How the calculator works
Each hole is a cylinder, and the post takes up part of it. The concrete you need is the cylinder's volume minus the post's volume:
Concrete per hole = π × (hole diameter ÷ 2)² × depth − (post width² × depth)
The result is converted from cubic inches to cubic feet (÷1728) and then to bags. An 80 lb bag yields 0.60 cu ft, a 60 lb bag 0.45 cu ft, and a 50 lb fast-setting bag 0.375 cu ft.
Worked example: ten 4×4 fence posts (3.5″ actual) in 10″-diameter holes, 24″ deep. Hole volume: π × 5² × 24 = 1,885 cu in. Post volume: 3.5 × 3.5 × 24 = 294 cu in. Concrete per hole: 1,591 cu in = 0.92 cu ft ≈ just over two 60 lb bags. Ten posts: 21 × 60 lb bags.
Digging the right hole
- Diameter: three times the post width. A 4×4 wants a 10–12″ hole. Too narrow and the concrete collar cracks.
- Depth: one-third of the post's above-ground height, and always below your local frost line. A 6 ft fence wants at least 24″ in the ground.
- Drainage: throw 3″ of gravel in the bottom before the post goes in. It drains water away from the end grain and adds years to the post's life.
Fast-setting vs standard mix
Fast-setting mix (the red bags) sets in 20–40 minutes and can be poured dry around the post, then soaked. It costs more per bag but saves the mixing step entirely. For a fence line of 20 holes, that's a real afternoon saved. Standard mix is fine when you can brace posts and leave them overnight.