Post Hole Calculator

How Deep to Set a Fence Post — The 1/3 Rule and Frost Depth

Depth is the most important decision when setting a post, and two rules govern it. The 1/3 rule says to bury at least one-third of the total post length: a 9-foot post wants 3 feet underground, leaving 6 feet above grade for a tall fence. The frost line is the depth to which the ground freezes in winter; set the bottom of the post below it so seasonal frost heave cannot lift the post and rack the fence.

• Use the deeper of the two — whichever is greater between the 1/3 rule depth and the frost line depth is the minimum you should dig.
• Frost line varies widely — about 12 inches across the deep South, 24–36 inches through the central states, and 36–48 inches in the upper Midwest and northern New England. Your building department publishes the local figure.
• End, corner, and gate posts — these carry the heaviest lateral loads, so go 6 inches deeper than the line posts and one hole size wider.

This calculator subtracts the buried portion of the post from the hole, so you order only the concrete that actually surrounds the post. To total an entire run of identical fence holes at once, use the fence post concrete calculator; to lay out the panels and post spacing for the whole fence, use the fence calculator.

Hole Diameter — The 3x Rule

Make the hole about three times the width of the post. That collar of concrete is what resists wind load, the weight of panels, and the swing of a gate; a hole barely wider than the post has almost no lateral strength. The 3x rule also leaves room to plumb and brace the post before the concrete sets.

• 4×4 post (3.5 in actual) — dig a 10–12 inch hole
• 6×6 post (5.5 in actual) — dig a 16–18 inch hole
• 4 in round post — dig a 12 inch round hole
• Gate posts — go one size larger; gate weight and swing create significant lateral force

Dry-Pack vs. Wet-Pour — Two Ways to Set a Post

There are two accepted methods for getting concrete around a post, and the right one depends on how structural the post is and how fast you want to work.

• Dry-pack (pour dry) — Brace the post plumb, pour bagged fast-setting concrete dry around it, then add water on top and let ground moisture finish the cure. No mixing, no wheelbarrow, and the post reaches walk-away strength in 20–40 minutes. This is the standard, fastest method for fence and mailbox posts.
• Wet-pour (mix first) — Mix the concrete to a workable consistency in a tub or mixer, then shovel it into the hole around the braced post. You get full control over the water-cement ratio and a stronger, more uniform result, which is why it is preferred for structural posts like pergola or carport posts.
• Either way, crown the top — finish the top of the concrete so it slopes away from the post on all sides so water sheds instead of pooling against the wood.

For a typical residential fence run, the dry-pack method is fast and more than strong enough. Reserve wet-pour for posts carrying real structural load.

Gravel Base and Post Rot Prevention

The reason most wood posts fail is not weak concrete — it is trapped moisture at the buried end. A few details dramatically extend post life:

• Gravel base — put 3–6 inches of compacted gravel or crushed stone in the bottom of the hole and set the post on it. Water that reaches the post base drains down through the gravel instead of soaking into the end grain.
• Crowned collar — slope the concrete up to the post so rain runs off; a flat or dished collar holds a ring of water against the wood.
• Collar above grade — keep the top of the concrete an inch or two above the surrounding soil so the post base is never sitting in wet ground.
• Ground-contact lumber — use UC4B (ground-contact) rated pressure-treated wood for anything buried; above-ground-rated lumber is not adequate for a post set in soil.

Gravel-Set vs. Concrete-Set Posts

Concrete is not the only way to set a fence post. For non-structural fence posts in stable soil, a fully gravel-set post is a legitimate alternative.

• Gravel-set posts — Fill the entire hole with pea gravel or crushed stone, tamping every 6 inches. Gravel drains freely, so the post base never sits in water, and the post is far easier to pull and replace later. Best for light wood fence posts in firm, well-draining soil.
• Concrete-set posts — Provide maximum lateral strength for heavy panels, gates, and high-wind exposures. Concrete is essentially permanent — replacing a post means breaking out the footing.

For most privacy and gate posts, concrete wins on strength; for low picket or wire fences in good soil, gravel can outlast concrete because it keeps the post dry.