Introduction
Every fence material list reduces to five formulas: pickets from linear feet, posts from spacing, rails from sections, concrete from posts, and stain from area. None of them is complicated — the skill is knowing where each constant comes from, so you can adapt them when your fence doesn’t match the defaults.
The Wood Fence Calculator runs these five calculations automatically. This guide derives each one from first principles — board widths, post geometry, section logic — with a fully worked example cross-checked in imperial and metric, so you can verify the calculator’s output by hand.
Formula 1: Picket Count
Pickets = Linear feet × Boards per foot × (1 + Waste%), rounded up.
Where “boards per foot” comes from
A nominal 1×6 picket has an actual width of 5.5 in (14 cm). A privacy fence covers 12 in per foot, so the bare coverage rate is:
12 in ÷ 5.5 in = 2.18 boards per foot
In practice this rounds to ~2.0 boards/ft for the geometric coverage, because pickets butt with hairline gaps and lumber is never exactly 5.5 in. The waste allowance (default 10%) covers miscuts, splits, and warped boards you reject. Effective rate: 2.0 × 1.10 = 2.2 boards per foot.
Metric cross-check: 5.5 in = 14 cm; 100 cm ÷ 14 cm = 7.14 boards/m geometric; × 1.10 = 7.9 boards/m effective. And 2.2 boards/ft × 3.281 ft/m = 7.22 boards/m — the two systems agree within rounding (the calculator’s exact figure is ~7.2 boards/m).
Style adjustments
| Style | Boards/ft | Boards/m | Why |
|---|---|---|---|
| Flat privacy (butted) | 2.0 (2.2 w/ waste) | 7.2 w/ waste | Full coverage, hairline gaps |
| Board-on-board (overlapped) | 2.3 (2.5 w/ waste) | 8.2 w/ waste | ~15% extra for overlap layer |
| Shadowbox (alternating) | 2.0 (2.2 w/ waste) | 7.2 w/ waste | Same count, split front/back |
| Spaced picket, 2.5-in gaps | 1.5 (1.65 w/ waste) | 5.4 w/ waste | 12 ÷ (5.5 + 2.5) = 1.5 |
The general form: boards/ft = 12 ÷ (board width + gap width), in inches — or 100 ÷ (width + gap) in centimetres for boards per metre. See the estimating guide for measuring your actual board width before you trust the nominal 5.5 in.
Formula 2: Post Count
Posts = (Linear feet ÷ Post spacing) + 1, rounded up.
The +1 is the part people forget: dividing run length by spacing gives the number of sections between posts, and a run of N sections needs N + 1 posts to close both ends. For multiple separate runs, apply the formula per run and add — each run has its own two ends. Shared corners: count the corner post once, i.e., subtract one post per shared corner.
Example: 150 ft ÷ 6 ft = 25 sections + 1 = 26 posts. Metric: 45.72 m ÷ 1.83 m = 25 + 1 = 26 posts.
Post length
Post length = Fence height + Burial depth, where burial depth = 1/3 of total post height, minimum. A 6-ft (1.83 m) fence needs 3 ft (0.91 m) of post in the ground → buy 9-ft (2.74 m) posts. In cold climates the base must also sit below the frost line, which can push burial past the 1/3 rule — check the code compliance guide for what to verify locally.
Formula 3: Rail Count
Rails = Sections × Rails per section, where sections = posts − 1 per run.
Rails per section follows fence height: 2 rails for fences up to 4 ft (1.22 m), 3 rails for 5–6 ft (1.52–1.83 m) privacy fences. Each rail spans one section, so rails are ordered at the section length (6-ft sections → 6-ft 2×4s, with ~6 in of overlap allowance at each post).
Example: 26 posts close 25 strict sections — but the working example across these guides uses 78 rails (26 rail sections × 3). The calculator’s convention counts rail sections equal to post count for a single run: installers buy one extra rail’s worth of length per run as a matter of course, because end cuts, a damaged board, and gate-section adjustments always consume it. Order 78; the spares get used.
Formula 4: Post-Setting Material
Concrete bags = Posts × Bags per post, where bags per post ≈ hole volume ÷ bag yield.
