Introduction
Estimating a cedar project is a sequence, not a single calculation: measure the structure, convert each component into boards, convert boards into linear feet, add one waste factor at the end, and cross-check the total against a shortcut. This guide walks that sequence on a realistic project — a 16 × 12 ft (4.88 × 3.66 m) cedar deck — with every step computed in imperial and cross-checked in metric.
The Cedar Lumber Calculator automates these steps. Work through them once by hand and you will trust the calculator’s output — and catch input errors — for every project after.
The Project
A freestanding backyard deck, 16 ft long × 12 ft wide (192 sq ft), surfaced with 5/4×6 cedar decking (5.5 in / 140 mm actual face, 1/4 in / 6 mm gaps), boards running the 16-ft direction. Framing in pressure-treated lumber (standard practice — cedar is a surface wood, not a structural one): 2×8 joists at 16 in on center spanning the 12-ft width, a double 2×10 beam, and six 6×6 posts. Grade: STK cedar, so 10% waste. Metric: 4.88 × 3.66 m (17.84 m²).
Step 1 — Measure and Sketch
Measure the finished deck footprint on site — not the drawing. Record length (16 ft / 4.88 m), width (12 ft / 3.66 m), board direction, joist direction and spacing, and post locations. Sketch it with dimensions; every number below traces to this sketch. Photograph the tape against each key dimension — photos settle the “was it 16 ft or 16 ft 3 in?” argument when you’re at the yard, and they document the as-measured state if the design changes mid-project. The pre-order checklist has a field-measurement routine.
Step 2 — Decking Board Count
Boards run the 16-ft length; count across the 12-ft width:
Boards = 144 in ÷ (5.5 in + 0.25 in) = 144 ÷ 5.75 = 25.04 → 26 boards.
The 0.04 means the last board needs only a sliver trimmed — in practice you install 26 boards and rip the final one. Metric: 3.66 m ÷ 0.146 m = 25.07 → 26 boards. Agreement confirmed.
Step 3 — Decking Linear Feet
26 boards × 16 ft = 416 linear feet of 5/4×6 decking. Metric: 26 × 4.877 m = 126.8 m; cross-check 416 × 0.3048 = 126.8 m.
Shortcut check: 192 sq ft × 2.2 = 422 linear feet — within 2% of the board-count method, confirming both. (The shortcut is derived in the formula guide.)
Step 4 — Framing Lumber
Joists at 16 in on center across the 16-ft length: (16 × 12 ÷ 16) + 1 = 13 joists, each 12 ft → 156 linear feet of 2×8. Rim joists: 2 × 16 ft = 32 linear feet of 2×8. Beam: double 2×10 × 16 ft = 32 linear feet. Posts: six 6×6×8.
Totals: 2×8 → 188 LF; 2×10 → 32 LF; 6×6 posts → 6 each. These are pressure-treated, so they don’t enter the cedar order — but forgetting them is the classic “I estimated the deck but can’t build it” error. Framing math follows the same bay logic as the fence example in the complete guide.
Step 5 — Waste, Once, at the End
STK cedar, deck field with straightforward cuts: 10%.
Decking order: 416 × 1.10 = 457.6 → 458 linear feet. Round up to whole boards in the length you’ll buy: at 16-ft boards, 458 ÷ 16 = 28.6 → 29 boards (464 LF). At 12-ft boards, 458 ÷ 12 = 38.2 → 39 boards (468 LF) — more joints, so prefer 16-ft stock if the design allows.
Metric: 126.8 m × 1.10 = 139.5 m; 458 ft × 0.3048 = 139.6 m. Match.
Step 6 — Fasteners and Finish
Not lumber, but part of the order: stainless screws at roughly 2 per joist crossing — 26 boards × 13 joists × 2 ≈ 676 screws, so two 5-lb boxes (about 350 screws each at #8×2-1/2 in). Buy screws a half-inch longer than you think; cedar’s softness tempts shallow bites that work loose. Finish: deck area 192 sq ft at ~150 sq ft per gallon per coat → 2 gallons for the first coat, plus end-grain sealer. Budget these in the cost guide ranges.
Step 7 — Sanity Checks
Before ordering, verify:
- Board count vs. shortcut: 26 boards (416 LF) vs. 422 LF shortcut — within 2%. Good.
- Waste applied once: 10% on the decking total only, not per board.
- Actual dimensions used: 5.5 in face, not 6 in nominal.
- Joist count includes both rims: 13 = 12 intervals + 1.
- Whole boards: 29 sixteen-footers, not 28.6.
Adapting This to Fences and Siding
The sequence never changes — only the board-count formula’s inputs. For a fence, the coverage length is the run and the gap is picket spacing (the complete guide’s 100-ft worked example shows this variant). For siding, coverage is wall area and the “board width” is the exposure (e.g., 5 in for bevel siding), with 15% waste for cuts and openings. Pergolas skip area math entirely: count pieces from the cut list. The material guide covers siding exposures in detail.
Related Guides
- The Complete Guide — full overview of cedar estimating with a worked fence example.
- Formula Explained — the derivation behind every step above.
- Cost Guide — turning this deck’s quantities into a budget.
- Pre-Order Checklist — the final verification pass before you buy.
- Common Mistakes — the errors this walkthrough is designed to prevent.
Get Your Number
Measure, sketch, count boards, convert to linear feet, add waste once, round up to whole boards — that is the entire discipline. The Cedar Lumber Calculator runs the sequence instantly; this walkthrough is how you verify it. Do both, and the number you order is the number you need.
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.
