How to calculate deck boards
Deck boards run along the deck, so the number of rows depends on the width across them:
Rows = deck width ÷ (board width + gap), rounded up
Decking = rows × deck length
Boards = decking × (1 + waste) ÷ board length, rounded up
Example: a 16 × 12 ft deck in composite boards 5 1/2 in wide with 3/16 in gaps. Each row takes 5 11/16 in, so the 144 in width needs 144 ÷ 5.6875 = 25.3, rounded up to 26 rows. That’s 26 × 16 = 416 ft of decking. With 10% waste, 457.6 ft ÷ 16 ft boards = 28.6, so buy 29 boards. With 12 ft boards it would be 39.
Joists and screws
Joists run across the deck under the boards, one at each end and one every spacing in between:
Joists = deck length ÷ spacing, rounded up, + 1
Screws = rows × joists × 2
For the same deck at 16 in on center: 192 in ÷ 16 = 12, plus 1 = 13 joists, each 12 ft long. With 26 rows crossing 13 joists and two screws at each crossing, that’s 676 screws.
| Joist spacing | Joists for a 16 ft deck |
|---|---|
| 12 in | 17 |
| 16 in | 13 |
| 24 in | 9 |
How far apart the joists can be depends on the decking. Trex’s 2025 guide allows 16 in for its 1 in composite boards and 24 in for its 2 × 6 boards on residential decks, and 4 in less when boards run at 45°. The Southern Forest Products Association rates 5/4 and 2 × 6 pine decking to span 24 in, says many professionals limit 5/4 boards to 16 in to reduce bounce, and spaces joists 16 in apart under diagonal decking.
Board sizes
Composite boards from Trex and TimberTech are 5 1/2 in wide and come in 12, 16 and 20 ft lengths; Trex lists its boards at 0.82 to 0.94 in thick. Wood decking is sold by nominal size: a 5/4 × 6 board is 1 × 5 1/2 in once dried and planed, and a 2 × 6 is 1 1/2 × 5 1/2 in, according to the USDA’s Wood Handbook. Construction lumber usually comes in even 2 ft lengths.
Pick board lengths that suit the deck: a 16 ft deck uses 16 ft boards with no joints, while a 24 ft deck may work better with 12 ft boards than with 20s. Butt joints have to land on a joist.
What this calculator leaves out
It counts the decking, the joists and the screws. Beams, posts, footings, ledger attachment and joist sizes depend on spans and loads, so follow your local code or the American Wood Council’s DCA 6 deck guide for those, and use the stair calculator for steps and the concrete calculator for footings.