Enter the size of the deck and the boards you plan to use. The calculator works out the rows of boards, how many boards to buy, the joists underneath and the face screws.
How to use the deck calculator
- Enter the deck length in the direction the boards run and the width across them.
- Pick the board width. Nominal 5/4 × 6 and 2 × 6 boards are 5.5 in wide; a 2 × 4 is 3.5 in.
- Choose a board length that matches the deck, so each row uses whole boards with as few joints as possible.
- Enter the gap the board maker asks for and the joist spacing.
Deck formulas
Rows = (deck width in inches + gap) ÷ (board width + gap), rounded up Boards per row = deck length ÷ board length, rounded up Boards = rows × boards per row × (1 + waste %) Joists = deck length in inches ÷ joist spacing + 1 Face screws = rows × joists × 2
TimberTech says the space between joists should never exceed 16 in on center for a residential deck, 12 in for a stiffer feel or commercial use, and 12 in for diagonal boards. Joists run at right angles to the boards, so the joist count comes from the deck length.
Worked example: a 16 × 12 ft deck
- Rows: (144 + 0.25) ÷ (5.5 + 0.25) = 25.09, so 26 rows.
- 16 ft boards on a 16 ft deck: one board per row.
- With 10% waste: 29 boards × 16 ft.
- Joists at 16 in on center: 192 ÷ 16 + 1 = 13 joists, each 12 ft.
- Face screws: 26 × 13 × 2 = 676.
Boards for common deck sizes
5.5 in boards, 1/4 in gap, joists at 16 in, 10% waste:
| Deck | Rows | Boards to buy | Joists |
|---|---|---|---|
| 10 × 10 ft | 21 | 24 × 10 ft | 9 |
| 12 × 12 ft | 26 | 29 × 12 ft | 10 |
| 16 × 12 ft | 26 | 29 × 16 ft | 13 |
| 20 × 12 ft | 26 | 29 × 20 ft | 16 |
| 16 × 16 ft | 34 | 38 × 16 ft | 13 |
| 20 × 16 ft | 34 | 38 × 20 ft | 16 |
Framing and footings
Joist size, beam size and footing spacing depend on the span, the load and your local code; a building permit usually needs them on a plan. The post hole concrete calculator covers footing concrete, and the board foot calculator prices framing lumber.
Tips for buying decking
- Stagger joints on long decks. If the deck is longer than your boards, butt joints must land on a joist; stagger them row to row.
- Check the gap. Wet pressure-treated boards shrink as they dry; composite boards expand with heat. Follow the maker’s spacing.
- Buy hidden fasteners by the box count. Clip systems have their own coverage per box; the screw count here is for face screwing.
- Add fascia and stairs separately. Measure the rim and the stair treads on their own.
Questions
How many deck boards do I need for a 12 × 12 deck?
About 29 boards × 12 ft with 5.5 in boards, a 1/4 in gap and 10% waste.
How far apart should deck joists be?
16 in on center at most for most residential decks, and 12 in for diagonal decking.
How wide is a 5/4 × 6 deck board?
5.5 in actual width, about 1 in thick.
How many screws per deck board?
Two at every joist the board crosses.
Sources
- Deck Floor Joist Spacing 101: How Far Apart to Place Your Joists, TimberTech.
Used for: Joist spacing: 16 in maximum residential, 12 in for diagonal decking and commercial decks. - NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically, National Institute of Standards and Technology.
Used for: Exact inch and foot definitions.