How Much Concrete Do I Need for Fence Posts?
Setting fence posts in concrete keeps a fence plumb for decades. The amount of concrete depends on hole size, post size, and how many posts you are setting. Plan it once and you buy the right number of bags in one trip.
What drives the concrete volume
Concrete fills the gap between the post and the hole wall, so the volume is the hole volume minus the part of the post buried in it. A wider or deeper hole uses more concrete; a thicker post uses less, because it displaces more of the hole. For the math behind subtracting the post, see How to Calculate Concrete for a Post Hole.
Choose hole size and depth
A common rule is to dig the hole three times the post width and one-third to one-half of the above-ground post height deep. For a 4-inch post, that means about a 12-inch hole. A 6-foot fence post often sits in a 24 to 30 inch hole. In frost-prone areas, dig to or below the local frost line so heaving does not lift the post. Add a few inches of gravel at the bottom for drainage before setting the post.
| Post | Hole diameter | Typical depth |
|---|---|---|
| 4×4 (4 in) | 12 in | 24–30 in |
| 6×6 (6 in) | 18 in | 30–36 in |
| Round 4 in | 12 in | 24–30 in |
From volume to bags
Once you have the concrete volume per hole, multiply by the number of holes and add a waste allowance for spillage and uneven holes. Bag counts use approximate yields: a 40 lb bag yields about 0.30 cubic feet, a 50 lb bag about 0.375, a 60 lb bag about 0.45, and an 80 lb bag about 0.60. Always confirm the yield printed on the bags you buy. For a single-hole reference table, see How Many Bags of Concrete Per Post Hole.
Worked example
A 100 foot fence with posts 8 feet apart needs about 13 posts (12 sections plus an end). Each hole is 12 inches wide and 24 inches deep for a 4×4 post embedded the full depth. Hole volume = π × (0.5 ft)² × 2 ft = 1.571 cu ft. Post volume = (4 × 4 × 24) ÷ 1728 = 0.222 cu ft. Concrete per hole = 1.349 cu ft. For 13 holes with 10% waste: 1.349 × 13 × 1.10 = 19.3 cu ft = 0.71 cu yd. In 60 lb bags (0.45 cu ft each): 19.3 ÷ 0.45 = about 43 bags.
Practical cautions
- Call before you dig to mark buried utilities — hitting a line is dangerous and costly.
- Gate posts carry extra load; use a wider hole and more concrete for gates.
- Slope the concrete crown away from the post so water runs off rather than pooling.
- Keep a waste allowance: hole walls are rarely straight and some concrete stays on tools.
For the full fence material list — posts, panels, rails — use the Fence Calculator, and run the concrete numbers in the Post Hole Concrete Calculator. For larger pours, the Concrete Calculator handles slabs and footings.