Concrete for 16 4×4 fence posts in 8″ holes, 24″ deep

Concrete for 16 posts

1950 lb bags

7.04 ft³ total — 0.44 ft³ per hole

50 lb bags0.375 ft³ per bag	19bags
60 lb bags0.45 ft³ per bag	16bags
80 lb bags0.6 ft³ per bag	12bags
Total volume0.26 yd³	7.04ft³
Per hole8″ hole, 20″ of concrete above the gravel	0.44ft³
Gravel base (4″)about 4 × 50 lb bags of ¾″ drainage rock	1.86ft³
Hole width vs post4×4 wants a hole of at least 10.5″ (3× the post)	Too narrow
Max post height above grade24″ buried carries about 72″ standing proud	6ft

Notes

A 8″ hole is narrower than the 3× rule for a 4×4 post. Go to at least 10.5″ or the collar is too thin to hold the post against wind load.
Depth beats width for stability. Bury about a third of the post: at 24″ deep this post tops out near 6 ft above grade. Gate posts and corner posts want more.
24″ clears the frost line across most of the southern and central US but not the northern tier — northern Minnesota, Maine and the mountain states run 48–60″. A post set above the frost line will heave, and no amount of concrete stops it.
The gravel is not optional on wood posts. Concrete poured straight to the bottom of the hole traps water under the post and rots it from the end grain up.

Per hole, then multiplied

Each 8″ hole 24″ deep holds 0.44 ft³ of concrete once the 4×4 post itself and the gravel at the bottom are taken out of the volume. Across 16 posts that is 7.04 ft³ — 19 bags of 50 lb fast-setting mix, 16 of 60 lb, or 12 of 80 lb.

Most people buy fast-setting mix for posts and pour it in dry, then add water to the hole. That is what the bag is designed for, and it is why the 50 lb count is the one shown first.

The two rules that decide whether it holds

A hole should be about three times the width of the post. For a 4×4 post that means at least 10.5″. Anything narrower and the collar of concrete is too thin to resist the leverage of wind on the fence.

Depth does more than width. Bury roughly a third of the post: at 24″ in the ground this one carries about 6 ft standing above grade.

Gravel first

1.86 ft³ of ¾″ drainage rock goes in before the concrete. On wood posts this is not optional — concrete poured to the bottom of the hole traps water underneath and rots the post from the end grain up.

Frost line

24″ clears the frost line across most of the southern and central US. The northern tier does not: parts of Minnesota, Maine and the mountain states run 48–60″. A post set above the frost line will heave, and concrete does not stop it.

Nearby sizes

Concrete per Fence Post: The Third-Depth Rule and What It CostsA fence post hole is one of the few places where pouring concrete dry out of the bag is correct practice rather than a shortcut.
Bags or a Truck — The Crossover Is About One Cubic YardThe arithmetic is not really about price. It is about how long a slab can sit half-poured before the joint between the halves becomes permanent.
Rebar in a Slab — Spacing, Cover, and Why Position Matters MoreRebar laid on the ground and left there does nothing. It has to be held up into the concrete, and that is the step that gets skipped.
Concrete Slab Cost by State — 400 ft², All 51The figures price a 4-inch slab on grade. A driveway wants six inches and rebar, and that is not a small adjustment.