# How much concrete for a fence post?

A 4x4 post in a 12-inch hole 30 inches deep takes about 1.75 cubic feet of concrete, or three 80-lb bags. For a whole fence, multiply that by your post count: a 100-foot line with posts 8 feet apart needs roughly 14 posts and 42 bags.

## Short version

- Size the hole at about three times the post width, so a 4x4 gets a 12-inch hole.
- Bury about one third of the post's total length, and never less than your local frost depth.
- One 4x4 post in a 12-inch by 30-inch hole takes roughly 1.75 ft³, about three 80-lb bags.
- Count your posts first, then multiply. Corners and gate posts get bigger holes and more concrete.
- Buy one or two extra bags. Running out with wet concrete in the hole is a bad afternoon.

## How big should a fence post hole be?

The working rule is a hole three times the width of the post, dug to a depth of about one third of the post's above-ground height. A 4x4 post, which actually measures 3.5 inches, gets a 12-inch-diameter hole. A 6x6 post gets an 18-inch hole.

The three-times rule exists because the concrete collar, not the post, resists the wind load and the leaning force on a fence. A narrow hole gives the collar no width to push against the surrounding soil.

Depth is the other half. A 6-foot fence on an 8-foot post puts roughly 2 feet in the ground, which is where the common 30-inch and 36-inch hole depths come from. Add 6 inches of gravel at the bottom for drainage and dig that much deeper.

Gate posts and corner posts carry more load than line posts. Bump those up a size in both diameter and depth.

## How much concrete does one post hole take?

A 12-inch hole 30 inches deep holds about 1.96 cubic feet, and the post itself displaces roughly 0.21 cubic feet, leaving about 1.75 cubic feet of concrete. The math is the cylinder volume π × radius² × depth, minus the volume of the buried post.

| Post | Hole diameter | Hole depth | Concrete needed | 80 lb bags | 50 lb bags |
|---|---|---|---|---|---|
| 4x4 | 12 in | 30 in | 1.75 ft³ | 3 | 5 |
| 4x4 | 12 in | 36 in | 2.10 ft³ | 4 | 6 |
| 6x6 | 18 in | 36 in | 4.67 ft³ | 8 | 13 |
| 6x6 | 18 in | 42 in | 5.45 ft³ | 10 | 15 |

Bag counts assume an 80 lb bag yields about 0.60 ft³ and a 50 lb bag about 0.375 ft³. Yields differ between manufacturers and between products, so check the data sheet on the bag you are actually buying.

Where the post is set into wet concrete rather than the concrete poured around a braced post, displacement changes slightly and some material rises around the post. Treat the table as a purchase quantity, not an exact fill volume.

For the single-hole calculation in more detail, see [how much concrete per fence post](/questions/fence-posts-and-setting/how-much-concrete-per-fence-post/).

## How do you figure concrete for a whole fence line?

Count the posts first, then multiply by the per-hole figure. Divide the fence length by your post spacing, add one for the closing post, and add extras for every corner, gate, and end.

Take a 100-foot fence with posts 8 feet on center. That gives 12.5 spaces, rounded up to 13, so you need 14 posts. At three 80-lb bags each, that is 42 bags, or about 24.5 cubic feet of concrete.

Once a job passes roughly 1 cubic yard, which is 27 cubic feet or about 45 bags of 80 lb mix, compare the bag price against a ready-mix delivery or a rented trailer mixer. Mixing 45 bags by hand is a long day.

Order 5 to 10 percent extra. Holes are never perfect cylinders, augers wander, and soft spots in the soil swallow more concrete than the geometry predicts.

## Should you use fast-setting or standard concrete mix?

Fast-setting mix is the usual choice for fence posts because it sets in minutes rather than hours, so you can plumb a post, brace it briefly, and move on. Many fast-setting products are designed to be poured into the hole dry and then have water added on top, which removes mixing from the job entirely.

Standard concrete mix costs less per bag and reaches higher strength, but it has to be mixed and it needs the post braced far longer. For a long fence line where you are setting all the posts in one pass and coming back the next day, standard mix is reasonable.

Follow the bag directions for water volume either way. Both mixes lose strength when they are over-watered, and a soupy collar around a post never develops the strength the label promises.

## How deep do post holes need to be in your area?

Frost depth governs, and it varies enormously across the country, from a few inches in the Gulf states to several feet in the northern tier. A post set above the frost line will heave upward each winter as the soil freezes and expands, and no amount of concrete prevents it.

Frost depth requirements, fence height limits, setbacks, and permit rules are all jurisdiction-specific. Your local building department publishes the frost depth that applies to your address, and codes are adopted locally through the ICC framework. Call them before you dig, and call 811 to have utilities located.

*General information, not engineering advice. Structural work should be designed by a licensed engineer.*

## Common mistakes

- **Digging a hole barely wider than the post** — the concrete collar needs width to resist leaning; a tight hole produces a fence that racks within a season.
- **Setting posts above the frost line** — the ground lifts them every winter and the fence goes out of level permanently.
- **Skipping gravel at the bottom of the hole** — water sits against the post end and rots wood from the bottom up.
- **Buying bags for the exact calculated volume** — irregular holes and soft soil consume more than the cylinder math predicts.
- **Adding extra water to make the mix easier to pour** — a wet collar sets weaker and lets the post work loose.