Fence Post Spacing, Depth and Concrete: A Practical Guide

Three numbers decide how well a wood fence stands up: how far apart the posts are, how deep they are set, and how they are anchored. The usual answer is posts 8 feet apart on center, buried about one third of the fence height (and at least 2 feet), in a hole about three times the width of the post. Here is how that works out in practice.

Post spacing

• 8 ft on center is standard for wood fences. It matches common rail lengths, and uses fewer posts.
• 6 ft on center makes a stronger fence. It is a good choice for tall privacy fences, windy sites, heavy materials, or panels that are 6 ft wide.
• Prefab panels set the spacing: measure the panel width and add the post thickness.
• Measure from the center of one post to the center of the next, not the gap between them.

To calculate how many posts you need, divide the length of the run by the spacing and round up. A straight run needs one more post than sections, and each gate adds another. A 100 ft fence with one 4 ft gate and 8 ft spacing: the 96 ft of fence is 12 sections, so 12 + 1 + 1 = 14 posts. Add one post at each corner.

Post depth

A common rule of thumb is to bury about one third of the fence height, and no less than 2 feet. A 6 ft fence uses 8 ft posts with 2 ft in the ground. Taller fences need proportionally deeper posts: an 8 ft fence typically uses 11 or 12 ft posts with about 3 ft underground. Burying closer to one third of the whole post (so 2½ to 3 ft for an 8 ft post) is better still on soft soil or windy sites, where a solid privacy fence acts like a sail.

In cold climates, the bottom of the hole should reach below the frost line (often 30 to 48 in, but much more in some places), so that freezing soil does not heave the post. Ask your building department what frost depth applies to you. Gate posts and corners carry more load, so set them deeper, or use larger posts such as 6×6.

Hole diameter

A hole about three times the width of the post is a standard recommendation: 10 to 12 inches for a 4×4 post and 16 to 18 inches for a 6×6. Wider holes put more concrete around the post and resist leaning better, but the volume of concrete grows quickly with the diameter, so there is a cost.

How much concrete per post?

For a 4×4 post in a 10 in hole with 24 in of concrete depth (a 30 in hole with a 6 in gravel base), the volume works out to 0.92 ft³ per post, which is roughly 1½ bags of 80 lb or 2 to 3 bags of 50 lb. For a whole fence, multiply by the number of posts and add 10% for waste. The 100 ft example with 14 posts uses about 24 bags of 80 lb.

Enter your own hole size, depth and post type into the fence post concrete calculator for an exact count.

Setting the post

1. Mark and dig. Call 811 (in the US) before you dig, so buried utilities are marked. Check the property line, and what your local rules say about setbacks and permits.
2. Add drainage. Put 4 to 6 inches of gravel in the bottom of each hole and tamp it down. This keeps the end grain of the post out of standing water.
3. Set the post. Stand it in the hole, plumb it with a level on two adjacent faces, and brace it.
4. Add the concrete. Pour in mixed concrete, or add dry rapid-set mix and water as directed. Rod or tamp it to remove air pockets.
5. Finish the top. Slope the concrete so that it sheds water away from the post, and recheck plumb before it sets.
6. Wait. Allow the concrete to cure for the time stated on the bag before attaching rails, and longer before putting weight or wind load on the fence.

Rails and pickets

Most 6 ft privacy fences use three horizontal rails (2×4s) per section, and shorter fences use two. Pickets are cut to length and attached to the rails. To count them, divide the run length in inches by the picket width plus any gap. For 1×6 pickets (5½ in actual) with no gap, a 96 ft run takes 96 × 12 ÷ 5.5 = 210 pickets. The fence calculator counts posts, rails, pickets and concrete for a whole run.

Common mistakes

• Posts that are too shallow. They lean, especially under the wind load on a solid privacy fence.
• Not checking for a slope. On a hill, either step the fence in sections (each rail level) or rack it to follow the ground. Posts are set to the same spacing measured along the horizontal, so count carefully.
• Burying untreated posts. Use pressure-treated lumber rated for ground contact (or a rot-resistant species such as cedar heartwood).
• Setting posts before checking the line. Run a string line along the whole fence first, and keep the post tops consistent.
• Forgetting the permit. Many places set maximum heights, and require a permit and an inspection for fences.