Fence calculator: posts, pickets, rails and concrete

About 18 boards 5 1/2 in wide, if they are butted tight across a 96 in section measured post center to post center: 96 ÷ 5.5 = 17.5, rounded up to 18. With 3 1/2 in boards it is 28, and with 7 1/4 in boards 14. If the boards sit between the posts instead of across them, measure the clear opening, about 92 1/2 in with 4x4 posts, and divide that. Board-on-board with a 1 1/2 in lap takes 24 boards per section. Add about 10% for splits, knots and cuts when you order for a whole fence rather than one section.

Work from the clear opening between posts and solve for the gap. Pickets = (opening − gap) ÷ (picket width + gap), rounded to a whole number; then the real gap = (opening − pickets × width) ÷ (pickets + 1). For a 92 1/2 in opening and 3 1/2 in pickets with a 2 1/2 in target gap, that is 15 pickets and gaps of exactly 2 1/2 in. With 5 1/2 in pickets it is 11 pickets and gaps of about 2 11/16 in. Rip a spacer block to the real gap and use it for every picket, so the last gap matches the first.

Divide the fenced length in inches by the board width plus any gap, round up and add about 10%. Buying by the bundle, cedar versus pine and whole-yard examples are in how many fence boards, pickets and panels you need.

Usually 6 to 8 ft on center for wood fences, matched to the rail or panel length, with 6 ft for tall or windy fences. Depth, height and spacing rules in detail are in fence post spacing, depth and height.

At least 24 in, and about one third of the post's height above ground, so roughly 24 in for a 6 ft fence and 32 in for an 8 ft fence. That is a trade rule of thumb, not a code figure. Where the ground freezes, the hole should reach below the local frost depth so frost heave cannot jack the post up, which can mean 36 in or more in cold regions. Gate posts and corner posts take more load, so set them deeper or in a wider hole. Your post length is the fence height plus the hole depth, so deeper holes also mean longer posts.

For a 4x4 post in a 10 in hole 24 in deep, about 0.92 cu ft once the post is subtracted, which is 2.5 bags of 50 lb or 1.6 bags of 80 lb. In a 12 in hole 30 in deep it rises to about 1.75 cu ft, or 3 bags of 80 lb. Round up per hole if you mix one hole at a time; across many posts the pooled total is lower. Bag yields come from the bag label: 80 lb makes about 0.60 cu ft and 50 lb about 0.375 cu ft. The calculator prints both the per-post and the total count.

Yes. The post fills part of the hole, so the concrete is the hole volume minus the post volume. A 4x4 post is 3 1/2 in square, and in a 10 in hole 24 in deep it takes up 0.17 cu ft of the 1.09 cu ft hole, about 16%. Over the 14 posts of a 100 ft fence that is roughly 6 bags of 50 lb you would otherwise overbuy. Larger posts in small holes matter even more: a 6x6 removes 0.42 cu ft per 24 in of depth. Any gravel at the bottom of the hole also comes off the concrete, which the calculator handles with its gravel input.

Two rails for fences up to about 4 ft, three for a 6 ft fence and four for 7 to 8 ft fences, which works out to about one rail for every 2 ft of height. Each section gets one rail per row, cut to the post spacing, so a 100 ft fence at 8 ft spacing has 13 sections and needs 39 rails at three per section before waste. A 6 ft board fence on only two rails tends to cup and warp between them, and the calculator warns about it. Place the top and bottom rails a few inches in from the ends of the boards so the board ends stay flat.

Each side of the fence is divided into sections on its own, and each gate adds one post. A straight 200 ft run at 8 ft spacing with one 4 ft gate needs 27 posts, but a yard of 100, 50 and 50 ft sides with one gate needs 28, because each 50 ft side ends at a corner and cannot share a partial section with the next side. A closed loop needs one post fewer than an open run of the same sections, since it ends where it starts. Enter up to four sides in the calculator to count corners correctly.

About 10% extra on pickets and rails is usual, and 5% is enough for straight runs of good stock. The extra covers split boards, knots, cut-offs at gates and corners, and boards that are badly bowed. Do not add waste to the fence length itself, or the post count goes up for no reason. Posts, panels and concrete bags are exact counts, but one spare post covers a cracked one or a hole that hits a rock. The calculator applies the waste setting to boards and rails only.

Often not for a low fence, but check locally. The 2021 International Residential Code exempts "Fences not over 7 feet (2134 mm) high" from a building permit in Section R105.2, and many towns adopt that line, while some lower it to 6 ft. Zoning is separate: most towns set height limits by location, commonly lower in front yards, and may require a zoning permit or a survey of the property line even when no building permit is needed. Pool barriers and homeowner association rules add their own limits.

A 1x6 fence board is about 5 1/2 in wide and a 1x4 about 3 1/2 in. Dressed 1x lumber is 3/4 in thick, but many fence pickets are milled to about 5/8 in, and rough-sawn cedar can run a little wider. Pickets are commonly sold in 4, 5, 6 and 8 ft lengths with flat, dog-ear or gothic tops. Because the count is length divided by width, using 6 in instead of 5 1/2 in undercounts a 100 ft fence by about 19 boards. Measure a board from the bundle and type that width into the calculator.

Boards = (fenced length − board width) ÷ (board width − overlap) + 1, rounded up, counting both layers together. For an 8 ft section of 5 1/2 in boards with a 1 1/2 in overlap on each edge that is 90.5 ÷ 4 + 1 = 24 boards; for 100 ft with a 4 ft gate it is 288 boards, or 317 with 10% waste. The back layer is spaced with gaps narrower than the boards, and the front layer covers each gap. Keep the overlap under half the board width. Shadow box fences are counted the same way.

Fence height plus hole depth. A 6 ft fence on 24 in holes needs 8 ft posts, a 6 ft fence on 30 in holes needs 8 ft 6 in, which means buying 10 ft posts, and an 8 ft fence on 36 in holes needs 11 ft, so 12 ft posts. Posts are stocked in even lengths, so round up to the next one. Leave a little extra if the post will be trimmed to height after the concrete sets, or if the ground slopes and the downhill posts of a stepped fence stand taller. The calculator prints both the exact length and the stock length.

For 5/8 to 3/4 in pickets, 2 in (6d) to 2 1/2 in (8d) ring-shank nails or 2 in exterior screws are the usual choice, a rule of thumb that keeps the fastener about two and a half to three times the board thickness. Use hot-dip galvanized or stainless steel, because treated lumber, cedar and redwood corrode plain steel and leave dark streaks. Put two fasteners per board at each rail. The default 100 ft fence with three rails needs about 1,386 fasteners for the boards alone, plus a few at each rail end.

Measure the length along the ground, then decide between racked and stepped. A racked fence follows the slope with plumb boards, so materials stay close to a flat fence. A stepped fence keeps each section level and drops it at the posts, so the downhill posts must be longer by the drop per section, and the triangular gaps under each step need filling or longer boards. For a 2 ft fall across four 8 ft sections, each step drops 6 in. Order post lengths for the tallest post on each step.

About 835 ft for a square acre. An acre is 43,560 sq ft, so a square acre is about 208.7 ft on each side, and four sides make about 835 ft. A half acre square takes about 590 ft and a quarter acre about 417 ft. Real lots are rarely square: a long, narrow acre needs more fence than a square one, so measure the actual sides. Subtract any side formed by a building, then enter the sides in the calculator to count posts, rails and boards for that length.