A wood fence is a repeating pattern of posts, rails and pickets, so estimating it comes down to counting sections. This fence calculator takes the total fence length, post spacing and height and returns the number of posts, horizontal rails and vertical pickets, plus how much concrete to set the posts. It works for privacy fences (pickets butted tight), spaced picket fences and shadowbox styles if you count both faces.
Most residential wood fences use posts 6 to 8 feet apart on center, which matches the common 8-foot length of rails. A widely used installation rule, repeated in guides such as This Old House's post-setting instructions, is to bury about one-third of the post's length and add roughly 6 inches of gravel at the bottom of each hole for drainage. If you plan to stain or paint the finished fence, the paint calculator can estimate coverage; for hardwood or custom lumber pricing, see the board foot calculator.
How to use the fence calculator
- Measure the full fence line in feet. Measure each run separately if it turns corners and add them together.
- Enter the post spacing. 8 ft is standard with 8 ft rails; use 6 ft for heavier gates, windy sites or tall fences.
- Enter the fence height and how many rails each section has: two rails for fences up to about 4 ft, three for 5–6 ft privacy fences.
- Enter the actual picket width (a nominal 1×6 is 5.5 in) and the gap. Use 0 for a privacy fence.
- Add gates, pick the post size and enter how deep the posts will be buried and the hole diameter to estimate concrete.
Formula
Post length is the fence height plus the buried depth. Concrete bags use typical yields of 0.6 ft³ per 80 lb bag and 0.375 ft³ per 50 lb bag; check your product's label.
Worked example
A 100 ft long, 6 ft tall privacy fence with posts 8 ft apart, 3 rails, 5.5 inch pickets with no gap, 4×4 posts buried 3 ft in 10 inch holes.
- Sections: 100 ÷ 8 = 12.5, rounded up to 13.
- Posts: 13 + 1 = 14, each 6 + 3 = 9 ft long (buy 10 ft stock and trim, or 8 ft posts if you bury only 2 ft).
- Rails: 13 × 3 = 39 eight-foot rails.
- Pickets: 100 × 12 = 1,200 in ÷ 5.5 = 218.2, so 219 pickets.
- Concrete per hole: π × (5 ÷ 12)² × 3 − (3.5 ÷ 12)² × 3 = 1.636 − 0.255 = 1.38 ft³. For 14 posts: 19.3 ft³ ≈ 33 bags of 80 lb.
Nominal vs actual lumber sizes
Lumber is sold by nominal size, but the finished dimensions are smaller. The American Softwood Lumber Standard (PS 20) sets the dressed sizes: a 4×4 post is 3½ × 3½ inches, a 6×6 is 5½ × 5½, a 2×4 rail is 1½ × 3½, and a 1×6 board is ¾ × 5½. Always enter the actual picket width, or the picket count will be about 9% too low.
| Nominal | Actual (dry, dressed) |
|---|---|
| 1×4 | ¾ × 3½ in |
| 1×6 | ¾ × 5½ in |
| 2×4 | 1½ × 3½ in |
| 4×4 | 3½ × 3½ in |
| 6×6 | 5½ × 5½ in |
Shadowbox and board-on-board fences
On a shadowbox fence, pickets alternate on each side of the rails, and on board-on-board they overlap. Both use more pickets than a flat privacy fence. A simple way to estimate is to enter the gap between pickets on one face, then double the result. For example, 5.5 inch pickets with a 3.5 inch gap on each face gives 134 pickets per face over 100 ft, or 268 in total.
Caveats and assumptions
- Local codes and HOA rules may limit fence height and set back fences from property lines. Call 811 before you dig so buried utilities can be marked.
- Frost depth varies widely. In cold climates, posts may need to go below the local frost line regardless of the one-third rule.
- Uneven or sloped ground changes picket lengths and may need stepped or racked sections.
- Gates usually need stronger posts and hardware. This calculator adds one post per gate but does not size gate frames.
Frequently asked questions
How many fence posts do I need?
Divide the fence length by the post spacing, round up, and add one. A 100 ft fence with posts every 8 ft needs 13 sections and 14 posts, plus one more for each gate and extra posts at corners if runs are measured separately.
How many pickets for 100 feet of fence?
With 5.5 inch pickets butted tight: 1,200 ÷ 5.5 = 219 pickets. With a 2 inch gap: 1,200 ÷ 7.5 = 160 pickets.
How deep should fence posts be?
A common rule of thumb is about one-third of the post length in the ground, so a 6 ft fence uses 9 ft posts set 3 ft deep, plus about 6 inches of gravel at the bottom. In frost-prone areas, follow local frost-depth requirements.
How many bags of concrete per fence post?
For a 4×4 post buried 3 ft in a 10 inch hole, about 1.38 ft³ — a little over two 80 lb bags. Wider or deeper holes need more.
What spacing should fence posts have?
Usually 6 to 8 ft on center. Eight feet matches standard rail lengths; closer spacing makes a stiffer fence in high wind.
Sources
- NIST — Voluntary Product Standard PS 20 (American Softwood Lumber Standard) — nominal and dressed lumber sizes
- This Old House — How to Install a Fence Post — post depth rule of thumb and gravel at the bottom of the hole
- Call 811 — Know what's below — free utility locating before digging
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.