Deck Material Calculator
Enter your deck size and board specs to get the number of boards, total linear feet, screw count and cost — before you buy a single thing.
The short answer
Formula. Number of boards = (deck area in sq ft × 12 ÷ (board width + gap, both in inches)) × (1 + waste %) ÷ board length in feet — always rounded up to a whole board.
Example. A 12 ft × 12 ft deck (144 sq ft) in 5.5-in boards with 1/4-in gaps and 10% waste needs about 301 linear feet of decking — 21 sixteen-foot boards and roughly 496 screws.
How this calculator works
A deck board covers a strip of deck equal to its face width plus the gap you leave between boards. So the total length of board you need is the deck area multiplied by 12, divided by that combined width in inches. We then add the waste factor you choose, divide by the length of board you plan to buy, and round up — because nobody sells you 0.66 of a board.
The screw estimate assumes the most common fastening plan: two screws where each board crosses each joist. With 16 in joist spacing and 5.5 in boards, that works out to roughly 3 screws per square foot of deck. Hidden fastener clips for grooved composite boards usually need one clip per joist crossing instead — count the crossings, not the screws, when you buy clips.
Buying tip from the arithmetic: choose a board length that divides into your deck's long dimension with little left over. A 12 ft deck built from 16 ft boards throws away 4 ft per board line; 12 ft boards may cost more per piece but far less per usable foot.
Sources & review date
- Screw count assumes face-fastening at 2 screws per joist crossing on 16-in joist spacing — standard framing practice, stated and editable on the page.
- Board widths are dressed-lumber actual sizes (a nominal 6-in deck board measures 5.5 in).
Last reviewed: October 2026. Numbers that drift (fees, prices, densities) are editable inputs on this page.
Deck board FAQs
How many deck boards do I need for a 12×12 deck?
About 21 boards in 16 ft lengths — roughly 331 linear feet — assuming standard 5.5 in boards, a 1/4 in gap and 10% waste. Enter your real numbers above; different board lengths change the count a lot.
How many screws will a 12×12 deck take?
Close to 500 screws at 16 in joist spacing (2 per crossing, including waste). Deck screws usually sell by count — jars of 100 to 1,000 — so two 250-count jars cover a 12×12 with spares left for stairs or repairs.
Does this work for composite decking?
Yes. Most composite boards are also 5.5 in wide, so the board count math is the same. Two differences: composite brands specify an exact gap (often 3/16 in) — use it — and many composites need hidden clips rather than screws, so buy one clip per board-to-joist crossing.
What about stairs, skirting and picture framing?
Calculate the flat field here, then add those pieces by hand: each stair tread is a short board run across the step width, and picture-frame borders add about 2× your deck perimeter in linear feet. Bump the waste factor to 15% if those are in your plan.