How Many Bags of Concrete Per Fence Post

A typical 10 inch diameter, 24 inch deep post hole takes roughly three 60 lb bags of concrete. The exact count depends on hole diameter and depth, both of which you control, so here is the actual formula rather than a single number to memorize.

The volume formula

A post hole is a cylinder, so its volume follows the standard cylinder formula:

volume = pi x radius^2 x depth

With diameter and depth in inches, convert to feet first (divide by 12) so the result comes out in cubic feet. For a 10 inch diameter, 24 inch deep hole:

radius = 10 / 2 / 12 = 0.417 ft
depth = 24 / 12 = 2 ft
volume = pi x 0.417^2 x 2 = 1.09 cubic feet

Converting volume to bags

Bagged premix concrete lists a yield, the volume one bag makes once mixed. Published yields are roughly:

Bag size	Yield (cubic feet)
50 lb	0.375
60 lb	0.45
80 lb	0.6

For the 1.09 cubic foot hole above:

1.09 / 0.45 = 2.4 -> 3 bags of 60 lb concrete

The same hole needs about 3 bags of 50 lb concrete or 2 bags of 80 lb, always rounding up since a partial bag still has to be opened.

Bigger posts need more

A 6x6 post commonly sits in a 12 inch diameter hole rather than 10 inches. At the same 24 inch depth, that larger diameter alone raises the volume to about 1.57 cubic feet, roughly 44 percent more concrete than the 10 inch hole, even though the depth did not change. Diameter matters more than people expect, since it is squared in the formula.

Calculate it for every post on your fence

The fence post concrete calculator runs this formula for your actual hole diameter and depth, then multiplies by your total post count and shows bag counts in 50, 60 and 80 lb sizes plus the cubic yard equivalent for ready-mix. If you have not worked out post depth yet, start with how deep should a 6 foot fence post be or the fence post depth calculator.