Fence Post Concrete Calculator

Free fence post concrete calculator. Enter number of posts, hole diameter and depth, get 40/60/80 lb bag counts. Subtracts post volume automatically.

A fence post concrete calculator figures how much concrete you need to set fence posts. The standard approach: dig a hole 3× the post width in diameter and 1/3 to 1/2 the above-ground post height deep, drop the post in, fill with concrete, and let it cure.

The calculator subtracts the post's volume from the hole's volume automatically — that matters because a 4×4 post in a 12″-diameter hole displaces about 1/3 of the concrete you'd otherwise need. For a 4″ square post, that's a real saving.

Typical fence post dimensions

| Fence type | Post size | Hole diameter | Hole depth | Concrete per hole (80 lb bags) | |---|---|---|---|---| | Garden / decorative | 4×4 | 8″ | 18″ | 1 bag | | Privacy (6 ft) | 4×4 | 10″ | 24″ | 2 bags | | Privacy (6 ft, heavy wind) | 4×4 | 12″ | 30″ | 3 bags | | Privacy (8 ft) | 4×4 or 6×6 | 12″ | 36″ | 4 bags | | Livestock / farm | 6×6 or 8×8 | 18–24″ | 36–48″ | 6–10 bags |

For 10 posts on a 6-ft privacy fence: roughly 20–30 bags of 80 lb concrete, depending on hole size and post height.

How deep should fence posts be?

The rule of thumb is 1/3 of the post length in the ground, 2/3 above. A 6-ft post needs 2 ft in the ground; an 8-ft post needs 2.5–3 ft. In cold climates, dig below the frost line (typically 30–48″ in the northern US) so frost heave doesn't push the posts out of the ground each winter.

Gravel base or no gravel base?

The standard practice is to put 4–6 inches of gravel at the bottom of the hole for drainage. The post sits on the gravel (not in the concrete), so water that gets in through the top of the post drains out the bottom instead of pooling and rotting the buried end. The calculator above does not include this gravel — figure ~0.5 ft³ of gravel per hole.

Setting posts in concrete

1. Dig the hole to the right diameter and depth (use a post-hole digger or an auger)
2. Pour 4–6 inches of gravel into the bottom; tamp it flat
3. Set the post in the hole, level it both ways
4. Brace the post with two angled stakes so it stays plumb while the concrete cures
5. Mix and pour the concrete around the post, leaving the top slightly above grade so water drains away from the post
6. Slope the top of the concrete away from the post so water runs off (don't leave a bowl that holds water)
7. Wait 24–48 hours before hanging fence boards or applying lateral load

Common mistakes

• Pouring concrete all the way to the top of the hole: leaves the post end sitting in water trapped by the concrete bowl. The buried end of the post will rot in 5–10 years.
• Skipping the gravel base: same problem, just faster.
• Not bracing the post: the post shifts while the concrete is wet and you end up with a leaning fence.
• Over-digging the hole: the wider the hole, the more concrete. Use the right-sized post-hole digger.