Deck calculator
Enter the deck's length and width, the board width and gap, the board length and which way the boards run, and the joist spacing. The calculator counts the decking boards with the cuts (every butt joint on a joist), the joists and rim joists, and the screws or hidden clips, and draws every board to scale.
Deck calculator
5.5 in (140 mm) for 1×6 composite and 2×6 lumber.
Trex's rule of thumb: 5% extra decking for waste and mistakes. Allow more for diagonal boards or a picture-frame border.
Trex asks for at least a 3/16 in (5 mm) gap side to side, and 3/8 in (10 mm) for docks and decks under trees. Screws: two per board at every joist, and two in each board end at a butt joint, over a 2 × 4 nailer beside the joist. Clips are counted one at every board-and-joist crossing; Trex sells its Hideaway clips by the box for 50 ft².
Decking boards
28 × 16 ft
- Joists
- 13 × 12′
- Rim joists
- 2 × 16′
- Screws
- 676
- Pass: Joists 16 in on centre Trex's maximum for 1 in boards laid square to the joists on a residential deck. At 45°, use 12 in.
- Warning: Last row 1 13⁄16″ wide A thin strip to rip. Widen the gap a little, or change the deck width, so the last board is closer to whole.
The deck from above
Lumber, in linear feet
- Decking448 ft70%
- Joists156 ft25%
- Rim joists32 ft5%
The same deck in each board length
| Board length | Boards | Joints | Bought (ft) | Offcut and spares (ft) |
|---|---|---|---|---|
| 8 ft | 55 | 26 | 440 | 24 |
| 10 ft | 55 | 26 | 550 | 134 |
| 12 ft | 37 | 26 | 444 | 28 |
| 16 ft | 28 | 0 | 448 | 32 |
The method
- Rows of boards: the side the boards cross, divided by one board plus its gap, rounded up. The last row is ripped to fit, and the calculator says how wide it is.
- Boards per row: a row longer than a board is cut so that every butt joint lands on a joist: each piece runs as far as a board reaches, back to the last joist (on joists 16 in apart, a 10 ft board is cut to 9 ft 4 in). Alternate rows are cut from opposite ends so the joints stagger, and the last joint moves back a joist if the final piece would rest on fewer than three. The pieces are then cut from as few boards as fit, offcuts reused for shorter pieces. The table under the result compares every board length for the same deck.
- Waste: Trex's rule of thumb, 5% more boards, rounded up to whole boards. Change it for a diagonal deck or a picture-frame border.
- Joists: the side the boards run along, divided by the spacing, rounded up, plus one, so there is a joist at each end. Each is as long as the side the boards cross.
- Rim joists: one across each end of the joists, as long as the boards run (against a house, one of them is the ledger).
- Fasteners: two screws where each board crosses each joist and two more at each butt joint (two in each board end), or one hidden clip at each crossing.
Worked example
A 16 × 12 ft deck with the boards running along the 16 ft length: the 12 ft (144 in) width takes (144 + 3/16) ÷ (5.5 + 3/16) = 25.4, so 26 rows, the last ripped to 1 13/16 in. Each row is one 16 ft board: 26 boards, and Trex's 5% (1.3, rounded up to 2) makes 28 boards, 448 linear ft. Joists at 16 in: 192 ÷ 16 = 12, plus one, 13 joists of 12 ft (156 ft), and two 16 ft rim joists. The boards cross the joists 26 × 13 = 338 times, so 676 screws, or 338 hidden clips (four 50 ft² boxes for 192 ft²). In 12 ft boards each row is a 12 ft and a 4 ft piece, joined on a joist, the joint 12 ft along in one row and 4 ft along in the next: 35 boards (three 4 ft pieces from each extra board), 37 with the waste, and 26 joints that take 52 more screws. A 10 ft board would end between joists, so each is cut to 9 ft 4 in: 52 boards, 55 with the waste.
| Deck | Boards | Joists | Rim joists | Screws |
|---|---|---|---|---|
| 10 × 10 ft | 24 × 10 ft | 9 × 10 ft | 2 × 10 ft | 396 |
| 12 × 10 ft | 24 × 12 ft | 10 × 10 ft | 2 × 12 ft | 440 |
| 12 × 12 ft | 28 × 12 ft | 10 × 12 ft | 2 × 12 ft | 520 |
| 16 × 12 ft | 28 × 16 ft | 13 × 12 ft | 2 × 16 ft | 676 |
| 16 × 16 ft | 36 × 16 ft | 13 × 16 ft | 2 × 16 ft | 884 |
| 20 × 12 ft | 35 × 16 ft | 16 × 12 ft | 2 × 20 ft | 884 |
Several of these leave a thin last row (10 ft wide decks end on a 9/16 in strip). Widening the gap slightly or changing the deck by an inch or two gives a last board closer to whole; the calculator flags any last row under half a board.
Before you buy
- Butt joints must land on a joist. The calculator cuts the rows so they do and staggers them from row to row (the drawing marks each one). Trex asks for a 2 × 4 nailer beside the joist at each joint, so both board ends can be screwed: count those too.
- Picture frames and borders use extra boards around the edge; count them separately.
- Stairs, railings, posts, beams and footings are not in this count. The concrete calculator works out the post holes.
- Diagonal boards need the closer joist spacing and give more offcut than square boards.
See it next to the house
"Open the deck as a plan" builds the deck as a room in FloorGen, where you can place it against the house, check the door swing and the stair landing, and see it in 3D. See how to draw a site plan for setbacks and lot coverage; check whether your zoning counts a deck.
Questions
How many deck boards do I need for a 12 × 12 deck?
With 5.5 in boards and a 3/16 in gap, 26 rows: 26 boards of 12 ft, the last ripped to about 1 13/16 in, and 28 with Trex's 5% for waste. A slightly wider gap or deck gets you a fuller last board.
How far apart should deck joists be?
For 1 in composite boards on a residential deck, Trex calls for joists no more than 16 in on centre, and 12 in when the boards run at 45°. Trex's 2×6 boards may span 24 in. Wood decking follows its own grade rules, and your code may be stricter.
How many screws do I need for a deck?
Two at every place a board crosses a joist, and two more at each butt joint, where each board end takes its own two. On a 16 × 12 ft deck in 16 ft boards, with 26 rows, 13 joists and no joints, that is 338 crossings and 676 screws, plus spares.
What gap should I leave between deck boards?
Trex's guide sets a minimum of 3/16 in (5 mm) side to side, in hot or cold weather, and recommends 3/8 in (10 mm) for docks and heavily wooded areas. Never more than 1/2 in.
Which way should deck boards run?
Across the joists. Running them along the longer side means fewer, longer rows and fewer butt joints if your boards are long enough; running them the short way can avoid joints altogether. The calculator shows both.