How the Fence Calculator works
The calculator counts the posts, sections, rails and pickets for a straight run of wood fence, with extra posts for gates. Turn on concrete to also get the concrete for each post hole and the number of 50, 60 or 80 lb bags.
Sections = fence length ÷ post spacing, rounded up. Posts = sections + 1, plus 1 for each gate so it has a post on both sides. Rails = sections × rails per section (about one rail per 2' of height). Pickets = fence length ÷ (picket width + gap), rounded up. Concrete per post = the round hole's volume minus the post; bags = total ÷ bag yield (50 lb fast-setting ≈ 0.375 cu ft, 60 lb ≈ 0.45, 80 lb ≈ 0.60).
Example: 100' of fence with posts 8' on center, one gate, 2 rails and 5 1/2" pickets butted tight. 100 ÷ 8 = 12.5, so 13 sections; 13 + 1 + 1 gate = 15 posts; 13 × 2 = 26 rails; 1,200" ÷ 5.5" = 218.2, so 219 pickets. Setting 4×4 posts (3 1/2" actual) in 12" holes with 24" of concrete: π × 6² × 24 = 2,714 cu in, minus the post 3.5 × 3.5 × 24 = 294, leaves 2,420 cu in = 1.40 cu ft per post. 15 posts take 21.0 cu ft, and 21.0 ÷ 0.60 = 35.02, so 36 bags of 80 lb.
Keep wood posts 8' apart or less, and closer to 6' for tall fences or windy sites. Dig holes about 3× the post width and 1/3 to 1/2 as deep as the post stands above ground, below the frost line where your local code requires it, and call 811 before you dig. For a fence with corners, work out each side separately so every corner gets a post, then count each shared corner post once.
Frequently asked questions
How many fence posts do I need?
Divide the fence length by the post spacing and round up to get the sections; you need one more post than sections, plus one per gate. A 100' fence at 8' spacing has 13 sections, so 14 posts, or 15 with one gate. If the fence turns corners, work out each straight side on its own and count shared corner posts once.
How far apart should fence posts be?
For a wood fence, 8' on center is the usual maximum, which matches common 8' rails. Tall fences and windy sites do better at about 6'. Vinyl and pre-built panels set the spacing by the panel width, so follow the maker.
How deep should a fence post hole be?
A common rule is 1/3 to 1/2 of the post's height above ground, so a 6' fence needs holes about 2' to 3' deep, and below the frost line where that is required. Make the hole about 3× the post width: roughly 10" to 12" across for a 4×4. Leave out any gravel at the bottom when you work out the concrete depth.
How many bags of concrete per fence post?
A 4×4 in a 10" hole with 24" of concrete takes about 0.92 cu ft: π × 5² × 24 = 1,885 cu in, minus 294 cu in for the post, = 1,591 cu in. That is about 1 1/2 bags of 80 lb mix (0.60 cu ft each) or about 2 1/2 bags of 50 lb fast-setting mix (0.375 cu ft each). Wider or deeper holes add up quickly, so total the whole job before you buy.
How many pickets do I need for an 8 foot section?
Divide the section length in inches by the picket width plus the gap. With 5 1/2" pickets butted tight, 96 ÷ 5.5 = 17.45, so 18 pickets; with a 1/4" gap, 96 ÷ 5.75 = 16.7, so 17. Nominal 6" pickets are usually 5 1/2" wide, so measure yours.