How Many Bags of Concrete Mix for Fence Posts?

Each post hole is a cylinder with the post itself taking up part of the space, so the concrete volume is the hole volume minus the post volume. Hole volume = π × radius² × depth, all in feet. A 10 in diameter hole is 5 in radius, or 0.417 ft, and a 30 in deep hole is 2.5 ft: 3.1416 × 0.417² × 2.5 = 1.37 ft³.

A 4×4 post is actually 3.5 in × 3.5 in, an area of 12.25 in² or 0.085 ft². Over 2.5 ft of embedment that is 0.21 ft³, leaving 1.16 ft³ of concrete per hole. Most people round up and call it 1.2 to 1.4 ft³ because the hole is never a perfect cylinder.

Convert to bags using the yield printed on the bag: a 50 lb bag of fast-setting mix yields about 0.375 ft³, a 60 lb bag about 0.45 ft³, and an 80 lb bag about 0.60 ft³. At 1.2 ft³ per hole that is roughly three 50 lb bags, or two 80 lb bags, per post.

Size the hole before you size the bag. The standard rules are: hole diameter three times the post width (12 in for a 4×4, 18 in for a 6×6), and hole depth one third of the above-ground post height or below the local frost line, whichever is deeper. A 6 ft fence therefore wants at least 24 in of embedment, and colder climates commonly go to 36–42 in.

Count posts with the fence formula: posts = (fence length ÷ post spacing) + 1, plus one extra post at every corner and gate. A 100 ft fence at 8 ft spacing needs (100 ÷ 8) + 1 = 13.5, rounded to 14 posts. At three 50 lb bags each that is 42 bags — and add 4–6 in of loose gravel in the bottom of each hole for drainage, which slightly reduces the concrete needed and greatly extends post life.