Fence calculator: posts, pickets and concrete per hole

How deep should fence posts go?

Below the frost line, and that is a local figure rather than a general one. It runs from about 12 inches in the southern United States to 60 inches or more in the northern states, and your building department publishes it.

This is the one part of fence building that is a genuine code input. Everything else on this page is trade practice.

If a post does not reach below the frost line, water in the soil freezes, expands and lifts the post. Over a few winters the fence goes visibly out of line and cannot be pushed back, because the hole below has filled in.

Code Frost depth is set locally by your building department. Convention The common rules of thumb, a third of the post height in the ground and a hole three times the post width, are trade practice with no published standard behind them.

How much concrete per post?

The volume of the hole less the volume of the post. A 4 by 4 post in a 10 inch hole 30 inches deep needs about 1.1 cubic feet, which is roughly two 80 pound bags.

A useful habit: put four to six inches of gravel in the bottom of the hole before the post. It lets water drain away from the end grain rather than sitting against it, and a post that stays wet at the bottom rots there first.

Do corner posts need to be different?

Yes, and this is where fences fail. Corner and gate posts carry loads that line posts do not: a corner post is pulled in two directions at once, and a gate post carries a swinging weight that works it loose over time.

Set them deeper and in a wider hole than the line posts, and use a heavier section where you can. A gate that drops after a year is almost always a gate post that was set like a line post.

Common questions

How far apart should fence posts be?

Six to eight feet is the usual range, and it is convention rather than code. Eight feet is the most common because rails come in eight foot lengths. Six feet is worth it for tall solid fences in windy places, because a solid fence is a sail and the load on each post rises quickly with height.

Should posts be set in concrete or gravel?

Concrete is stronger and standard for most fences. Well-compacted gravel drains better and lets a post be replaced without breaking out a footing, which is why it is common for cedar posts in wet ground. Concrete without a gravel base at the bottom is the worst of both, because it holds water against the post.

How many pickets do I need?

Divide the fence length by the picket width plus the gap. For a solid privacy fence the gap is nominally zero, though a small gap is normal to allow for swelling. Pressure-treated pickets shrink as they dry, so fitting them tight is usual and they open slightly on their own.

Which side does the good side face?

Traditionally the finished side faces your neighbour, with rails and posts on your side. Some local rules and homeowner associations require it. Worth checking before you build rather than after.