How deck boards are calculated
Rows = deck width ÷ (board width + gap), rounded up
Decking length = rows × deck length × (1 + waste %)
Boards = decking length ÷ board length, rounded up
Joists = deck length ÷ joist spacing, rounded up, + 1
Screws = rows × joists × 2Worked example: a 16 × 12 ft deck
This uses 5.5 in boards running along the 16 ft side, with 1/8 in gaps and joists 16 in apart.
- Rows: 144 in ÷ 5.625 in = 25.6, so 26 rows.
- Decking length: 26 × 16 = 416 ft. With 10% waste, that's 457.6 ft.
- In 16 ft boards: 457.6 ÷ 16 = 28.6, so buy 29 boards.
- Joists: 192 in ÷ 16 in = 12, plus 1, so the boards cross 13 joists.
- Screws: 26 × 13 × 2 = 676 screws.
Estimating the cost of deck boards
Prices vary widely between pressure-treated lumber, hardwood and composite, so enter the price per board for the product and length you've chosen. As an illustration only, the 29 boards in this example at $30 each would cost 29 × $30 = $870. That's the decking surface alone, so screws, framing and railings are extra.
Choose board lengths that suit your deck
Waste depends heavily on the lengths you buy. A 16 ft deck is efficient with 16 ft boards, with a trim cut on each. On a 14 ft deck, 16 ft boards leave a 2 ft offcut per row, which may be hard to use. If your boards won't span the deck in one piece, joints must land on a joist, so plan your layout before ordering. The calculator divides the total length by the board length, so check that the cut pieces will actually work for your layout.
Nominal and actual sizes
| Sold as | Typical actual width |
|---|---|
| 5/4 × 6 decking | 5.5 in |
| 2 × 6 lumber | 5.5 in |
| 2 × 4 lumber | 3.5 in |
Composite boards vary by brand, so use the size listed in the product specifications.
Board gaps
Spacing lets water drain and leaves room for movement. The right gap depends on the material and how wet the lumber is when installed. Pressure-treated boards may shrink as they dry, and composite boards expand and contract with temperature. Follow the manufacturer's spacing chart, as using the wrong gap may affect the warranty on composite products.
Screws and hidden fasteners
The screw count assumes two per joist crossing. Screws are usually sold by the pound or by box count, so check the count on the box. Hidden fastener systems use clips instead, and their coverage is listed per box. Use fasteners rated for your lumber, particularly with pressure-treated wood.
What this leaves out
Framing, joist hangers, posts, footings, stairs and railings aren't included, and they generally need to meet local building code. Many areas require a permit for attached or raised decks. Need footing concrete? Use the concrete calculator in round column mode.