How Many Bags of Concrete for a Fence Post?

The usual rules for a residential fence:

• Diameter: three times the post width. A 4×4 gets a 10–12 inch hole.
• Depth: one third of the above-ground post height, or below the local frost line — whichever is deeper.
• Gravel base: 4–6 inches at the bottom for drainage, which also reduces the concrete you need.

Frost depth is the one that catches people out. In cold climates it can be 36–48 inches, which more than doubles the concrete per post compared to a 24-inch hole.

A post hole is a cylinder with a square post sitting in the middle of it:

• Hole volume (ft³) = π × (diameter in ÷ 24)² × (depth in ÷ 12)
• Post volume (ft³) = (actual post width in ÷ 12)² × (depth in ÷ 12)
• Concrete = hole volume − post volume, then ÷ 0.45 for 60 lb bags

Remember a 4×4 actually measures 3.5 × 3.5 inches and a 6×6 is 5.5 × 5.5. The post hole concrete calculator handles the whole thing, including multiple holes at once.

Rounding up per hole wastes a lot across a long fence. Work out the total volume for every post, then divide once.

A 100 ft fence with posts on 8 ft centres needs 14 posts. At 0.92 ft³ each that is 12.9 cubic feet total — 29 bags of 60 lb, not the 42 you would get rounding each hole separately. That is a saving of about 13 bags.

Fast-setting concrete for posts is poured dry into the hole and water is added on top — no mixing, and it sets in 20–40 minutes. A 50 lb bag of fast-setting mix yields about 0.375 cubic feet, so the 10 in × 24 in hole above takes between two and three bags. Standard mix is cheaper and stronger, but you have to mix it and brace the post overnight.