Deck boards are sold in fixed lengths, so the calculator counts whole boards for each row rather than working in square feet. Lumber is labeled with a nominal width, and the real width is narrower. The starting value, 5.5 inches, is the actual width of a common wood deck board.
How to use this calculator
- Measure the length in feet along the direction the boards will run, and the width across them. For an L-shaped deck, split it into rectangles, measure each one the same way, and add the results.
- Enter the actual width of one board, measured across its face, not the nominal size on the label. Keep the starting value only if you are buying the same kind of board.
- Open More options to set the gap, and keep the starting gap for wood decking. For composite, use the gap the maker gives.
- Pick a board length from the list. Each row is rounded up to whole boards, so a row that runs a little past one board length needs another board. Compare a few board lengths before you buy.
- Set the joist spacing from the list, using one that your decking maker and local code allow. A tighter spacing needs more joists.
- Add spare boards to the count, since the result has no allowance for waste. Then check the framing, the guard and the permit requirements before you order.
Worked examples
Small corner deck
Deck: 10 × 8 ft · Board length: 12 ft
- Deck boards
- 18 boards of 12 ft
- Boards (12 ft)
- 18
- Joists
- 9
- Rows of boards
- 18
- Decking
- 180 linear ft
- Deck area
- 80 sq ft
Narrow walkway deck
Deck: 24 × 6 ft · Board length: 20 ft · Joist spacing: 24 in
- Deck boards
- 26 boards of 20 ft
- Boards (20 ft)
- 26
- Joists
- 13
- Rows of boards
- 13
- Decking
- 312 linear ft
- Deck area
- 144 sq ft
Large backyard deck
Deck: 20 × 16 ft · Board length: 20 ft · Joist spacing: 16 in
- Deck boards
- 35 boards of 20 ft
- Boards (20 ft)
- 35
- Joists
- 16
- Rows of boards
- 35
- Decking
- 700 linear ft
- Deck area
- 320 sq ft
Short boards on a standard deck
Deck: 16 × 12 ft · Board length: 8 ft
- Deck boards
- 52 boards of 8 ft
- Boards (8 ft)
- 52
- Joists
- 13
- Rows of boards
- 26
- Decking
- 416 linear ft
- Deck area
- 192 sq ft
Deck boards for common deck sizes, by board length
| Deck size | Board length: 12 ft | Board length: 20 ft |
|---|---|---|
| 10 × 8 ft | 18 | 18 |
| 12 × 12 ft | 26 | 26 |
| 16 × 12 ft | 52 | 26 |
| 20 × 16 ft | 70 | 35 |
Each cell: Deck boards (boards of 12 ft). Inputs not in the table use the calculator's starting values.
How the board and joist counts are built
The calculator works out how many boards, with a gap between each pair, cover the deck's width, and rounds up to whole rows. For each row, it divides the deck's length by the board length, rounds up, and multiplies by the rows.
Joists run across the boards, so they are spaced along the deck's length. The count puts a joist at each end and another at every spacing mark, so a tighter spacing needs more joists. Stairs down from the deck are counted separately, with the deck steps calculator.
Joists, beams, footings and guards
The calculator counts joists but does not size them. Joist size, beam size, footings, the connection to the house and the guard are structural. They depend on the local code and usually need a permit, so check with your building department before you buy framing lumber. Round footings are a separate job, and the concrete footings page covers round forms.
A guard is required where the deck surface is more than 30 inches above the ground, and a required guard is at least 36 inches high. Check the guard height and the spindle spacing against the local code before you buy the railing. The guard spindle spacing page works out the spindle layout.
Wood and composite boards
The starting gap, 0.125 inch, is the figure a wood deck construction guide gives for wood boards. Change it only when your lumber supplier or the guide calls for another gap, since the gap sets how many rows fit across the width.
Composite makers set their own gaps, and those can change with temperature. Follow the product's instructions and enter that gap under More options. Composite boards also have their own actual widths, so use the width printed on the product.
Mistakes to avoid
- Entering the nominal width from the label. The actual width is narrower, so the row count comes out low and the deck runs short.
- Ordering exactly the counted boards. The result has no allowance for waste, so add spares for cut-offs and rejected pieces.
- Picking a board length without comparing. A row that runs a little past one board length needs an extra board, so check a few lengths first.
Common questions
How many deck boards do I need?
Enter the deck's length and width, the actual board width, the gap and the board length you plan to buy. The main result is the number of boards, with each row rounded up. Square feet alone does not give the count, because the board width, the gap and the board length all change it.
How far apart should deck joists be?
The spacing you pick changes only the joist count. The spacing you can use depends on the decking maker's instructions and the local code, so check both before you choose.
Does the deck calculator include joists and footings?
It counts joists at the spacing you pick but does not size them. It also leaves out footings, beams and posts, which depend on the local code and usually need a permit.
Can I use this calculator for composite decking?
Yes, for the board count. Enter the width printed on the product and the gap the maker gives, and follow the maker's instructions for joist spacing.
Can a deck calculator tell me if my deck can hold a hot tub?
No. This calculator counts boards and joists but does not check weight limits. A hot tub is a heavy, concentrated load, so ask your building department or a structural engineer to check the framing and footings before you install one.
How many extra deck boards should I buy for waste?
Add extra boards for cut-offs, warped pieces and mistakes. One decking maker suggests 10 to 15 percent extra for a straight layout and 30 percent for a diagonal one. Round up to whole boards.
How many extra boards do I need for a diagonal deck pattern?
Diagonal boards mean more angled cuts and more scrap, so one decking maker suggests adding about 30 percent. Composite makers also want joists no more than 12 inches apart for diagonal boards. Check your decking maker's guide.
How many deck screws do I need?
Fasten each deck board with 2 nails or screws where it crosses a joist, and composite makers ask for 2 screws at each joist too. Multiply the boards by the joists they cross and by that number, then add a handful of spares.