How many bags of concrete per fence post?

For a typical 10" × 30" hole around a 4×4 post it is about 3 bags of 60 lb (or 2 of 80 lb, 4 of 50 lb). The exact number depends on your hole diameter and depth — the concrete-per-post calculator shows all sizes.

What is the concrete formula for a post hole?

Hole volume = π × (diameter ÷ 2)² × depth, minus the post cross-section × depth; then bags = ceil(volume ÷ bag yield). Work in feet.

Does the gravel base need concrete?

No — the ~6" gravel base under the post is for drainage and isn’t counted as concrete. The concrete column sits above it, so it is a little shorter than the full hole depth.

Should I round up bags per hole or for the whole job?

Round up per hole (you buy whole bags for each post), then multiply by the post count, and add a few spares for spillage.

Concrete per fence post by post & hole size

A fence post takes the volume of its hole minus the post itself — then you divide by the yield of your bag and round up. Here is the full calculation, for all four common bag sizes.

By Francesco Zinghinì

Setting posts in concrete is the most common way to anchor a fence, and “how many bags?” is one of the first questions on any fence project. The answer is a short volume calculation applied per hole, then multiplied by the post count.

The hole-volume formula

A post hole is a cylinder with the square post standing in the middle of it, so the concrete fills the cylinder minus the post:

hole volume (ft³) = π × (diameter ft ÷ 2)² × depth ft − post cross-section × depth ft

Work in feet: a 10" diameter is 0.833 ft, a 30" depth is 2.5 ft, a 4×4 post is 3.5" = 0.292 ft on a side. Then convert bags:

bags per post = ceil(hole volume ÷ bag yield)
total bags = bags per post × number of posts

Bag yields (labeled typicals)

A bag of mix yields a set volume of set concrete — confirm the exact figure printed on your bag:

40 lb ≈ 0.30 ft³
50 lb ≈ 0.375 ft³
60 lb ≈ 0.45 ft³
80 lb ≈ 0.60 ft³

Worked example — a 10" × 30" hole, 4×4 post

Cylinder: π × (0.417)² × 2.5 = π × 0.1736 × 2.5 = 1.363 ft³
Post occupies: 0.292² × 2.5 = 0.213 ft³
Net concrete: 1.363 − 0.213 = 1.15 ft³ per hole
Bags per post: 60 lb → ceil(1.15 ÷ 0.45) = 3; 80 lb → ceil(1.15 ÷ 0.60) = 2; 50 lb → 4; 40 lb → 4
Whole fence (26 posts, 60 lb): 26 × 3 = 78 bags

The concrete-per-post calculator shows all four bag sizes side by side, and the concrete-bags-per-post-hole table lists common post and hole combinations.

How hole size changes the count

Both diameter and depth drive the volume, but diameter is squared, so a wider hole costs concrete fast. A common planning rule sizes the hole diameter at about 3× the post width (a 4×4 post → ~10–12" hole) and the depth so that roughly 1/3 of the post is buried, plus a 6" gravel base under the post for drainage. The gravel isn’t concrete, so it doesn’t add bags, but it does mean the concrete column is a little shorter than the total hole. Check the depth with the post-hole depth reference.

Bags for the whole fence, quickly

Once you know bags-per-post, the job total is just multiplication — but the numbers add up faster than people expect. At 3 bags of 60 lb per hole, a 26-post fence is 78 bags (over a ton of mix), a 40-post run is 120 bags, and a 60-post acreage fence is 180 bags. That volume is why it is worth comparing bagged mix against a ready-mix delivery or renting a mixer for a big run: the per-hole method is convenient for a few dozen posts, while a large project may be cheaper and faster poured from a truck.

Dry-set, wet-mix and post foam

There is more than one way to get concrete into the hole:

Dry-set: pour the dry mix around a braced post and add water (or let groundwater and rain cure it). Fast for line posts; the bag count is the same.
Wet-mix: mix to a workable consistency first. Preferred for gate and corner posts where full strength matters.
Expanding post foam: a two-part foam sold by the post rather than by volume. It is not concrete, so it doesn’t use this formula — follow the product’s posts-per-kit rating instead.

Whatever the method, round every hole up to whole bags, buy a few spares for spillage and the odd deeper hole, and crown the top of the concrete slightly at grade so water sheds away from the post.

Weight, water and staging the job

Concrete is heavy, and a fence adds up fast: 78 bags of 60 lb is well over two tons of material to move, so plan the delivery and where you’ll stage the pallet before it arrives. Each bag also needs water — a rough guide is around a gallon per 60 lb bag — but always follow the ratio printed on the bag, because too much water weakens the set. For a big run this is one more reason to price a ready-mix truck or a rented mixer against mixing bag by bag. Keep unopened bags dry until use: mix that has drawn moisture and hardened in the bag is wasted, which is another argument for buying close to the calculated count rather than badly over.

This is a material-quantity guide built on stable geometry and labeled bag yields, not a structural or geotechnical design. Frost depth, soil, wind load and local code set the real post depth and footing; a licensed engineer sizes load-bearing or high-wind posts, and 811 locates utilities before you dig.