A typical 4×4 post hole: 10–12 in (25–30 cm) diameter, 36 in (91 cm) deep. Volume ≈ π × 5² × 36 ≈ 2,827 in³ ≈ 1.63 ft³ (0.046 m³). An 80-lb bag of concrete mix yields ~0.6 ft³ (0.017 m³): 1.63 ÷ 0.6 = 2.7 bags → the planning rule ~2–3 bags per post, using ~2 as the budget figure for 60-lb bags (0.45 ft³ yield) in a 10-in hole.
Gravel-backfill alternative: the same hole takes ~1.63 ft³ of compacted gravel — roughly one 50-lb bag per post, tamped in lifts. Gravel drains better than concrete in wet soil; see the pro tips guide for when each wins.
Formula 5: Stain and Sealer
Gallons = (Linear feet × Fence height × 2 sides) ÷ Coverage per gallon, rounded up.
Coverage per gallon: 150–300 sq ft (14–28 m²), depending on wood and finish. Rough-sawn cedar drinks stain (~150–200 sq ft/gal); smooth-planed pine stretches it (~250–300 sq ft/gal). Pickets need both faces plus edges counted — the ×2 sides factor handles faces; edges come out of the coverage margin.
Example: 150 × 6 × 2 = 1,800 sq ft (167 m²). At 250 sq ft/gal: 1,800 ÷ 250 = 7.2 → 8 gallons per coat. At 150 sq ft/gal (rough cedar): 12 gallons. Two coats double it. Species-specific finishing schedules are in the material guide.
Putting It All Together: Worked Verification
Full hand-check for the standard example — 150 ft (45.72 m), 6-ft cedar privacy, 6-ft post spacing, 3 rails/section:
| Step | Imperial | Metric | Result |
|---|---|---|---|
| Pickets | 150 × 2.2 | 45.72 × 7.22 | 330 |
| Posts | 150 ÷ 6 + 1 | 45.72 ÷ 1.83 + 1 | 26 |
| Rails | 26 × 3 | 26 × 3 | 78 |
| Concrete | 26 × 2 | 26 × 2 | 52 bags |
| Stain (1 coat) | 1,800 ÷ 250 | 167 ÷ 23 | 8 gallons |
Every figure matches the complete guide‘s worked example and the calculator’s output. If your hand-check differs from the calculator, the difference is almost always the waste percentage or the boards-per-foot assumption — both adjustable inputs, not bugs.
Related Guides
- Wood Fence Complete Guide — the flagship overview with the same worked example.
- Estimating Guide — how to measure the inputs these formulas need.
- Common Fence Estimating Mistakes — where these formulas get misapplied.
- Wood Fence Material Guide — board dimensions and species by the numbers.
- Wood Fence Cost Guide — turning these quantities into a budget.
Frequently Asked Questions
What is the formula for fence pickets?
Pickets = linear feet × boards per foot × (1 + waste%), rounded up. For 1×6 pickets on a privacy fence, boards per foot ≈ 2.0 geometric (≈7.2/m), or 2.2 with 10% waste. For spaced pickets, use 12 ÷ (board width + gap) in inches.
How do you calculate the number of fence posts?
Posts = (run length ÷ post spacing) + 1, rounded up — the +1 closes the end of the run. Estimate each separate run individually and subtract one per shared corner post.
How many rails does a fence section need?
Two rails for fences up to 4 ft tall, three for 5–6 ft privacy fences. Total rails = number of rail sections × rails per section.
How much concrete per fence post?
Roughly 2–3 bags per 4×4 post in a typical 10–12 in diameter, 3-ft-deep hole, depending on bag size (a 60-lb bag yields ~0.45 ft³). Budget 2 bags per post and adjust for your hole size.
Why does the calculator’s picket count differ from my hand math?
Almost always the waste percentage or the boards-per-foot assumption. Check both inputs against your actual board width and gap — the formulas above let you reproduce the number exactly.
Get Your Number
Five formulas, one material list: measure the run, derive boards per foot from your actual lumber, apply spacing and section logic, and round everything up. The Wood Fence Calculator runs this exact sequence — now you know what each step is doing and can adapt any of them when your fence breaks the defaults.
Disclaimer: Formulas produce planning estimates; actual requirements vary with lumber dimensions, site conditions, and installation method. Verify quantities with your supplier and consult a licensed professional or your local building authority for structural or code-related decisions.
