How to Set Fence Posts in Concrete

To set fence posts in concrete, dig a hole about three times the post's width. Make it deep enough that roughly one third of the post's above-ground height is buried, and go below your frost line where that is deeper. Tamp 4 to 6 inches of gravel into the bottom. Plumb and brace the post, then fill around it with concrete and slope the top away from the wood. Line posts can usually take a dry pour of fast-setting mix. Gate and corner posts deserve concrete mixed wet.

For the exact bag count on a standard 4×4 line post, the post hole concrete calculator already does the math. This guide is about the field procedure.

What you need before you dig

• Utility locate: In the US, dial 811 at least 2–3 business days before digging. Striking a gas or electric line is dangerous and expensive.
• Frost depth: Ask your building department for your region's frost line. Posts must extend below the frost depth to prevent frost heave from pushing them out of plumb.
• Layout stakes and string line: Establish end and gate post locations first, then pull a tight string to space line posts (typically 6 to 8 feet on center).
• Tools: Post-hole digger, auger, tamping bar, post level (2-vial rubber strap level), and 1x4 bracing lumber.

How deep and wide should fence holes be?

Make hole sides straight or slightly bell-shaped at the bottom. Avoid cone-shaped holes that taper narrower at the bottom, as frost heave easily grabs a cone and lifts it.

Gravel under fence posts: yes, but only at the bottom

Pour 4 to 6 inches of coarse gravel or crushed stone into the bottom of the hole and tamp it firm before setting the post. Standing the post on gravel allows ground water to drain freely away from the vulnerable end grain of the wood.

Crucial warning: Never wrap gravel up the sides of the post with concrete only at the top. Concrete must encase the entire sides of the post down to the gravel pad.

Dry pour vs wet mix fence posts

How to set fence posts in concrete, step by step

1. Dig and prepare: Dig the hole to the target depth plus 6 inches. Tamp 6 inches of gravel into the base.
2. Set and plumb: Place the post into the center of the hole. Use a 2-way post level to check both faces.
3. Brace securely: Screw two 1×4 boards to adjacent sides of the post and stake them into the turf with duplex nails.
4. Add concrete:
   • If wet mixing: Shovel concrete evenly around the post to within 1–2 inches of grade. Rod with a scrap stick to eliminate voids.
   • If dry pouring: Pour dry fast-setting mix into the hole in 6-inch lifts, adding the specified volume of water per bag as you go.
5. Crown the top: Trowel the top of the concrete so it slopes gently away from the post on all four sides. This sheds rainwater away from the wood.
6. Wait before hanging: Wait at least 24 hours for fast-setting mix and 48–72 hours for standard wet mix before hanging heavy rails, panels, or gates.

Worked example: one 6×6 gate post

For a heavy 6-foot wooden privacy gate post:

• Post: 6×6 (actual 5.5 × 5.5 in), 10 ft long (6 ft exposed, 4 ft buried).
• Hole: 16 inch diameter, 48 inches deep.
• Gravel base: 6 inches gravel → 42 inches (3.5 ft) of concrete depth.
• Total hole volume: π × (8/12)² × 3.5 = 4.89 cu ft.
• Post displacement: (5.5/12)² × 3.5 = 0.74 cu ft.
• Net concrete: 4.89 − 0.74 = 4.15 cu ft.
• 80 lb bags needed: 4.15 ÷ 0.60 = 6.9 → 7 bags of 80 lb concrete.

Mistakes that make posts lean or rot

• Leaving a depression around the post: If concrete finishes below ground level or slopes toward the wood, water pools against the post, causing rot in 2–3 years.
• Not digging below frost depth: Frozen ground grabs the concrete plug and levers the post out of level.
• Hanging gates too early: Fast-setting mix hardens in 30 minutes, but full strength takes days. Hanging a heavy gate on day one bends the green concrete collar.