Fence Calculator

Post depth is the whole fence

The working rule is that a third of the post length goes in the ground, or at least below the local frost line, whichever is deeper. A six-foot fence therefore wants nine-foot posts set three feet deep. Fences that lean after a few winters almost always have posts set too shallow — frost heave lifts them a little each year and nothing puts them back.

Spacing, and why eight feet is the default

Eight feet matches standard rail lengths, so nothing is cut to waste. Six feet is stiffer and is worth it for tall privacy fences or exposed sites, because a six-foot solid fence is a sail. Note that posts are counted as sections plus one: a 120-foot run at 8-foot spacing is 15 sections and 16 posts, plus one extra post per gate.

Picket count and the spacing that never divides evenly

Pickets needed is total length in inches divided by (picket width + gap). It essentially never comes out whole, so you either adjust the gap slightly across the run or accept a narrower final picket at one end. Adjusting the gap by a sixteenth of an inch over a long run is invisible; a half-width picket at the gate is not.

Posts fail first, so the post schedule decides the fence

Almost every fence that leans failed at the post, not the panel. Spacing is normally 6 to 8 feet on centre, and shorter spans are worth the extra posts on exposed sites where wind load is the governing force rather than gravity. A solid privacy fence behaves like a sail: it collects the full pressure of the wind and transfers it to the posts, which is why the same post spacing that works for a picket fence can be inadequate for a 6-foot board fence.

Depth is set by the local frost line, not by a rule of thumb, because a footing that stops above it will be lifted by frost heave and dropped out of alignment each spring. Local building departments publish the required depth, and many jurisdictions also cap fence height, regulate which side the finished face must show, and require a permit above a certain height. Gate posts deserve a larger footing than line posts: they carry the dead weight of the gate plus the repeated shock of it closing.

Frequently Asked Questions