Introduction
Behind every cedar order are three calculations: how many boards the structure needs, how many linear feet those boards represent, and how much extra to add for waste. None of them is complicated — but each has a trap: nominal dimensions, forgotten gaps, and one-size-fits-all waste factors. This guide derives each formula, shows where the numbers come from, and cross-checks everything in metric and imperial.
The Cedar Lumber Calculator runs these formulas automatically. What follows is the math it performs, so you can verify the output, adapt it to odd layouts, and understand exactly where each board in the count comes from.
Formula 1: Board Count
Boards = coverage length ÷ (board face width + gap), rounded up.
The “coverage length” is the run the boards must cover — fence length for pickets, deck width (perpendicular to the boards) for decking, wall length for siding. The denominator is the actual face width of one board plus the gap between boards. Use actual, surfaced dimensions: a 1×6 picket covers 5.5 in (140 mm), not 6 in; a 5/4×6 deck board covers 5.5 in (140 mm); a 2×6 covers 5.5 in as well.
Why the gap belongs in the denominator: each board “consumes” its own face width plus the space beside it. Omit the gap and you undercount by the gap’s share of each board’s footprint — about 4% for decking at 1/4 in gaps, more for fences with wide spacing.
Example: 100 ft fence ÷ (5.5 in + 0.25 in) → first convert to common units: 1,200 in ÷ 5.75 in = 208.7 → 209 pickets. Always round board counts up — 0.7 of a picket is still a picket you must buy.
Formula 2: Linear Feet
Linear feet = boards × board length (ft)
This converts the count into the unit lumber yards actually sell by. 209 pickets × 6 ft = 1,254 linear feet. Linear feet is additive across components: pickets + rails + posts (each in their own lengths) sum to the project’s total footage.
Metric equivalent: linear meters = boards × board length (m). 209 × 1.83 m = 382.5 m. Cross-check: 1,254 × 0.3048 = 382.2 m — the small difference is rounding of the 1.83 m conversion (6 ft = 1.8288 m exactly; using it gives 382.3 m either way).
Formula 3: Board Feet (Volume)
Board feet = (thickness in inches × width in inches × length in feet) ÷ 12
One board foot is 144 cubic inches. This is the volume measure rough lumber is priced in. Note the mixed units — thickness and width in inches, length in feet — which is the classic source of errors. Using nominal dimensions here is standard practice in the trade (a “2×6×8” is sold as 8 board feet: 2 × 6 × 8 ÷ 12 = 8), because board-foot pricing was built on nominal sizes.
Metric version: cubic meters = (thickness m × width m × length m) × boards. A nominal 2×6×8 (actual 1.5 × 5.5 in × 8 ft = 0.0381 × 0.1397 × 2.4384 m) contains 0.01298 m³; 8 board feet × 0.0023597 m³/bf = 0.01888 m³ nominal. The gap between nominal and actual volume is why you should confirm with your supplier whether board-foot pricing uses nominal or actual dimensions.
Formula 4: The Waste Multiplier
Order quantity = computed quantity × (1 + waste %)
Waste is multiplicative and applied once, at the end — not per line item. Adding 10% to pickets, then 10% to rails, then 10% to the total compounds the padding. The correct sequence: compute exact quantities for every component, sum them, then multiply the total by (1 + waste).
Cedar-specific waste guidance, because grade changes everything:
| Grade / situation | Waste factor | Why |
|---|---|---|
| Clear / A&BTR, straight runs | 5–10% | Few defects; losses are mostly end-trimming |
| STK, typical fence or deck | 10% | Sound knots; occasional board rejected or cut around |
| Knotty / #2, defect-heavy | 10–15% | Cutting around knots and wane consumes length |
| Pergolas, complex cut lists | 15% | Many short pieces; offcuts rarely reusable |
Why knotty grades waste more: a knot you cut around costs you the knot’s width plus the saw kerfs and the unusable stub on each side — typically 6–12 in (15–30 cm) of board per defect. A 12-ft board with three bad knots can lose 2–3 ft. Clear stock loses only end trim, usually 1–2 in per cut end. Grade choice is therefore a budget decision as well as an appearance one — the material guide maps grades to applications, and the cost guide shows what each grade adds to a project total.
Formula 5: The Decking Shortcut
Linear feet of decking ≈ deck area (sq ft) × 2.2
Derivation: a 5/4×6 board covers 5.5 in of face plus a 1/4 in gap = 5.75 in per board. One square foot of deck (12 in × 12 in) needs 12 ÷ 5.75 = 2.087 boards per foot of run, each 1 ft long — hence 2.087 linear feet per square foot, rounded to the trade’s 2.2 for convenience. Add waste on top of the 2.2 figure.
Metric: linear meters ≈ deck area (m²) × 7.2. Derivation: 140 mm face + 6 mm gap = 146 mm per board; 1 m ÷ 0.146 = 6.849 boards per meter of width, each contributing its run length — 6.85 m per m², rounded to 7.2. Check against imperial: 2.2 LF/ft² × 0.3048 ÷ 0.092903 = 7.216 m/m². The two shortcuts agree.
Adjust the factor if your gap differs: 3/16 in gaps push the factor toward 2.15; 3/8 in gaps toward 2.25. The estimating guide applies this shortcut to a full deck.
Formula 6: Posts and Rails from Bays
Fences and pergolas are bay structures, and bay math has an off-by-one trap:
Posts = (run length ÷ bay spacing), rounded up, + 1
A 100 ft run at 8 ft bays: 100 ÷ 8 = 12.5 → 13 bays → 14 posts. The “+1” is the end post — the mistake is counting 12.5 bays as 13 posts and forgetting the far end.
Rails = bays × rails per bay. 13 bays × 2 rails = 26 rails. Rail length equals bay spacing (8 ft boards for 8 ft bays), so 26 × 8 = 208 linear feet of rail stock.
Putting It Together: Full Derivation
The 100-ft fence from the complete guide, derived step by step:
- Board count: 1,200 in ÷ 5.75 in = 208.7 → 209 pickets.
- Linear feet: 209 × 6 ft = 1,254 LF.
- Waste: 1,254 × 1.10 = 1,379.4 → 1,380 LF (230 six-foot pickets).
- Posts: 100 ÷ 8 = 12.5 → 13 bays → 14 posts.
- Rails: 13 × 2 = 26 rails × 8 ft = 208 LF.
- Metric cross-check: 30.48 m ÷ 0.14605 m = 208.7 → 209 pickets; 209 × 1.8288 m = 382.2 m; × 1.10 = 420.4 m; and 1,380 ft × 0.3048 = 420.6 m. Agreement within rounding.
Every number the calculator returns traces back to one of these six formulas. When an estimate looks wrong, check the inputs in this order: actual vs. nominal width, gap included, waste applied once at the end, posts counted with the +1.
Related Guides
- The Complete Guide — full overview of cedar estimating with a worked fence example.
- Estimating Guide — step-by-step walkthrough applying these formulas to a real project.
- Material Guide — grades and species, and why grade changes the waste factor.
- Common Mistakes — the input errors that break these formulas in practice.
- Cost Guide — turning these quantities into a project budget.
Get Your Number
The formulas are simple; the discipline is in the inputs — actual dimensions, real gaps, one waste factor at the end. The Cedar Lumber Calculator runs all six formulas for you in metric or imperial. Enter your dimensions, verify the board count by hand with Formula 1, and you will know the number is right before you call the yard.
Disclaimer: Calculator results are planning estimates based on the dimensions you enter. Actual material needs vary with site conditions, product availability, and local pricing. Verify quantities with your supplier and consult a licensed professional for structural or code-related decisions.
