Fence Materials Calculator

A fence is mostly posts. They cost the most, they take the longest, and they are the part that decides whether the fence is still upright in ten years. Pickets and rails are straightforward once the posts are right, which is why this calculator starts there.

The post count, and the one everybody forgets

Divide the run by the spacing and you have the number of sections. The number of posts is sections plus one, because both ends of the run need a post. A 100 foot run at 8 foot spacing is 12.5 sections, rounded to 13, and 14 posts.

Each gate adds a post on top of that. A gate interrupts the line: the opening needs a hinge post and a latch post, and neither can be shared with the neighbouring section if the gate is to swing properly. Gate posts also want to be heavier than line posts, often a 6×6 where the rest of the fence uses 4×4, because a gate hangs its whole weight off one side and works that post loose over time.

Post length is not fence height

This is where orders go wrong. A 6 foot fence does not use 6 foot posts.

The common rule is to bury a third of the exposed height, so a 6 foot fence wants 2 feet in the ground and a 8 foot post at minimum, and 9 feet if you want the third exactly. Where the ground freezes there is a second rule that overrides it: the hole must reach below the frost line, or the ground will heave the post upward each winter until the fence is visibly out of line. Frost depth is published by the local building department and ranges from nothing in the far south of the United States to more than 6 feet in northern Minnesota.

Take whichever is deeper. On a sloping site, measure at the downhill end, because that is where the exposed height is greatest.

Concrete: why the bags run out

The concrete needed per post is the hole volume minus the post volume, and hole volume grows with the square of the diameter. Going from a 10 inch hole to a 12 inch one is a 20 percent increase in width and a 44 percent increase in concrete.

The usual guidance is a hole three times the post width, so 10 to 12 inches for a nominal 4×4 that actually measures 3½. At 2 feet deep that is roughly 1 cubic foot of concrete per post after the post itself is subtracted, which is about two 80 pound bags. Fourteen posts is therefore close to thirty bags, and that is the number that surprises people at the checkout.

Whether to use concrete at all is a genuine argument among fencers. Concrete gives immediate rigidity but can trap water against a wood post and rot it at the collar. Compacted gravel drains, is easier to remove later, and is preferred by many professionals for wood posts, though it takes more care to get plumb and hold. Metal posts are almost always set in concrete.

Pickets and coverage

A privacy fence has no gaps, so the picket count is the run length divided by the picket width. A picket fence has a deliberate gap, usually 2 to 3 inches, and the count is the run divided by the width plus the gap.

Use the actual width, not the nominal one. A 1×6 picket measures 5½ inches. Over a 100 foot run that half inch difference is nineteen extra pickets, which is neither trivial nor something you want to discover on the last section.

A refinement worth the trouble on a visible fence: adjust the gap slightly so the last picket in each section lands at full width. Ending a run on a 2 inch sliver is the detail that makes a fence look homemade.

Rails

Two rails is normal up to about 4 feet. Three is standard at 6 feet, and the middle rail is what stops long pickets bowing outward over time. Some builders add a fourth on 8 foot fences.

Rail stock is usually sold in 8 or 16 foot lengths, so it pays to set the post spacing to match rather than cutting every rail. Eight foot spacing with 16 foot rails means each rail spans two sections and breaks on a post, which is both stronger and faster than butting every joint.

Before you dig

Call the national dig line and have buried utilities located. In the United States that is 811, it is free, and it is required by law in most states. A post-hole auger meets a gas line at roughly the speed you were driving it, and the consequences are not financial.

Two other checks belong here. Confirm the property line with a survey rather than with the previous owner's assumption, because a fence built a foot inside your neighbour's land is a fence you may be made to move. And check the local height limit before buying: many jurisdictions cap a front boundary fence at 4 feet and a rear one at 6, and homeowner associations frequently add rules of their own about materials and which side faces out.