How this instrument works
This calculator is scoped to the framing skeleton of a wood fence — posts and horizontal rails — not the pickets or panels that get attached to it afterward. Posts follow the standard fence-estimating pattern: every panel span needs a post at each end, and adjacent spans share a post, so the count is one more than the number of spans (posts = ceil(fence length / post spacing) + 1). That '+1' is why a 100 ft fence on 8 ft spacing needs 14 posts, not 12 or 13 — 12.5 spans round up to 13, plus the extra shared post at the far end.
Rails are simpler: each horizontal rail is a full-length course that runs the entire fence, so the total linear feet of rail lumber is just the number of rails per section multiplied by the fence length. A 2-rail fence (typical for shorter picket or standard-height fences) needs two such courses; a 3-rail fence (typical for 6 ft+ privacy fences, which carry more weight and wind load) needs three. This is a linear-footage total — it doesn't pre-cut that total into your lumber yard's actual stock lengths (commonly 8 ft or 16 ft boards), so budget extra for joints, cuts, and waste, and add more for any gates built into the run.
Post spacing of 6-8 ft and a 2- or 3-rail count are common defaults, not fixed rules — adjust them for your fence's height, style, and local wind exposure, since taller or heavier fences typically need closer post spacing and more rails. This tool estimates materials only; it doesn't size posts, footing depth, or concrete, and it isn't a wind-load or structural design. For a tall privacy fence in a high-wind area, or wherever your local code sets a minimum post embedment depth, confirm your spacing and post size against your building department before you dig.
- Enter the total fence length in feet.
- Enter your post spacing in feet — 6 to 8 ft is typical for wood fences, but check your style, height, and local code.
- Enter how many horizontal rails each section uses — 2 is typical for shorter fences, 3 for taller privacy fences.
- Read the number of posts and the total linear feet of rail lumber needed.
- Round up to your lumber yard's standard board lengths and add margin for cuts, waste, and any gates before ordering.
A 100 ft backyard fence on standard 8 ft post spacing
A 100 ft run of wood fence, set on the common 8 ft post spacing with a standard 2-rail section: posts = ceil(100/8) + 1 = ceil(12.5) + 1 = 13 + 1 = 14 posts. Rail lumber: 2 rails per section x 100 ft = 200 linear ft of rail — two full-length courses running the entire run, before rounding to actual board stock and adding waste.
Compare a taller 6 ft privacy fence covering 150 ft: tighter 6 ft post spacing and a third rail (common for privacy-height fences carrying more weight and wind load) give posts = ceil(150/6) + 1 = 25 + 1 = 26 posts, and rail = 3 x 150 = 450 linear ft — more than double the shorter fence's rail total, from both the added rail course and the tighter spacing driving up the post count.
Questions
Why is the post count one more than fence length divided by spacing?
Because posts are shared between adjacent panel spans. A run divided into 13 spans has a post at the start of the first span, one at the end of every span after that, and one final post closing out the last span — 14 posts total for 13 spans. The '+1' accounts for that extra shared post at the ends of the run, the same convention used for picket and panel fence counts generally.
How far apart should wood fence posts actually be?
6 to 8 ft on-center is the common range for most wood fence styles, and it's a default you should adjust rather than treat as fixed — taller privacy fences, board-on-board designs, and high-wind sites typically call for the tighter end of that range (or closer) since more fence surface area means more wind load pushing on the posts. Check your fence style, height, and local code before finalizing spacing.
How many rails does my fence actually need?
Two rails is standard for shorter picket or standard-height fences (roughly 3-4 ft), while three rails (top, middle, bottom) is standard for 6 ft+ privacy fences, which carry substantially more dead weight and wind load and sag more without the extra support. This calculator leaves the rail count as an adjustable input rather than assuming one fixed number, since it varies by fence style and height.
Does this calculator include pickets, concrete, or post footings?
No — it's deliberately scoped to just posts and rails, the framing skeleton of the fence. Picket or panel-board counts are a separate calculation (see this site's picket and board-on-board fence calculators), and this tool doesn't estimate concrete volume or footing depth, which depend on your soil, frost line, and local code.
Does the rail total already account for standard board lengths?
No — it's a raw linear-footage total (rails per section x fence length), not pre-cut into 8 ft or 16 ft stock lengths. In practice you'll round each course up to your lumber yard's available board lengths, stagger the joints between posts for strength, and add extra for waste and any gates cut into the run.
Do I need a permit or engineered design for a wood fence?
Most standard backyard wood fences don't need a full engineered structural design, but that's a local-code question, not a universal answer. Height limits, permit requirements, and wind-load provisions for tall privacy fences vary by municipality — check with your local building department, especially for fences over 6 ft or in high-wind areas, before you build.