A deck’s square footage is the wrong number to take to the lumber yard. Boards are bought one at a time, so the count has to come from how many rows the deck needs across its width and how many stock boards each row takes along its length.
Rows = smallest whole number n where n × board width + (n − 1) × gap ≥ deck width
Boards per row = deck length ÷ stock board length, rounded up
Boards to buy = rows × boards per row × (1 + allowance), rounded up
A 16 × 12 ft deck
The boards run the 16 ft way, so the deck is 12 ft wide across the rows. With 5½ in boards and a 1/4 in gap, the row count is 26 (25.09 before rounding up). A 16 ft stock board covers the 16 ft run in one piece, so the planner’s working reads: 26 rows × 1 board per row = 26 boards.
This example assumes
- 5½ in actual board width and a 1/4 in gap, both example values: use the width and gap for your own product
- 16 ft stock boards
- 10% extra allowance, an editable planning default
Worked example 16 × 12 ft deck, 5½ in boards, 1/4 in gaps, 16 ft stock
| Material | Calculated | Buy |
|---|---|---|
| Deck boards5.5 in boards, 0.25 in gaps, 16 ft stock, 10% extra | 26 rows × 1 board per row = 26 boards | 29 boards |
Without an allowance that is 26 boards. The 10% allowance takes the count to 28.6 boards, and the list rounds that up to 29 boards.
What the stock length changes
The rows do not change with the stock length, but the boards per row do. The same deck with three stock lengths:
| Stock length | Calculated | Boards to buy |
|---|---|---|
| 12 ft | 26 rows × 2 boards per row = 52 boards | 58 boards |
| 16 ft | 26 rows × 1 board per row = 26 boards | 29 boards |
| 20 ft | 26 rows × 1 board per row = 26 boards | 29 boards |
A board shorter than the run needs a second piece in every row, and the planner counts a whole stock board for each piece. Whether you can use the offcut of one row to start another depends on how you stagger the joints, which this count does not try to optimize.
Why whole boards and an allowance
You cannot buy part of a board, and cuts and defects use some. Trex’s own count method adds about 5% for waste and rounds up. The planner starts at 10%, and you can set either: the right figure depends on the layout, such as a border or a diagonal pattern. Keep the extra editable and compare.
What changes the estimate?
- The actual board width, not the nominal name
- The gap between boards, taken from your decking maker’s instructions
- The stock lengths you can buy, since each short piece counts as a whole board
- The allowance for cuts, defects and layout
What this does not cover
This is a count of surface boards. It is not a cut plan, and it says nothing about the framing, fasteners or fascia under or around the deck. See how board width and gap change the number of rows if you are choosing between products.