Ordering too little concrete leaves your post holes half-filled while the mix dries, forcing frantic emergency runs to the lumberyard. Ordering too much leaves you with hundreds of pounds of rock-hard concrete bags taking up space in your garage or yard.
Accurately calculating the number of 50-lb, 60-lb, or 80-lb bags of concrete needed for your fence project requires basic cylindrical geometry, post volume deduction, and gravel base accounting.
Here is the step-by-step formula and reference takeoff chart used by professional fence contractors.
The Cylindrical Volume Formula
A fence post footing is a cylinder of concrete with a square or round post embedded through its center. To find the net concrete volume required per hole:
-
Calculate the gross hole volume (Cylinder):
Volume (cu ft) = pi * (Radius in feet)^2 * Depth in feetRemember that the radius is half the diameter. For a 10-inch diameter hole, the radius is 5 inches, or5 / 12 = 0.4167 feet. -
Deduct the gravel drainage bed: If you add 6 inches of crushed stone to the bottom of a 36-inch hole, the actual concrete pour depth is only 30 inches (
2.5 feet). -
Subtract the embedded post displacement: The post displaces volume inside the hole.
- For a standard 4x4 post (actual size 3.5 in x 3.5 in):
Displacement = (3.5/12) * (3.5/12) * 2.5 ft = 0.213 cu ft. - For a standard 6x6 post (actual size 5.5 in x 5.5 in):
Displacement = (5.5/12) * (5.5/12) * 2.5 ft = 0.525 cu ft.
- For a standard 4x4 post (actual size 3.5 in x 3.5 in):
-
Net Concrete per Hole:
Net Volume = Cylinder Volume - Post Displacement.
Yield per Bag of Pre-Mixed Concrete
Commercial concrete is sold in standard bags by weight. Each bag yields a predictable volume of mixed wet concrete:
- 50-pound bag (Fast-setting / Quick-cure): Yields roughly 0.375 cubic feet (requires 2.67 bags per 1 cu ft).
- 60-pound bag: Yields roughly 0.45 cubic feet (requires 2.22 bags per 1 cu ft).
- 80-pound bag: Yields roughly 0.60 cubic feet (requires 1.67 bags per 1 cu ft).
Quick Concrete Bag Lookup Table
The table below shows concrete bag counts for common residential post hole dimensions (after accounting for a 6-inch gravel base and 4x4 or 6x6 wood post displacement):
| Post Size | Hole Diameter | Hole Depth | Net Concrete Volume | 50-lb Fast-Set | 60-lb Standard | 80-lb Standard |
|---|---|---|---|---|---|---|
| 4x4 wood | 8 inches | 24 inches | 0.44 cu ft | 1.5 bags | 1.0 bag | 0.8 bags |
| 4x4 wood | 10 inches | 30 inches | 0.98 cu ft | 2.5 bags | 2.2 bags | 1.7 bags |
| 4x4 wood | 10 inches | 36 inches | 1.22 cu ft | 3.5 bags | 2.8 bags | 2.1 bags |
| 4x4 wood | 12 inches | 36 inches | 1.83 cu ft | 5.0 bags | 4.1 bags | 3.1 bags |
| 4x4 wood | 12 inches | 48 inches | 2.54 cu ft | 7.0 bags | 5.7 bags | 4.3 bags |
| 6x6 wood | 12 inches | 36 inches | 1.52 cu ft | 4.0 bags | 3.4 bags | 2.6 bags |
| 6x6 wood | 14 inches | 42 inches | 2.45 cu ft | 6.5 bags | 5.5 bags | 4.1 bags |
| 6x6 wood | 16 inches | 48 inches | 3.82 cu ft | 10.0 bags | 8.5 bags | 6.5 bags |
Note: These figures are illustrative, rounded examples of the formula above, not a substitute for running your own hole diameter, depth and post size through the fence post concrete calculator for an exact count. Always round up to the nearest whole bag, and add 10% to the total project order to account for uneven soil walls and minor over-digging.
Fast-Setting vs Standard Concrete: Which Should You Buy?
- Fast-Setting Concrete (Red bag): Formulated for dry-pour application. You dump dry powder directly into the hole around the plumbed post, pour clean water on top, and let it set in 20 to 40 minutes. It is ideal for line posts on small residential DIY projects because you do not need a wheelbarrow or mechanical mixer.
- Standard High-Strength Concrete (Yellow bag): Requires pre-mixing with water in a wheelbarrow, mortar tub, or portable mixer before pouring. It is generally cheaper per pound than fast-setting mix, and many contractors prefer it for corner posts, gate posts, and large commercial runs; check the specific product’s rated compressive strength, since this varies by brand and formulation.
Calculator Tools and Project References
Automate the math and customize parameters for your exact fence dimensions:
- Calculate concrete bags and total yardage with our fence post concrete calculator.
- Determine large footing dimensions using the concrete bags calculator.
- Learn bag yields in detail in how many bags of concrete per fence post.
- Compare foundation methods in setting fence posts in dirt vs concrete.
- Explore frost line engineering in our fence post depth and setting guide.