Fence Calculator: How Many Fence Posts and Pickets Do I Need?

Pickets per 8 ft section

Counts are rounded up per section. The 5.5 in pickets with a 2 in gap match the spacing example in Lowe's guide (7.5 inches per picket plus space).

Picket spacing, board-on-board, and pool fences

281 pickets for 100 ft of board-on-board fence with 5.5 in boards, against 230 with the same boards edge to edge. Lowe's shadowbox layout spaces the pickets 3 1/2 in apart on each side of the rails and overlaps them 1 in on each side, so every 9 in of fence takes 2 pickets. The calculator has no overlap field: enter the picket width minus the overlap (4.5 in) and a 0 gap. Pool fences have a limit instead, set by the CPSC's barrier guidelines:

To make the gaps come out even in an 8 ft section, divide 96 in by the picket width plus the gap you want, round up to whole pickets, then divide 96 by that count and subtract the picket width: 5.5 in pickets with a 2 in gap fit 13 per section at 1.88 in, and 3.5 in pickets fit 13 at 3.88 in, under the 4 in pool limit. The CPSC also calls for a pool barrier at least 48 in high with no more than 4 in of clearance under it, or 2 in over grass or pebbles.

Posts and pickets by fence length

Straight runs from the calculator, with 5.5 in pickets, no gap, and 5 percent extra. Lowe's fence guide adds 1 post for each gate; subtract 1 post for each corner that two runs share.

Fencing a pentagon or other odd-shaped yard

100 ft of fence for a pentagon with 20 ft sides: the perimeter of a regular pentagon is 5 times one side, and for an irregular one you add up its 5 sides. A closed fence needs a post at every corner, so its posts equal the sections on all the sides added together. With posts 8 ft apart:

With the calculator, run each side separately and subtract 1 post per corner: each 20 ft side of the pentagon gives 3 sections and 4 posts, so 5 × 4 = 20 posts, less 5 shared corners, is 15, and the rails are 5 × 6 = 30. That's one post more than a straight 100 ft run, because each 20 ft side needs its own short section. For pickets, enter the whole perimeter once: 100 ft takes 230. Lowe's adds 1 post for each gate.

If you know only the area of a regular pentagon, each side is the square root of the area divided by 1.7205, from MathWorld's area formula (about 1.7205 × side²): 1,000 sq ft gives sides of 24.1 ft and 120.5 ft of fence. For a slanted side, measure it on the string line, or square the two legs, add them, and take the square root: a corner cut 30 ft by 40 ft is 50 ft long.

Rails, post depth, and holes

Frost depth and local code can require deeper holes, so check with your building department. For tall fences or windy, exposed sites, a fence contractor can confirm post size and depth.

Fence post depth by fence height

24 to 36 in for a 6 ft fence and 16 to 24 in for a 4 ft fence: Quikrete's post-setting guide puts the hole at 1/3 to 1/2 of the post's height above ground. Quikrete's data sheet and Sakrete's give the deep end as 1/3 of the post's overall length, the same as the 1/2 column, so a 9 ft post goes 3 ft into the ground under a 6 ft fence:

The post lengths are the fence height plus the hole depth. Quikrete puts about 6 in of gravel in the bottom of the hole, so a post that stands on it needs a hole 6 in deeper to keep the same depth in the ground. Height isn't the only limit: Lowe's notes that some codes require post holes below the frost line, and University of Wisconsin-Extension's fencing guide says a post about 4 ft deep usually gets below it in most northern climates, where freezing and thawing in silt and clay soils can push posts out of the ground. The calculator doesn't size the hole; for a deeper one, raise the bags per post, using the chart in the next section.

How much concrete per fence post?

The calculator defaults to 2 bags per post, but the right number depends on your hole. Quikrete's data sheet for 50 lb Fast-Setting Concrete Mix gives a yield of about 0.375 cu ft per bag and says two bags will set a 3 in square post in a 9 in diameter hole 30 inches deep.

Hole width matters more than most people expect, because the volume grows with the square of the diameter. A 4x4 post actually measures 3.5 × 3.5 in, so Quikrete's "3 times the post width" rule gives a hole about 10.5 in across. Scaling from Quikrete's two-bag figure, that hole takes roughly 3 bags at 30 inches deep. A 12 in hole holds nearly twice as much concrete as the 9 in hole, or about 4 bags. Set the bags-per-post field to 3 and the example fence needs 42 bags instead of 28; at 4 bags it needs 56. With regular 60 or 80 lb bags instead, the concrete calculator guide lists their yields.

Quikrete's post-setting guide, which pours the dry mix into the hole and then adds water, calls for about a gallon of water per 50 lb bag, filling the hole to 3 to 4 inches below ground, and waiting about 4 hours before putting heavy loads on the posts. The mix sets in 20 to 40 minutes. For the gravel layer, Quikrete's shopping list for the job includes 50 lb bags of All-Purpose Gravel.

Bags of concrete per post by hole size

2.5 bags of 50 lb fast-setting mix for a 4x4 post in a 10 in hole 30 in deep, and 3.7 bags in a 12 in hole, with 6 in of gravel in the bottom and concrete to the top. Each cell is the hole's volume less the post's, divided by Quikrete's yield of 0.375 cu ft per bag:

A 4x4 measures 3 1/2 in square and a 6x6 5 1/2 in, according to the Southern Forest Products Association, and the 16 in hole is about 3 times the 6x6's width. The method agrees with the makers' figures: it gives 2.02 bags for Quikrete's example of a 3 in post in a 9 in hole 30 in deep (the data sheet says two), and Sakrete's chart for an average 10 in hole, at 0.38 cu ft per 50 lb bag, lists 2, 2.5, and 3 bags at 18, 24, and 30 in deep. Stopping 3 to 4 in below ground, as Quikrete's guide does, takes a little off. In the calculator, round up to the half bag: at 2.5 bags per post, the 100 ft example fence needs 35 bags.

Tips for buying fence materials

• Order extra pickets. Lowe's recommends increasing the picket count by 10 percent. The calculator defaults to 5 percent; raise it if you expect to cull warped or split boards.
• Buy rails at least as long as your spacing. With 8 ft on center, 8 ft rails span post to post.
• Plan the gates first. If you buy a prebuilt gate, subtract the opening from the length you enter for pickets. If you build the gate from the same pickets, leave it in.
• Count hardware. Lowe's guide lists one post cap per post. Gates need their own hardware, and fasteners depend on your rail and picket count.
• Use the actual picket width. Measure a board at the store. A small difference adds up over 100 feet.

Common mistakes to avoid

• Forgetting extra posts. Each corner, end, and gate side needs one.
• Double-counting shared corners when you add up several runs.
• Using nominal picket width. A "1x6" is 5.5 inches wide, not 6.
• Undersizing concrete. Wider holes take far more concrete than the 2 bag rule of thumb.
• Digging before utilities are marked. Call 811 a few business days ahead.