Concrete for Post Holes Calculator: How Many Bags Per Post?

Quick answer: a standard fence post hole, 10" wide and 24" deep with a 4x4 post, takes about 0.9 cu ft of concrete. With 10% extra, that is 2 bags of 80 lb or 3 bags of 60 lb per post. Use the calculator for your holes.

Concrete for post holes calculator

Hole diameter (in)

Hole depth (in)

Gravel at bottom (in)

Post 4x4 (3.5" x 3.5") 6x6 (5.5" x 5.5") Round 4" Steel pipe 2-3/8" None (post on bracket, hole fully filled)

Number of posts

Extra for waste 0%10%15%

Bag yields used: 80 lb = 0.60 cu ft, 60 lb = 0.45 cu ft, 50 lb = 0.375 cu ft, 40 lb = 0.30 cu ft (typical for standard and fast-setting mixes; check the bag). Holes are rarely perfectly round, so keep the extra for waste.

Bags of concrete per post (chart)

80 lb bags per post, with a 4x4 post, no gravel and 10% extra.

Tips for setting posts in concrete

• Hole width: about 3 times the post width is a common rule, so 10–12" for a 4x4 and 14–18" for a 6x6.
• Hole depth: about one third of the post's height above ground, and below the frost line in cold climates.
• Gravel base: 4–6" of gravel under a wood post helps water drain away from the end grain.
• Crown the top: slope the concrete away from the post so rain runs off.

Common questions

How many 50 lb bags of concrete per fence post?

For a 10" x 24" hole with a 4x4 post, about 3 bags of 50 lb (with 10% extra). For a 12" x 36" hole, plan on about 7 bags.

How many bags of concrete are in a cubic yard?

About 45 bags of 80 lb, 60 bags of 60 lb, or 72 bags of 50 lb. For more than about 1 cubic yard, ready-mix delivery is often cheaper.

Do I need concrete for deck posts?

Most codes want deck posts on a concrete footing with a metal post base, not buried in concrete, because buried wood rots faster. Use the deck footing size calculator to size the footing.

Checked by: Engr. Abdul Latif, civil and structural engineer. Volumes are calculated from the hole size minus the post, using typical bag yields. Estimates for planning only.