How Much Concrete for Fence Posts?

The amount depends on post size, hole diameter and hole depth.

Common rules of thumb

• Hole depth: about one third of the post’s total length, deeper in frost-prone areas.
• Hole diameter: about three times the post width.

Worked example

A 4×4 post (3.5 in) in a 10 in hole, 24 in deep. Hole volume is about 1,885 cubic inches; the post takes up about 294. Concrete needed is about 1,590 cubic inches, or 0.92 cubic feet.

Bags

Bag size	Yield (approx.)
50 lb	0.375 cu ft
60 lb	0.45 cu ft
80 lb	0.60 cu ft

The example needs about 1.5 bags of 80 lb, so buy 2 per post. Multiply by your post count from How Many Fence Posts Do I Need? Check the yield printed on your bag, as brands vary.

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