How this calculator works
A straight run is split into equal sections, and each section needs a post at each end, so the post count is the number of sections plus one. Each section carries two or three rails. Pickets are counted along the whole run at the width of a picket plus its gap. The concrete figure is the ring of space between the post and the wall of the hole, multiplied by the hole depth, so a thick post leaves less concrete to pour.
Formula
sections = ceil(run ÷ post spacing); posts = sections + 1; rails = sections × rails per section; pickets = ceil(run ÷ (picket width + gap)); concrete per post = π × (r hole² − r post²) × depth; bags = ceil(concrete ÷ 0.60 ft³).
Worked example
A 100 ft privacy fence with 8 ft sections has 13 sections, so 14 posts and 26 rails. With a 5.5 in picket and a 2 in gap it takes 160 pickets. A 4 in post in a 12 in hole 24 in deep needs about 1.40 ft³ of concrete per post, which is 3 bags of 80 lb mix.
Advertisement
Frequently asked questions
Why is the concrete based on a ring instead of the whole hole?
The post already fills the middle of the hole. Pouring the full cylinder volume would over-order, especially for thick posts, so the ring around the post is the amount you actually mix.
Do gates need anything extra?
Yes. A gate hangs from a hinge post, which takes the swinging load, so use a deeper hole and a larger post. Add the gate posts and any mid-run gate hardware outside this count.
How deep should post holes be in Canada?
Below the local frost line, which is 4 ft or more in much of the country, so the 24 in default is a starting point for mild climates only. Local building departments publish the required footing depth.