What a deck takes
The surface of a deck is a series of boards laid side by side across joists. The number of rows depends on the board width and the gap between boards; the number of joists depends on their spacing. This calculator counts both, adds a waste factor and estimates screws and board cost.
The formulas
- Rows of boards = deck width ÷ (board width + gap), rounded up.
- Linear feet = rows × deck length × (1 + waste).
- Boards = linear feet ÷ board length, rounded up.
- Joists = deck length ÷ joist spacing + 1, each as long as the deck is wide.
- Screws = rows × joists × 2.
Worked example: a 16 × 12 ft deck
Boards run along the 16 ft side. They are 5.5 in wide with a 3/16 in gap, 16 ft long, over joists at 16 in on center.
- Rows: 144 ÷ 5.6875 = 25.3, so 26 rows.
- Linear feet: 26 × 16 × 1.10 = 458 ft.
- Boards: 458 ÷ 16 = 28.6, so 29 boards.
- Joists: 192 ÷ 16 + 1 = 13 joists of 12 ft.
- Screws: 26 × 13 × 2 = 676 screws.
- At $22 a board, the decking costs $638, or about $3.32 per square foot.
Boards by deck width
| Deck width | Boards | Screws | Board cost |
|---|---|---|---|
| 8 ft | 19 | 442 | $418 |
| 10 ft | 25 | 572 | $550 |
| 12 ft | 29 | 676 | $638 |
| 14 ft | 33 | 780 | $726 |
| 16 ft | 38 | 884 | $836 |
Picking board lengths
Choose board lengths that match the deck, so each row is one board or splices fall on a joist. A 16 ft deck uses 16 ft boards with almost no waste; a 14 ft deck cut from 16 ft boards wastes 2 ft per row. When boards have to be joined, stagger the joints across the deck and add a doubled joist or blocking under each seam.
Gaps and spacing
- Gap: about 1/8 to 3/16 in lets water drain and leaves room for expansion. Wet pressure-treated wood can be laid tight because it shrinks as it dries.
- Joist spacing: 16 in on center for 5/4 wood and most composites; 12 in for diagonal layouts and many composites in hot climates; 24 in only for 2× boards.
- End gaps: composites expand with heat and need gaps at the board ends. Follow the manufacturer’s chart.
What is not counted
This calculator covers the decking surface and joists. Beams, posts, footings, ledger board, joist hangers, stairs and railings depend on the structure and on local code, which usually requires a permit for attached decks or decks more than 30 in above grade. Have the framing plan checked before you buy lumber for it.
Hidden fasteners
If you use hidden clips instead of face screws, count one clip per joist crossing per row: 338 clips for the example deck, plus starter and end clips.