Fence calculator

How many fence posts do I need?

Divide each side by the maximum post spacing, round up to whole sections, and add one post for an open fence. A closed loop needs one post per section, because the last side ends on the first post.

posts = sections + 1, where sections = length ÷ maximum spacing (rounded up)

A 100 ft run with posts no more than 8 ft apart is 100 ÷ 8 = 12.5, so 13 sections and 14 posts, spaced 7.69 ft apart so every section is equal. Each corner of a multi-sided fence is rounded separately and shares one post. The fence post spacing guide covers how to choose the maximum spacing.

Fence pickets and boards: how many?

Rails = sections × rails per section, and pickets = fence length ÷ (picket width + gap), rounded up. A wood preservative maker's installation guide uses three rails when pickets are 60 inches or longer, with the top and bottom rails within 6 inches of the picket ends, and centers each line post where the rails meet.^[1] Because rails meet on a post, the rail length sets the post spacing.

• Privacy fence: enter a gap of 0. A 100 ft run of 5.5 inch boards takes 1,200 ÷ 5.5 = 218.2, so 219 boards.
• Spaced pickets: add the gap to the width. 3.5 inch pickets with a 2 inch gap on 100 ft take 1,200 ÷ 5.5 = 219 pickets.
• Wire fence: choose no rails and leave the picket width blank.

How deep should fence posts be?

For wood fence posts, bury at least a third of the post and never less than 2 ft, with the bottom of the hole a few inches below your local frost line.^[1] The same guide puts 4–6 inches of packed gravel at the bottom of each hole and finishes the concrete above the soil line, sloped to shed water.^[1]

Concrete for fence posts

Each hole takes about 1 cu ft of concrete at 10 inches across and 30 inches deep, with 4 inches of gravel and a 3½ inch post. The concrete fills the hole above the gravel, less the space the post takes up. To add gate posts, footings or a slab to the same order, use the concrete calculator.

concrete per hole = (3.1416 × diameter² ÷ 4 − post width²) × (depth − gravel)

For the 14 posts in the example that is 14 cu ft. An 80 lb bag of standard concrete mix yields about 0.60 cu ft and a 50 lb bag about 0.375 cu ft,^[4] so the job takes 24 bags of 80 lb, or about 1.7 per post. Heavier jobs are easier to compare in the concrete cost calculator, and the concrete weight page lists bags per cubic yard.

Wire farm fences

Field fences use much wider spacing than wood fences. NRCS specifications allow line posts up to 14 ft apart for conventional woven wire and 25 ft for high-tensile fixed-knot wire in Kentucky,^[2] and a general maximum of 20 ft for standard fences in Missouri, with corner bracing wherever the fence turns 25 degrees or more.^[3] Enter that spacing as the maximum, choose no rails, and the calculator counts the posts. Lengths convert to meters exactly.^[5]

Common fence calculator mistakes

The most common mistake is forgetting the last post: an open fence always needs one more post than it has sections.

• Measuring the gap instead of on center. Spacing runs from the center of one post to the center of the next.
• Rounding sections down. That makes every section longer than your rails. Always round up and space the posts evenly.
• Setting posts too shallow. Bury wood posts at least a third of their length and not less than 2 ft, below the frost line.^[1]
• Skipping gravel. Put 4–6 inches of packed gravel at the bottom of each hole.^[1]
• Too few rails on a tall fence. Pickets 60 inches or longer use three rails.^[1]