How the fence calculator works
A straight fence is a row of sections with a post at every joint and at both ends, so there is always one more post than there are sections. How the sections are laid out depends on the type of fence.
Boards & rails: sections = length ÷ max post spacing, rounded up posts = sections + 1 boards = (length + gap) ÷ (board width + gap), rounded up rails = sections × rails per section Panels between posts: each bay = panel width + post width Panels on post faces: posts one panel width apart, center to center Either way: full panels until the last bay, which is cut to fit
Examples
Board fence: 20 ft, posts at most 8 ft apart
20 ÷ 8 = 2.5, which rounds up to 3 sections and 4 posts, spaced 6 ft 8 in apart. A 6 ft privacy fence with 3 rails per section uses 9 rails. Its 5½ in boards cover 240 in: 240 ÷ 5.5 = 43.6, so 44 boards, or 49 with 10% extra.
Panel fence: 20 ft of 8 ft panels with 4×4 posts
Each bay takes 8 ft plus a 3½ in post. Two full panels and three posts use 16 ft 10½ in. The rest of the run needs a fourth post and a last panel cut to 2 ft 10 in. Spreading the posts evenly instead would mean cutting every panel.
Choosing your inputs
Length
Measure each straight run from the outside of the first post to the outside of the last. On a keyboard you can type it as you read it from the tape, for example 98' 6". Calculate each straight run separately; corners and gates start new runs.
Post spacing and panel width
For boards and rails, fence installation guides commonly call for posts no more than 8 ft apart, with closer spacing for a stronger fence (for example this redwood fence guide). Enter the most you are willing to use and the calculator spaces posts evenly at or below it. For panels, measure the actual panel and choose how it attaches: between posts puts posts one panel width apart, clear between posts; to post faces puts them one panel width apart, center to center.
Board and post sizes
Lumber is sold by nominal size, which is larger than the real board. Under the American Softwood Lumber Standard (NIST PS 20), a dry “1×6” is ¾ × 5½ in, a “4×4” is 3½ × 3½ in and a “6×6” is 5½ × 5½ in. Fence pickets vary more: many are ⅝ in thick and 5½–5⅝ in wide, so measure the boards you buy.
Post depth and concrete
By default the calculator follows QUIKRETE’s post-setting instructions: set about one-third of the post’s total length in the ground (half its height above ground), in a hole about three times the post’s width, with about 6 in of gravel below the post. Frost depth, soil and local rules can require more, so check with your local building department, and call 811 before you dig. Concrete per post is the round hole minus the space the post takes up; the gravel is not included.
Common mistakes
- The fence-post error: counting one post per section. You always need one extra post to close the run.
- Using nominal widths. Dividing by 6 instead of 5½ for 1×6 boards leaves you about 8% short.
- Forgetting the post width with panels. Twelve 8 ft panels need more than 96 ft once the posts are added.
- Forgetting gates and corners. Each gate needs its own posts and hardware.
Limitations
This version handles one straight run on level ground. Slopes, stepped fences, gates and corners are not calculated yet. Results are material estimates for planning, not structural or code advice.
Frequently asked questions
How many posts do I need for a 100 ft fence?
For a board fence with posts at most 8 ft apart: 100 ÷ 8 = 12.5, so 13 sections and 14 posts, about 7 ft 8 in apart. With 8 ft panels and 4×4 posts, 12 panels and 13 posts fill 99 ft 9½ in; the last 2½ in is spread across the bays.
How many pickets are in 8 feet of fence?
8 ft is 96 in. With 5½ in boards and no gap: 96 ÷ 5.5 = 17.5, so 18 boards. With 3½ in pickets and a 2½ in gap: (96 + 2.5) ÷ 6 = 16.4, so 17 pickets.
How much concrete per fence post?
For a 6 ft fence, a 10 in hole set 3 ft deep (the manufacturer guidance above) around a 4×4 post takes about 1.38 cu ft: roughly 2.3 bags of 80 lb mix or 3.7 bags of 50 lb fast-setting mix. Choose your bag size underPosts & concrete.
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