Concrete for Fence Posts

How big should the hole be?

The usual guidance is a hole diameter about three times the width of the post, so a 4×4 post (3.5 in actual) goes in a hole of 10 to 12 in. The wider concrete collar resists the post tipping. Depth is generally one-third to one-half of the post’s total length, or at least 2 ft, whichever is greater. For a 6 ft fence that means a post at least 8 ft long with 2 to 3 ft buried. In regions where the ground freezes, depth below the frost line helps prevent heaving; check your local requirement.

Bags per post

The numbers subtract the volume of the post itself. The bag counts are fractional so you can scale them; with several posts, round up the total rather than each post. Add 10% for spillage and irregular holes. The post hole calculator does the same math for any hole and post.

Worked example: ten 4×4 posts

1. Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Hole diameter 10 in = 0.8333 ft; Hole depth 30 in = 2.5 ft; Post size (actual) 3.5 in = 0.2917 ft; Post depth in the hole 24 in = 2 ft
2. Hole volume = π × 0.4167² × 2.5 = 1.3635 ft³
3. Post volume = 0.1701 ft³
4. Concrete per post = 1.3635 ft³ − 0.1701 ft³ = 1.1934 ft³
5. Total = 1.1934 ft³ × 10 = 11.934 ft³
6. Net volume = 11.934 ft³ ÷ 27 = 0.442 yd³ (× 0.0283168 = 0.3379 m³)
7. Order quantity = 0.442 yd³ × (1 + 10%) = 0.4862 yd³ = 13.1274 ft³
8. 80 lb bags = 13.1274 ft³ ÷ 0.6 ft³ per bag = 21.88 → 22 bags (always round up)
9. Weight = 13.1274 ft³ × 150 lb/ft³ = 1,969 lb

Ten posts in 10 in holes, 30 in deep with 24 in of post buried need 11.93 ft³ of concrete, 22 bags of 80 lb mix including 10% waste, or 30 bags of 60 lb mix. That is a job for a wheelbarrow and a helper. The weight of those bags is 1,760 lb.

Step by step

1. Dig the hole to the planned diameter and depth. Straight sides hold better than a bell shape.
2. Put 4 to 6 in of gravel at the bottom and tamp it. This gives drainage.
3. Set the post on the gravel and brace it plumb in two directions.
4. Mix the concrete as the bag directs, or use a fast-setting post mix according to its label.
5. Fill the hole in layers, tamping or rodding to remove air, up to a few inches below grade or to grade.
6. Slope the top surface away from the post so water drains.
7. Check plumb again, and leave the post braced until the concrete has set. Do not hang the fence panels until the concrete has gained enough strength.

Concrete or gravel?

Some installers backfill with compacted gravel instead of concrete, which allows the post to drain and be replaced. Concrete holds better in soft soil, for gate posts and for tall or solid fences that catch the wind. Concrete around a wood post can trap water at the surface, which is why the top is sloped and the base is on gravel. Whatever you choose, treated posts rated for ground contact last longer.

Posts that take the most load

Gate posts, corner posts and end posts carry more load than line posts. Use a larger hole and more concrete for them, and consider larger post sizes. Tall privacy fences are like sails in the wind, so take extra care with depth. The structural design of retaining walls or very tall fences should come from an engineer.

Mistakes to avoid

• A hole that is too narrow, so there is no concrete collar.
• Burying the post too shallow.
• No gravel base.
• Letting the concrete mound around the post so water pools.
• Hanging panels too early.
• Not checking for utilities before digging.

Depth by fence type

Use these as rough guides only. The frost line, soil type, wind exposure and local code decide the actual depth.

Soil and frost

Clay holds water against the post and can heave in winter; sand drains but offers less side resistance. Rocky ground may call for a smaller hole filled with concrete. In freezing climates the hole should extend below the frost line so that the ground cannot lift the post. Backfill with gravel for drainage in wet soil, and slope the top of the concrete away from the post.

Example: an eight-post privacy fence

Eight 4×4 posts in 12 in holes, 36 in deep with 30 in of post buried, need 17.15 ft³ of concrete net. With 10% waste that is 32 bags of 80 lb mix, or 42 bags of 60 lb mix. Add a gravel base and a few extra bags for the gate posts and corners. Digging the holes is the harder part: rent a power auger for more than a few holes, and call before you dig to check for buried utilities.

Frequently asked questions