Answer: Enter your values and the Fence Calculator returns the exact result instantly — formula, worked example, and a plain-English explanation are included below the tool.
Posts, rails, and pickets from perimeter length
This fence calculator estimates posts, sections, rails, pickets, and the concrete needed to set posts for a linear run of fencing. Enter the fence length, post spacing, rails per section, picket width, and gap; the tool rounds each material to whole purchasable units the way a fence supplier would.
The math is straightforward linear arithmetic: posts = ceil(length ÷ spacing) + 1 (one extra post to close the run), sections = posts − 1, rails = sections × rails per section, and pickets = ceil(length ÷ ((picket width + gap) ÷ 12)) for widths entered in inches. Post concrete is estimated at two 80 lb bags per post, the standard setting for a 6-foot fence in most soils — about 1.2 ft³ of mixed concrete per post from two bags (each 80 lb bag yields ≈ 0.60 ft³, with the two-bag figure allowing for a partially filled hole).
This page explains each formula, provides a material table for common fence lengths, works a full example for a 100-foot run, and answers the questions that come up when ordering fence materials.
Posts: a run of L feet with posts spaced S feet apart needs ceil(L ÷ S) + 1 posts — the division counts the gaps, the +1 closes the far end. Spacing is measured on-center. The 8 ft default matches the longest common pressure-treated post spacing; 6 ft is typical for wind-exposed or picket fences.
Rails: each section between two posts carries the same rail count — 2 for picket and privacy fences under 6 ft, 3 for taller or heavy fences. Rails (in linear feet of lumber) = sections × rails per section × spacing. A 100 ft run at 8 ft spacing has 13 sections × 2 rails × 8 ft = 208 linear feet of rail boards.
Pickets: spacing is set by picket width plus gap, both in inches — privacy style forces gap = 0. A 5.5-inch picket with no gap gives 12 ÷ 5.5 = 2.18 pickets per foot, so 100 ft needs ceil(100 × 12 ÷ 5.5) = 219 pickets. Style 'rail' fences skip pickets entirely. Post-hole geometry sits behind the concrete estimate. A typical 4×4 post in a 6-foot privacy fence uses an 8-to-10-inch diameter hole dug 24 to 36 inches deep — below the frost line in cold climates. Two 80 lb bags yield about 1.2 ft³ of mixed concrete, right for a 10-inch hole at 30 inches minus the post's own volume, with enough left to crown the top so water runs off.
Setting technique matters as much as count: place and plumb each post before the concrete stiffens, slope the crown, and brace tall corner posts until cure. Standard bagged concrete reaches fixture strength in 24 to 48 hours, but most crews wait three to seven days before hanging rails — schedule the second visit rather than trusting an early cure in cool weather.
The table below shows calculator output for 50, 100, and 200-foot privacy fence runs at 8 ft post spacing, 2 rails per section, and standard 5.5-inch pickets butted with no gap. Posts and pickets are buy-list numbers; rails are shown in linear feet of board.
Concrete is two 80 lb bags per post. For gates, add one extra pair of posts at the gate opening and a gate frame kit; the calculator models a single continuous run. Fasteners and hardware complete the buy list: figure roughly 2 exterior-grade screws or nails per picket per rail, a bracket or bracket-and-screw set for each rail-to-post connection, and a gate kit with hinges if your run includes one. Treat these as a hardware-store run sized from your rail and picket counts — the arithmetic above gives the quantities they attach to.
| Fence length | Posts (8 ft spacing) | Sections | Rails (2/section) | Rail lumber (LF) | Pickets (5.5", no gap) | 80 lb concrete bags |
|---|---|---|---|---|---|---|
| 50 ft | 8 | 7 | 14 | 112 | 110 | 16 |
| 100 ft | 14 | 13 | 26 | 208 | 219 | 28 |
| 200 ft | 26 | 25 | 50 | 400 | 437 | 52 |
Posts: ceil(100 ÷ 8) + 1 = 14 posts, forming 13 sections. Rails: 13 × 2 = 26 boards = 208 linear feet. Pickets: ceil(100 × 12 ÷ 5.5) = 219 five-and-a-half-inch pickets butted edge-to-edge. Concrete: 14 × 2 = 28 eighty-pound bags — just over half a cubic yard of mixed concrete across the run.
At a 6 ft spacing the same run needs ceil(100 ÷ 6) + 1 = 18 posts (17 sections, 34 rails) — a 29% increase in posts and concrete in exchange for a stiffer fence. Use the tighter spacing in high-wind areas or where the fence carries shadow-box or heavy cedar pickets. Corners and grade changes add material quietly. Every 90-degree corner wants its own braced post, and a run that steps down a slope needs posts cut to different heights, which shortens usable rail stock. A practical rule is to add 5% to posts and 10% to pickets for runs with more than two corners or visible slope, then let the whole-number round-up absorb the remainder.
How many fence posts do I need for 100 feet?
At 8-foot spacing: ceil(100 ÷ 8) + 1 = 14 posts (13 sections). At 6-foot spacing: ceil(100 ÷ 6) + 1 = 18 posts. Always add one post beyond the gap count to close the run, plus an extra pair if you're framing a gate.
How far apart should fence posts be?
8 feet is the maximum for most wood privacy fences with 2 rails; 6 feet gives a stiffer fence for high-wind sites, shadow-box styles, or 3-rail designs. Chain-link and vinyl typically run wider spacing per their system specs.
How do I calculate pickets for a fence?
Pickets = fence length in feet × 12 ÷ (picket width in inches + gap in inches), rounded up. For a 100 ft privacy fence with 5.5-inch pickets and no gap: 100 × 12 ÷ 5.5 = 218.2 → 219 pickets.
How many bags of concrete per fence post?
Two 80 lb bags per post is the standard for a 6-foot fence in typical soil — roughly 0.6 ft³ of mixed concrete per bag. In loose sand or frost-heave climates, use three bags or a 12-inch-diameter hole; in firm clay, one bag plus tamped backfill may suffice.
How much rail lumber does a fence need?
Rails = sections × rails per section × post spacing. A 100 ft run at 8 ft spacing with 2 rails per section: 13 × 2 × 8 = 208 linear feet of rail board. Add 5–10% for cutting waste and culls at joints and corners.