Fencing & Outdoor calculators
Posts, pickets, rails and the concrete that holds them up.
What is in this category
The fence calculator works out posts, rails, pickets and concrete from a run length and a post spacing. It pairs with the post hole calculator for what goes in the ground, and the footing calculator if you are building a wall rather than a fence.
Posts are the whole job
Everything visible about a fence — the pickets, the rails, the caps — is cosmetic and replaceable. The posts are the fence. A fence fails at the posts, always, and almost always in one of two ways: the post rots at ground level, or frost lifts it.
Both are set by how the hole is dug and filled, not by what you bolt to the post afterwards:
- Depth. A third of the post's total length in the ground is the traditional rule, and below the frost line is the requirement. For a 6 ft fence that usually means an 8 ft post set about 30 inches deep — deeper in the north.
- Gravel at the bottom. Four to six inches of compacted gravel under the post gives water somewhere to go. A post sitting in a concrete cup full of water rots from the base up.
- Crown the concrete. Slope the top of the collar away from the post so rain runs off rather than sitting against the lumber.
Spacing, and why 8 feet
Eight foot spacing is standard because rails come in 8 ft lengths and span post to post with no joint. Six feet is stronger and uses more posts; ten is doable with heavier rails but starts to sag.
The run rarely divides evenly, and the usual answer is to make every bay equal and slightly under your target rather than running a short odd bay at one end. It looks deliberate, and it is what the calculator assumes.
Before you dig
Two things to settle first, and both are cheaper now than later. Call 811 — or your local equivalent — and have the utilities marked. It is free, it is usually required, and hitting a gas or fiber line with a post hole auger is a very expensive afternoon.
And check the boundary and your local rules. Height limits, which side the finished face has to point, setbacks from the sidewalk and how close you can build to a corner all vary, and a fence in the wrong place is a fence you take down.