Fence Calculator

Formula

Worked example

How to use this calculator

1. Walk the full fence line with a 100-foot tape and measure corner to corner, including anywhere a gate will go. Don't subtract the gate openings from the length — the calculator handles gates separately, and it needs the true run to space the sections.
2. Enter the section length. Eight feet on center is the wood-fence standard because rails and pre-made panels come in 8-foot lengths. Shorter sections cost more posts; longer ones let the rails sag.
3. Enter the actual picket width. A '1x6' cedar picket measures 5.5 inches wide, and dog-ear pickets are no exception. Enter 6 and you'll underbuy by roughly 9 percent — that's 20 missing pickets on a 100-foot run.
4. Set the gap between pickets. Zero for a privacy fence. Two to four inches for a spaced or picket style. Measure the gap you want with a scrap block and live with it for a minute — gaps look wider once they're on the fence.
5. Choose rails per section: two for fences up to 5 feet tall, three for 6-foot fences. The third rail keeps tall pickets from bowing, and bowing is what makes a five-year-old fence look fifteen.
6. Count your gates and enter the number. Each gate adds one extra post for the hinge side, and gate posts should be bigger and deeper than line posts — the calculator adds the count; you supply the beefier lumber.
7. Press Calculate. Sanity-check the post count against your mental picture of the run, then add 10 percent to the picket order. Pickets warp, split, and arrive with bark edges; the cull pile is real.

Formula

Worked example

Measurement tips

• Call 811 before you dig — it's free, it's the law in most states, and the utility locate takes a couple of business days. A post-hole digger through a gas line turns a fence project into an emergency, and the fine lands on you.
• Set line posts 2 feet deep as a minimum, and go below your local frost line if you get real winters — 3 to 4 feet in the northern states. Frost heave pushes shallow posts up a little every year until your fence looks like a piano keyboard.
• Drop 6 inches of gravel in the bottom of each hole before the post goes in. It drains water away from the end grain, which is where rot starts. Posts set straight into concrete with no drainage rot faster than posts in plain dirt.
• Keep pickets 2 inches off the ground. Pickets that touch soil wick moisture and rot from the bottom up; that two-inch shadow line at the base is the sign of a fence built by someone who's replaced one before.
• Run a mason's string line between your end posts and set every line post to it. Eyeballing post alignment over 150 feet produces a fence with a visible wave that you'll notice every single day.
• Build gates with a diagonal brace running from the bottom hinge corner to the top latch corner, and hang them on 6x6 posts. Gates sag — it's what they do — and the brace plus the beefier post buys you years before it happens.
• Face the finished side of the fence toward the neighbor (or the street) if your HOA or city requires it — many do. Nothing sours a fence project like a neighbor complaint filed the week after you finish.

Common mistakes

• Setting gate posts like line posts. A gate is a swinging, wind-catching lever bolted to one post. Skimp with a shallow 4x4 and no extra concrete, and within a year the gate drags on the ground. Gate posts want 6x6 lumber, 3 feet deep, and generous concrete.
• Digging post holes by eye instead of measuring equal sections. The eye is terrible at 8-foot intervals over distance; you end up with sections ranging from 7 to 9 feet, rails that don't land on posts, and a fence that looks drunk.
• Forgetting that the gate opening still needs its posts. People measure the run, subtract the gate width from the length, and then wonder why they're two posts short. Measure the full run; enter the gates; let the math add the posts.
• Butting pickets tight edge-to-edge in a wet climate with zero gap. Wood swells when it rains. Tight pickets buckle, pop nails, and bow outward in waves. Even a privacy fence benefits from a hairline gap or properly dried lumber.
• Mounding concrete above grade around the post like a little volcano. That dome holds water against the wood exactly where rot starts. Slope the concrete away from the post so water runs off, and keep it a couple inches below grade.
• Skipping the 811 call because 'I know where my lines are.' Nobody knows where their lines are. The locate is free, the repair bill for a severed cable or gas line is thousands, and in many states digging without a ticket is a citable offense.

Factors that affect accuracy

• Fence height. A 4-foot fence takes two rails per section; a 6-footer needs three to keep pickets from bowing. Taller fences also catch more wind, which means deeper posts and more concrete per hole.
• Terrain. Flat ground gives you clean 8-foot sections. Slopes force stepped sections — each step down needs its post set to full depth while the rails step — and rolling ground can add posts where a straight run would have had one.
• Soil type. Sandy or loose soil won't grip a post, so holes go wider and deeper with more concrete. Rocky soil fights the digger and slows you to one hole an hour. Clay holds posts well but traps water, so drainage gravel matters more.
• Fence style. A privacy fence with no gap eats roughly 30 percent more pickets than a spaced style on the same run. Board-on-board looks great from both sides but nearly doubles the picket count — budget accordingly.
• Gate count and width. Every gate needs its own posts, hinges, and latch hardware, none of which is in the picket math. Wide double gates for trailers need 6x6 posts and sometimes a center drop rod — plan the hardware before you set the posts.
• Post material. Pressure-treated 4x4s are the default; 6x6s for corners and gates resist twist far better. Cedar posts look nice but rot faster in the ground than treated pine — many builders use treated posts with cedar pickets and never look back.

Frequently asked questions