Updated · Built and checked by the PlinzoTools team, formulas tested against published sources
Calculate how many fence posts you need and how much concrete each post hole requires.
Each hole: 12in diameter, 1.35 cu ft of concrete.
This solves the classic "fence post problem": the number of posts along a fence run is always one more than length divided by spacing, because a fence needs a post at both the start and the end, not just one every "spacing" feet along the way. It also estimates the concrete needed per post hole.
Enter your total fence length and post spacing (6-8ft is typical for most residential fencing). For the concrete estimate, enter your post's width and how deep you'll dig each hole.
Posts = ROUNDUP(Fence Length ÷ Spacing) + 1
Hole Diameter = Post Width × 3 (a standard rule of thumb)
Concrete per Hole = Hole Volume − Post Volume
The "+1" is the key detail people often miss: a 100ft fence at 8ft spacing needs 13 posts (ROUNDUP(100÷8)=13), not 12 -- one extra post closes out the final section.
Hole depth should account for your local frost line in cold climates -- a post set too shallow can heave with seasonal freeze-thaw cycles. Check your local building code for the required minimum depth in your area.
The formula assumes one straight run with a post at each end. Real fences usually have corners and gates, and each of those affects the count. A simple way to handle this is to split the fence into straight segments, run the calculator for each one, and then adjust for shared posts. At a corner, two segments share one post, so you subtract one post for every corner. For example, an L-shaped fence with a 40-foot leg and a 30-foot leg at 8-foot spacing: the first leg needs ROUNDUP(40/8) + 1 = 6 posts, the second needs ROUNDUP(30/8) + 1 = 5 posts, and the shared corner post means 6 + 5 - 1 = 10 posts in total.
Gates typically need heavier posts on both sides because of the swing load. The calculator does not treat these differently, so add extra posts for gates and choose stronger post sizes if your plan requires.
The tool computes each hole as a cylinder with a diameter of three times the post width, then subtracts the post's own volume. Take a hypothetical 4-inch post in a hole 24 inches deep. The hole is 12 inches across, so its volume is about pi x 6 x 6 x 24 = 2,714 cubic inches. The post occupies 4 x 4 x 24 = 384 cubic inches. The difference is roughly 2,330 cubic inches, or about 1.35 cubic feet per hole, which lines up with the figure in the page example.
To turn per-hole volume into bags, divide the total volume by the yield printed on the bag. Bag sizes and yields vary by brand, so read the label. Multiply your per-hole volume by the number of posts, then add a margin because holes are rarely perfect cylinders and some concrete is lost or spilled.
Total Concrete = Concrete per Hole × Number of Posts
Bags = ROUNDUP(Total Concrete ÷ Bag Yield)
ROUNDUP(100÷8) = 13 sections' worth of spacing, and a fence needs one more post than it has sections (a post at both ends of every section) -- so 13 + 1 = 14 posts total.
About 3 times the post's width is a standard rule of thumb -- wide enough for the post to be centered with concrete filling around it.
Deep enough to clear your local frost line, generally at least a third of the post's above-ground height -- check your local building code for the specific requirement in your area.
A closed loop has no free ends, so the post count equals the number of sections. Calculate each side and remember that every corner post is shared.
Usually yes. Gate posts are often heavier and set deeper. Add them separately based on your gate design.
Some installers use compacted gravel for drainage around a post. The concrete estimate here only applies if you set posts in concrete.
The calculator rounds up, so your actual spacing will be slightly narrower than entered. Adjust the spacing until sections look even.