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How many bags of concrete per fence post?

Short answer: Bags per post = (hole volume − post volume) ÷ bag yield. A 10-inch hole filled 24 inches deep around a 4×4 post holds about 0.92 cu ft of concrete. With 10% waste that is 1.7 bags of 80 lb mix (0.60 cu ft each), so plan about 2 bags per post; 17 bags covers 10 posts.

The formula

Hole volume (cu in) = π × (diameter ÷ 2)² × depth
Post volume (cu in) = post width × post width × depth (use actual size: a 4×4 is 3.5 × 3.5 in)
Concrete (cu ft) = (hole − post) ÷ 1,728
Bags = concrete × (1 + waste) ÷ bag yield

Bag yields from the Quikrete Concrete Mix data sheet: 80 lb = 0.60 cu ft, 60 lb = 0.45, 50 lb = 0.375, 40 lb = 0.30. A 10% waste allowance covers uneven hole walls and spillage.

Worked example

10 posts, 10-in holes, 24 in of concrete, 4×4 posts, 80 lb bags, 10% waste:

• Hole: 3.1416 × 5² × 24 = 1,885 cu in
• Post: 3.5 × 3.5 × 24 = 294 cu in
• Concrete: (1,885 − 294) ÷ 1,728 = 0.92 cu ft per post
• Bags: 0.92 × 1.10 ÷ 0.60 = 1.69 per post → 10 posts × 1.69 = 16.9 → 17 bags

FAQ

How deep should a fence post hole be?

A common rule is one-third of the post height above ground, plus about 6 inches of gravel at the bottom for drainage. For a 6-ft fence that is about 2 ft of concrete plus gravel. Always go below your local frost line and check local code.

How wide should the hole be?

About three times the post width: roughly 10 inches for a 4×4 and 12 inches for a 6×6.

Should I use fast-setting concrete?

Fast-setting mix is poured dry and soaked with water, which is quick for single posts. Check its bag yield, since a 50 lb bag of fast-setting mix yields about 0.375 cu ft, not 0.60.

Do I need to subtract the post?

Yes. A 4×4 takes up about 15% of a 10-inch hole, so ignoring it overestimates the concrete.