Fence Calculator

The formula

A fence takeoff is a division problem followed by a plus-one:

Sections = total run ÷ post spacing, rounded up

Posts = sections + 1, plus one extra post per gate opening

The plus-one is the part people forget. Ten sections need eleven posts, because every section needs a post at each end and the last one has nowhere to borrow from. Miss it on a 40-post job and you make a second trip for one post.

Rails = sections × rails per section. Three rails per section on a 6 ft privacy fence, two on a low picket, three on split rail, and a single top rail on chain link.

Pickets = run in feet × pickets per foot. A 5½ in picket butted tight is 2.18 per foot; spaced at a picket's width apart it is about 1.1 per foot. The calculator uses 2.1 as a snug default.

A worked example

A 150 ft run of 6 ft privacy panels, 8 ft spacing, one gate, two corners, two ends, posts set in concrete.

• Sections: 150 ÷ 8 = 18.75, round up to 19
• Posts: 19 + 1 = 20, plus 1 for the gate = 21 posts
• Panels: 19
• Rails inside the panels: 19 × 3 = 57
• Post depth: 6 ÷ 3 = 2 ft
• Hole volume at 10 in diameter, 2 ft deep, less the post: about 1.1 cu ft
• Concrete: 21 × 1.1 ÷ 0.6 cu ft per bag = 39 bags of 80 lb mix

That is roughly two bags a hole, which matches what any fence crew loads onto the truck.

Post spacing is not a preference

This is the single most common mistake on a DIY fence. People pick 8 feet because it sounds right, then buy 6-foot panels, and end up with a two-foot gap they have to fill with something ugly.

Spacing is dictated by the material. Pre-built privacy panels are almost always 8 ft wide, so posts go at 8 ft centres and the panel spans between them. Split rail comes in 10 ft rails with the tenons already cut, so posts are at 10 ft. Chain link uses 10 ft as the maximum by code in most places and the fabric will simply sag beyond that. Only on a stick-built picket or board fence, where you cut your own rails, do you get to choose — and even then 8 ft is the practical maximum for a 2x4 rail before it bows in the middle.

Measure your run, decide the material, and let the material set the spacing. Then absorb the odd remainder in one short section, ideally at a corner where the eye does not read it.

Corners and ends are different posts

A line post has equal tension pulling from both sides and effectively cancels out. A corner post has two runs pulling on it at ninety degrees, and an end or gate post has an unbalanced pull from one side only, plus, in the case of a gate, a swinging load that tries to lever it out of the ground every time someone opens it.

So corner, end and gate posts get upgraded. On a wood fence that means a 6x6 instead of a 4x4, or at minimum a deeper and wider footing — plan on a hole a third deeper and an extra bag of concrete. On chain link, terminal posts are a heavier gauge and larger diameter than line posts, and every fitting is different; ordering all line posts is a classic first-time mistake. A gate post that was set like a line post is the reason most gates drag on the ground within two years.

The one-third rule and the frost line

The working rule for post depth is one third of the above-ground height, so a 6 ft fence goes 2 ft down, an 8 ft fence goes about 2 ft 8 in. That is the stability requirement — enough embedment that wind load cannot lever the post over.

The frost line is a separate and often deeper requirement. Where the ground freezes, water in the soil expands and heaves anything shallower than the frost depth straight up out of the ground, a little each winter. Northern states commonly run 36 to 48 inches, and local code will state the number. Whichever is deeper — one third or the frost line — wins.

Two details make concrete footings last: crown the top of the concrete so it sheds water away from the post rather than holding a puddle against the end grain, and put three or four inches of gravel in the bottom of the hole so the post foot drains instead of sitting in a concrete bathtub. In well-drained soil, tamped gravel alone is a legitimate alternative and makes the post far easier to replace later.