Deck Board Calculator
Calculate how many deck boards your deck needs.
Standard deck boards are 5.5 in wide with a 1/4 in gap. 5% waste included; add more for diagonal decking.
Deck boards are priced high enough that overbuying stings, but a mid-project lumber run — with the inevitable color mismatch between batches of composite — stings more. Our deck board calculator above converts your deck dimensions into board rows, total linear feet, and the exact count of boards at your chosen length, with 5% waste built in.
How the Math Works
Rows = Deck width (in inches) ÷ (Board width + Gap), rounded up
Total linear feet = Rows × Deck length × 1.05
Boards = Total linear feet ÷ Board length, rounded up
A 16 × 12 ft deck with standard 5.5-inch boards and a 1/4-inch gap needs 26 rows. That’s 26 × 16 ft × 1.05 ≈ 437 linear feet — 28 boards at 16 ft.
Board Width and Gap: The Two Numbers That Matter
Standard deck boards (wood 5/4×6 or composite) measure 5.5 inches actual width — the “6” in the name is nominal. If you’re using 2×4s (3.5 in) or wide-format composite (7+ in), change the width field.
The gap does real work: it drains rain, sheds debris, and gives boards room to move.
- Wet pressure-treated lumber: install nearly tight (1/16 in) — the boards shrink as they dry, opening the gap for you.
- Dry wood and cedar: gap 1/4 inch at install.
- Composite/PVC: follow the manufacturer, typically 3/16–1/4 in side-to-side — required for warranty.
Picking Board Length
The goal is minimal cutting and minimal butt joints (where two boards meet mid-deck — the weakest visual point of any deck):
- If a single board can span the full deck length (16 ft deck → 16 ft boards), do it. Zero joints, fastest install.
- For decks longer than available boards, plan a staggered joint pattern and buy lengths that divide cleanly — a 20 ft deck runs nicely with alternating 12 ft + 8 ft rows.
- Joints must land on a joist. If your math puts joints between joists, add blocking or shift lengths.
Diagonal decking looks sharp but changes the math: add 10–15% waste instead of 5% and expect every board end cut at 45°.
Wood or Composite?
Pressure-treated pine — cheapest upfront; needs cleaning and re-staining every 2–3 years; 15–20 year life.
Cedar/redwood — beautiful, naturally durable, moderate price; still wants regular sealing.
Composite — 2–3× the material cost of treated lumber, near-zero maintenance, 25–50 year warranties. Because color varies subtly by production run, buy all boards at once — a mid-project top-up may not match.
PVC — fully synthetic, lightest and most stain-proof, top of the price range.
The structure below (joists, beams, posts) stays pressure-treated regardless of what you walk on.
Fasteners and Extras
- Screws: roughly 2 screws per board per joist. A 16 ft board over joists at 16 in centers crosses 13 joists → 26 screws. Plan on ~350 screws per 100 sq ft of deck.
- Hidden fasteners for grooved composite: sold in boxes rated per square foot — one clip per board per joist.
- Fascia and stair treads are separate line items; measure their linear feet independently.
- Our stair calculator handles the steps; our concrete calculator covers the footings.
Frequently Asked Questions
How many deck boards for a 12×12 deck? With 5.5 in boards + 1/4 in gap: 26 rows × 12 ft × 1.05 ≈ 328 linear ft → 28 boards at 12 ft.
Which way should boards run? Perpendicular to the joists — always. If you want a different visual direction, the joists must be framed for it first.
Bark side up or down? The old rule (“bark side up”) matters less than picking the best-looking face and fastening well. Cupping is controlled far more by drainage, gap, and sealing than by ring orientation.
Estimates are for planning purposes. Deck structure is code-regulated in most areas — confirm framing, footings, and railing requirements with your local building department.
Notes from building this calculator
Deck board coverage depends on a dimension nobody quotes: the gap. A 5/4×6 board is 5.5 inches of actual face width, and a quarter-inch gap makes effective coverage 5.75 inches. Ignoring the gap overestimates board count by about 4 percent, which sounds trivial until you are pricing a 400-square-foot deck. We calculate on effective width and account for butt joints needing blocking underneath.
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Sources and Further Reading
Coverage figures and material constants are cross-checked against manufacturer technical data sheets before publication. Assembly and dimensional references come from applicable model building codes. See our Methodology page for how figures are selected and verified.
Planning estimate only. Results assume standard conditions and are intended for material planning. Verify against your product’s technical data sheet and your local building code before purchasing. Structural, electrical, and gas work should be specified by a licensed professional.
Last updated: August 7, 2026