How many deck boards do I need?
Enter the size of the deck, the width and length of your boards, the gap and the joist spacing. See how many boards to buy, the rows they make, and how many joists sit under them.
- Deck boards
- 29
A 192 ft² deck needs 29 boards in 26 rows, over 13 joists.
- Rows of boards
- 26
- Linear feet of board
- 416 ft
- Joists
- 13
- Deck area
- 192 ft²
Deck boards: 29. A 192 ft² deck needs 29 boards in 26 rows, over 13 joists.
How to calculate
Computes the rows of deck boards across a deck, the linear feet and number of boards to buy with extra for waste, the joists under them, and the cost.
Example with the default inputs (Deck length 16 ft, Deck width 12 ft, Board width 5.5 in, Gap between boards 0.1875 in, Board length 16 ft, Joist spacing 16 in, Extra for waste 10%): A 192 ft² deck needs 29 boards in 26 rows, over 13 joists.
Method: rows = ⌈(width + gap) ÷ (board width + gap)⌉; linear = rows × length; boards = ⌈linear × (1 + waste ÷ 100) ÷ board length⌉; joists = ⌈length ÷ spacing⌉ + 1
- Boards run along the deck length and the joists run across it, square to the boards. The last row is ripped to fit.
- Boards are counted by total length, not by a cutting plan. Offcuts and end joints come out of the waste allowance.
- The joist count spaces joists from one end, with one at each end. It does not size the joists, beams or footings.
- A count within 0.000000001 of a whole number counts as that whole number, so exact fits are not rounded up.
Worked examples
Each example is checked against the calculator on every build.
- Deck length 16 ft, Deck width 12 ft, Board width 5.5 in, Gap between boards 0.1875 in, Board length 16 ft, Joist spacing 16 in, Extra for waste 10%, Price per board $42.00 gives Rows of boards 26, Linear feet of board 416 ft, Deck boards 29, Joists 13, Cost of boards $1,218.00.Source: hand calculation in content.mdx: (144 + 0.1875) ÷ (5.5 + 0.1875) = 25.35, so 26 rows; 26 × 16 = 416 ft; × 1.10 ÷ 16 = 28.6, so 29 boards; 192 ÷ 16 + 1 = 13 joists; 29 × $42 = $1,218
- Deck length 20 ft, Deck width 10 ft, Board width 5.5 in, Gap between boards 0.25 in, Board length 12 ft, Joist spacing 12 in, Extra for waste 5% gives Rows of boards 21, Linear feet of board 420 ft, Deck boards 37, Joists 21.Source: hand calculation in content.mdx: (120 + 0.25) ÷ 5.75 = 20.91, so 21 rows; 21 × 20 = 420 ft; × 1.05 ÷ 12 = 36.75, so 37 boards; 240 ÷ 12 + 1 = 21 joists
- Deck length 10 ft, Deck width 0.9583 ft, Board width 5.5 in, Gap between boards 0.5 in, Board length 10 ft, Joist spacing 24 in, Extra for waste 0% gives Rows of boards 2, Deck boards 2, Joists 6.Source: hand calculation in content.mdx: (11.5 + 0.5) ÷ (5.5 + 0.5) = 2 rows exactly; 20 ft ÷ 10 = 2 boards; 120 ÷ 24 + 1 = 6 joists
How it works
The boards run along the deck length; the joists run across it at right angles to the boards.
- Rows = (deck width + gap) ÷ (board width + gap), rounded up. n rows cover n board widths and n − 1 gaps; the last row is ripped to fit.
- Linear feet = rows × deck length.
- Boards = linear feet × (1 + waste ÷ 100) ÷ board length, rounded up.
- Joists = deck length ÷ joist spacing, rounded up, + 1 (one at each end).
- Deck area = length × width. Cost = boards × price per board.
A count within 0.000000001 of a whole number counts as that whole number, so an exact fit is not rounded up.
The board count is by total length. It does not plan the cuts, so a board shorter than the deck is assumed to join end to end over a joist, and offcuts come out of the waste allowance.
Worked examples by hand
A 16 ft × 12 ft deck, 5 1/2 in boards, 3/16 in gaps, 16 ft boards, joists 16 in apart, 10% waste, $42 a board.
- Rows: (144 + 0.1875) ÷ (5.5 + 0.1875) = 144.1875 ÷ 5.6875 = 25.35, so 26 rows.
- Linear feet: 26 × 16 = 416 ft.
- Boards: 416 × 1.10 ÷ 16 = 28.6, so 29 boards.
- Joists: 192 in ÷ 16 in = 12, + 1 = 13 joists.
- Cost: 29 × 42 = $1,218.
A 20 ft × 10 ft deck, 5 1/2 in boards, 1/4 in gaps, 12 ft boards, joists 12 in apart, 5% waste. Rows: 120.25 ÷ 5.75 = 20.91, so 21. Linear feet 21 × 20 = 420 ft. Boards 420 × 1.05 ÷ 12 = 36.75, so 37. Joists 240 ÷ 12 + 1 = 21.
A 10 ft walkway 11 1/2 in wide, 5 1/2 in boards, 1/2 in gap, 10 ft boards, joists 24 in apart, no waste. Rows: 12 ÷ 6 = exactly 2. Boards: 2 × 10 ÷ 10 = 2. Joists: 120 ÷ 24 + 1 = 6.
Other questions people ask
How do I calculate how many deck boards I need?
Divide the deck width, plus one gap, by the board width plus the gap, and round up: that is the number of rows. Multiply by the deck length for the linear feet, add waste, and divide by the board length. A 16 × 12 ft deck with 5 1/2 in boards and 3/16 in gaps takes 26 rows, 416 linear feet, and 29 boards of 16 ft with 10% waste.
How wide is a deck board?
Deck boards are sold by nominal size. Under the American Softwood Lumber Standard (PS 20), a nominal 6 in board is 5 1/2 in wide when dressed, and 5/4 × 6 decking is about 1 in × 5 1/2 in. Composite boards are usually close to 5 1/2 in; check the product.
How big should the gap between deck boards be?
Follow the board maker. Trex, for example, recommends 3/16 in between the long edges of its composite boards in all weather, and 1/8 in to 3/16 in end to end depending on the temperature at installation.
How far apart should deck joists be?
The International Residential Code (IRC 2021, Table R507.7) allows joists up to 16 in on center under 1 1/4 in (5/4) wood decking laid square to the joists, and up to 24 in under 2 in wood decking; diagonal decking needs closer joists (12 in and 16 in). Composite boards have their own limits in the maker’s instructions.
How much extra decking should I buy?
Enter your own allowance in Extra for waste. The calculator counts boards by total length, so end joints, offcuts and bad boards all come out of that allowance; a deck with angles or a picture frame border uses more.
Does this size the joists and beams?
No. It counts joists at the spacing you choose. Joist and beam sizes, spans, ledgers and footings come from your local building code (IRC section R507 in many US areas).