Understanding your result
The headline is the number of deck boards to buy, in the length you chose, with the waste allowance included. Below it is the total length of decking in linear feet, which is how lumber yards usually quote a price.
Rows of boards is the deck dimension across the boards, divided by the width of one board plus one gap, rounded up. The last row is nearly always ripped to a narrower width so that the deck finishes where it should.
Joists are counted from your spacing, plus one to close the far end. They run at right angles to the deck boards, so if you change the board direction, the joist length changes with it. The count covers the regular joists only. Rim joists, beams and blocking are extra.
The drawing shows the deck from above. Solid lines are deck boards, and dashed lines show the direction of the joists underneath.
How we calculate this
For a diagonal layout there are no simple rows, so the calculator works from the area instead:
The area method gives the same total length as the row method for straight boards, give or take the last partial row. The difference with diagonal decking is waste. Every board is cut at an angle at both ends, and the offcuts are triangles that can rarely be reused, which is why the allowance should go up to 15%.
The assumptions behind the numbers
| Assumption | Value used | Why |
|---|---|---|
| Board width | Actual width, 5.5 in unless you change it | Nominal sizes are larger than the real board |
| Gap | 1/8 in (3 mm) unless you change it | Typical for wood. Follow the maker for composite |
| Waste | 10% unless you change it | End cuts and defects. Use 15% for diagonal layouts |
| Board count | Total length ÷ board length | Assumes offcuts are reused where they reach a joist |
| Joists | Row length ÷ spacing, plus one | Regular joists only. No rim joists, beams or blocking |
| Screws | 2 per board at each joist | Standard for boards up to 6 in wide |
| Hidden clips | 1 per board at each joist | First and last boards are face-screwed |
Assumptions last reviewed October 3, 2026.
The board count is a material estimate, not a cutting plan. If your boards are shorter than the run, every row has a butt joint, each joint must sit on a joist, and joints in neighboring rows should be staggered. Buying boards long enough to span the deck in one piece costs a little more per foot and saves most of that trouble.
Two worked examples
A deck with boards along the length
A deck is 20 feet long and 14 feet wide. The boards are 5.5 inches wide with an eighth-inch gap, bought in 20 foot lengths and laid along the length. Joists are at 16 inches.
- Board coverage: 5.5 + 0.125 = 5.625 in
- Rows: 14 × 12 ÷ 5.625 = 29.9, so 30
- Decking: 30 × 20 = 600 linear ft
- With 10% waste: 660 linear ft
- Boards of 20 ft: 660 ÷ 20 = 33 boards
- Joists: 20 × 12 ÷ 16 = 15, plus 1 = 16 joists, each 14 ft
- Screws: 30 × 16 × 2 = 960
Thirty rows need thirty full-length boards, so the waste allowance here amounts to three spare boards. That is about right. In any delivery of lumber, one or two boards are twisted or split badly enough to set aside.
A small diagonal deck
A square deck 12 feet by 12 feet is laid diagonally with the same 5.5 inch boards, bought in 16 foot lengths. Joists are closed up to 12 inches, and the waste allowance is raised to 15%.
- Area: 12 × 12 = 144 sq ft
- Decking: 144 × 12 ÷ 5.625 = 307 linear ft
- With 15% waste: 353 linear ft
- Boards of 16 ft: 353 ÷ 16 = 22.1, so 23 boards
- Joists: 12 × 12 ÷ 12 = 12, plus 1 = 13 joists, each 12 ft
Laid straight on joists at 16 inches, the same deck would take 22 boards and 10 joists. The diagonal look costs one extra board and three extra joists. The joists are the real expense, and they are easy to overlook when comparing layouts.
Common mistakes
- Using the nominal board width. A “6 inch” board covers 5.5 inches. Work from the real width or you will come up two or three rows short.
- Forgetting the gap. Small as it is, an eighth of an inch per row adds up to a few inches across a deck.
- Keeping 16 inch joists under diagonal or composite decking. Both usually need 12 inch spacing. Check before the frame is built, not after.
- Buying boards exactly the length of the deck. Board ends are rarely square or clean. Allow enough to trim both ends.
- Ignoring where joints fall. A joint that lands between joists has nothing under it. Plan the board lengths around the joist layout.
- Treating the joist count as a frame design. The count is for ordering. Beam and joist sizes come from span tables and code.
- Skipping the waste on a diagonal layout. Ten percent is not enough when both ends of every board are cut at 45 degrees.
Printable shopping list
Print shopping list gives a single page with the boards, joists and fasteners, the deck size and the settings you used. Lumber yards price decking by the linear foot or by the board, so the breakdown table has both. Ask for boards to be picked straight, and stack them flat and off the ground until you lay them.
Questions people ask
- How many deck boards do I need?
- Divide the width of the deck by the board width plus the gap to get the number of rows, then multiply by the length of each row and add about 10% for waste. A 16 × 12 foot deck with 5.5 inch boards and an eighth-inch gap needs 26 rows of 16 feet, which is 29 boards of 16 feet once waste is added.
- How wide is a deck board really?
- A board sold as "5/4 × 6" or "2 × 6" is 5.5 inches wide, not 6. A "2 × 4" is 3.5 inches. Composite boards are usually close to 5.5 inches as well, but check the maker's sheet. Using the nominal width in the sum leaves you about 8% short.
- How much gap should I leave between deck boards?
- About an eighth of an inch (3 mm) for most wood decking, so that water drains and the boards can swell. Wet pressure-treated lumber is often laid tight because it shrinks as it dries. Composite makers specify their own gaps, which vary with temperature.
- How far apart should deck joists be?
- Sixteen inches on center is standard for wood decking laid straight. Diagonal decking and most composite boards need joists at 12 inches. Twenty-four inches is only suitable for thick 2-inch lumber decking, and many codes do not allow it for anything thinner.
- How many screws do I need for a deck?
- Two screws wherever a board crosses a joist. For a 16 × 12 foot deck with joists at 16 inches, that is 26 rows crossing 13 joists, or 676 screws. As a rule of thumb, allow about 350 screws for every 100 square feet of deck.
- Does this calculator design the deck frame?
- No. It counts joists from the spacing you choose, but beam sizes, post positions, footings and the connection to the house depend on spans, loads and local building code. Use it for the decking order, and get the framing from your building department's span tables or a designer.
Built and reviewed by Bertha
Last updated · How we work out our numbers