag yields approximately 0.60 cubic feet. For post hole depth, a commonly cited rule is to bury one-third of the total post length. In cold climates, holes must extend below the local frost line to prevent frost heave. Check your local building department for the required frost depth before digging. Structural posts for decks and pergolas may have specific minimum depth requirements in your local building code. Always add a small extra bag or two to account for spillage and variations in hole size.
Post hole concrete formula
Radius (ft) = Diameter (in) / 2 / 12
Volume per hole (cu ft) = pi x Radius^2 x Depth (ft)
Total volume = Volume per hole x Number of holes
50-lb bags = Ceiling(Total / 0.375)
80-lb bags = Ceiling(Total / 0.60)
Worked example
8 holes, 10 in diameter, 36 in deep.
- Radius = 10 / 2 / 12 = 0.417 ft
- Volume per hole = pi x 0.417^2 x 3 = 3.1416 x 0.174 x 3 = 1.64 cu ft
- Total = 1.64 x 8 = 13.09 cu ft
- 50-lb bags = Ceiling(13.09 / 0.375) = 35 bags
- 80-lb bags = Ceiling(13.09 / 0.60) = 22 bags
Frequently asked questions
How much concrete do I need per post hole?
The volume of a cylindrical hole is pi x (radius squared) x depth. For a 10-inch diameter hole that is 36 inches deep: pi x (5/12)^2 x 3 = 1.636 x 0.174 x 3 = approximately 0.854 cubic feet. Divide by 0.375 (yield of a 50-lb bag) to get about 2.3 bags, so 3 bags per hole.
How deep should post holes be?
A common rule of thumb is to bury at least one-third of the post length below ground. For frost protection in cold climates, holes should extend at least 6 inches below the local frost line. Contact your local building department for the frost depth in your area. Fence posts are typically 24 to 36 inches deep; deck posts often require deeper holes.
How much concrete does a 50-lb bag yield?
A 50-lb bag of premixed concrete (such as Quikrete or Sakrete) yields approximately 0.375 cubic feet. An 80-lb bag yields approximately 0.60 cubic feet. This calculator provides counts for both bag sizes.
Should I set posts in concrete or gravel?
Setting posts in concrete provides maximum stability and is required for structural applications like decks, pergolas, and retaining walls. Gravel-set posts allow drainage and may extend post life in some soil conditions, but are less rigid. Most building codes require concrete for structural posts. Consult your local building department.
Do I subtract the post volume from the concrete needed?
For most practical purposes, the volume displaced by the post itself is small relative to the hole volume and is typically ignored. For large posts (6x6 or 8x8) in narrow holes, you may want to subtract the post cross-section volume from the total hole volume for a more precise estimate.
Sources
- Portland Cement Association: Concrete basics.
- American Wood Council: Prescriptive Residential Wood Deck Construction Guide (DCA6).
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. Check local building department for frost depth and required hole dimensions.