How Much Concrete Do I Need for Fence Posts?
Fence-post holes eat more mix than they look. Here is the hole size rule, the cylinder math, how to turn it into bags, and when a row of posts is still bag territory.
A post hole looks like a small job until you multiply it by a run of fence. Each hole is a cylinder of concrete, and ten of them add up faster than a patio you can see. Get the diameter and depth right first; the bag count follows.
How big the hole should be
A common field rule: hole diameter about three times the post width, and buried depth about one-third of the above-ground height - plus extra in frost country so the bottom sits below the frost line. A 4x4 that stands 6 feet out of the ground often wants roughly 2 feet in the ground and a 12-inch-wide hole. Put a few inches of gravel in the bottom for drainage before you pour; that depth still counts in the cylinder if you fill to grade.
Do not skimp the diameter to save mix. A skinny collar around a 4x4 will not lock the post when the gate or the wind leans on it.
The cylinder math
Treat each hole as a round column:
Volume (cu ft) = π × radius² × depth, with radius and depth in feet.
A 12-inch hole is a 0.5 ft radius. Two feet deep: 3.14 × 0.25 × 2 ≈ 1.6 cubic feet per hole. Ten posts ≈ 16 cubic feet. Divide by 27 for cubic yards: about 0.6 yards for the run, before waste. Round holes are not perfect, and the post itself occupies volume, but homeowners usually ignore the post displacement and order a little extra rather than come up short.
Turning volume into bags
An 80 lb bag yields about 0.60 cubic feet; a 60 lb bag about 0.45. That 1.6 cu ft hole is roughly three 80 lb bags (or four 60s). Ten holes is on the order of 30 of the 80s - plus 5 to 10 percent waste. Fast-set mix is popular for posts because you can plumb and brace without waiting on a full cure the way a slab does; check the bag yield, which can differ slightly from standard mix. The broader bags-versus-yard picture is in how many bags are in a cubic yard and how much concrete you need for other shapes.
Still bag territory
A line of fence posts is the classic reason to stay on bags: you mix one hole at a time, set the post, and move down the line. You are well under a truck's minimum, and a ready-mix chute is awkward over a trench of holes. If the same weekend includes a slab, price the slab as a truck and keep the posts on bags.
Pour treats a post hole as a column: diameter (or radius) and depth in, cubic feet and bag count out, with a waste buffer. Run one hole, multiply by the number of posts, and buy once instead of sending someone back to the store at hole seven.