Decking Calculator
Plan a deck and get the full materials list: deck boards in the length you buy, joists, and the deck screws to fix them down.
How many deck boards do I need?
Deck boards are laid in rows across the width of the deck, with a small drainage gap between them. The row count is the width divided by one board plus one gap — round up, because a part-row still needs a full board — and each row runs the full length of the deck.
Rows = round up(deck width ÷ (board width + gap)) · Linear feet = rows × deck length
Example: a 12 × 10 ft deck with standard 5.5 in boards and a 1/4 in gap: each row takes 5.75 in, so 120 ÷ 5.75 rounds up to 21 rows. That is 252 linear feet of board — bought as 24 boards of 12 ft once the 10% waste allowance is in.
Joists and screws
Joists run under the boards at right angles, spaced 16 inches on center for most wood decking (12 inches for a stiffer feel or diagonal boards, 24 only where the board maker allows it). The count is the deck length divided by the spacing, plus one for the far end. Screws are two per board at every joist it crosses — 2 × rows × joists — which is why a modest deck eats screws by the hundreds.
| Deck | Boards (5.5 in, 1/4 in gap) | Joists at 16 in | Screws |
|---|
| 10 × 10 ft | 21 rows · 210 lin ft | 9 | 378 |
| 12 × 10 ft | 21 rows · 252 lin ft | 10 | 420 |
| 16 × 12 ft | 26 rows · 416 lin ft | 13 | 676 |
Posts and concrete footings are a separate count driven by your local code and soil — the concrete calculator here will size the footing pours once you know how many posts the design needs.
Frequently asked questions
Which way should deck boards run?
Boards usually run out from the house or along the long direction of the deck — whichever gives fewer joins. This calculator assumes boards run along the deck length you enter; swap length and width if your boards run the other way.
What gap should I leave between deck boards?
1/4 in (6 mm) is the standard for pressure-treated lumber, which shrinks as it dries. Kiln-dried and composite boards often specify 1/8–3/16 in — check the maker's sheet; the gap selector here covers the common range.
Can joists really be 24 inches apart?
Only with thick boards (true 2 in decking) or composite boards rated for it. Standard 5/4 decking at 24 in feels springy and can void the board warranty. 16 in is the safe default, 12 in for diagonal layouts.
How many screws are in a 5-lb box?
Roughly 400 for standard 2.5 in deck screws, but it varies by brand and screw size. This calculator gives you the exact screw count and an approximate box count — check the count printed on your box before you buy.
CalcCoverage