How to use the deck calculator
- Decide which way the boards run. Deck boards run at right angles to the joists. On a typical attached deck the joists run out from the house, so the boards run parallel to it. Enter the dimension along the boards as the length, and the dimension across them as the width.
- Enter the board's actual width. A 5/4×6 or 2×6 is about 5½ in wide, and a 2×4 is about 3½ in. Most "6-inch" composite boards are also about 5½ in, but check the spec sheet.
- Set the gap. 1/8–3/16 in is typical for kiln-dried wood. Wet pressure-treated boards are often laid tight because they shrink as they dry. Composite and PVC makers specify their own side and end gaps, which can depend on temperature.
- Pick a board length. Try a few lengths and compare. The best choice usually leaves the least offcut and puts every butt joint over a joist.
- Choose joist spacing based on what your decking allows (see the table below). If you pick 24 in, the result carries a reminder that only 2× wood decking spans that far; 5/4 wood and most composite boards need 16 in or less.
Deck board formulas
Boards per row = deck length ÷ board length, rounded up
Boards = rows × boards per row × (1 + waste %), rounded up
Joists = deck length (in) ÷ joist spacing, rounded up, + 1
Screws = 2 × rows × joists + 2 per butt joint
Each row of decking spans the full length of the deck. If one board isn't long enough, the row is made from several boards butted end to end, and that joint must land on a joist. The calculator doesn't plan a staggered joint layout or reuse offcuts from one row in another. If you mix lengths to stagger joints (for example 12 ft and 8 ft boards on a 20 ft deck), count those boards by hand. The waste allowance covers rejects, cuts and the occasional bad board.
Joists are counted from one end of the deck to the other, rounding up so there is always a joist at each end. If the length isn't an exact multiple of the spacing, the last bay is simply shorter. The count includes the two end joists but not a ledger, rim joists or blocking.
Worked example: a 16 × 12 ft deck
Using 5½ in boards with a 3/16 in gap, 16 ft boards, joists at 16 in on center and 10% waste:
- Rows: 12 ft × 12 = 144 in ÷ (5.5 + 0.1875) = 25.3, so 26 rows (the last board is ripped to fit).
- Boards per row: 16 ÷ 16 = 1, so 26 boards, plus 10% = 28.6, which rounds up to 29 boards (464 linear feet).
- Joists: 192 in ÷ 16 = 12, + 1 = 13 joists, each spanning the 12 ft width.
- Screws: 2 × 26 rows × 13 joists = 676 screws. There are no butt joints, so nothing extra is added.
- Area: 16 × 12 = 192 sq ft.
If a board is exactly as long as the deck, check that it can still cover any planned overhang past the rim joist. If it can't, choose the next length up.
Board sizes and joist spacing
The spacings below are common maximums for decking laid perpendicular to the joists. The wood values follow the IRC's decking table (Section R507). Composite and PVC boards are rated by their manufacturer, and many require 12 in spacing for diagonal decking or stair treads. Always check the manufacturer's installation guide and your local code.
| Decking | Actual width | Max joist spacing, perpendicular | Max joist spacing, diagonal |
|---|---|---|---|
| 5/4×6 wood | 5½ in | 16 in | 12 in |
| 2×6 wood | 5½ in | 24 in | 16 in |
| 2×4 wood | 3½ in | 24 in | 16 in |
| Composite or PVC | usually about 5½ in | per manufacturer (often 16 in) | per manufacturer (often 12 in) |
Butt joints and board length
With joists at 16 in on center, 8, 12, 16 and 20 ft boards all end exactly on a joist (96, 144, 192 and 240 in are all multiples of 16). A 10 ft board (120 in) doesn't, so it has to be trimmed or the joint needs extra blocking. Compare the options: a 20 ft deck with 12 ft boards uses 2 boards per row and throws away 4 ft per row. A 12 + 8 ft combination uses the same two boards with no offcut. Some builders avoid butt joints altogether with a "breaker board" laid across the deck over doubled joists.
Fasteners
Face-screwing uses two screws wherever a board crosses a joist, and two more at each butt joint. Use screws rated for your decking and for treated lumber. Composite makers often sell color-matched screws. Hidden clip systems use roughly one clip per board per joist crossing, which is about half the screw count, but the exact number depends on the system. Use the coverage on the box.
Practical tips
- Lay out the first and last boards. Divide the width before you start so you don't end with a sliver. Adjusting the gap slightly or ripping the first board can balance the layout.
- Crown and bark side. Sight down each wood board and install it consistently. Many builders also check the growth rings and lay boards with the bark side up.
- Order a few spares. Boards from a later batch can differ in color, which matters most with composite.
Frequently asked questions
How many deck boards do I need for a 12×16 deck?
With boards running the 16 ft way, 5½ in boards and a 3/16 in gap, you need 26 rows. Using 16 ft boards, that is 26 boards, or 29 with 10% extra.
How far apart should deck joists be?
16 in on center is the most common. Under the IRC, 5/4 wood decking laid square to the joists allows up to 16 in, and 2× decking up to 24 in. Diagonal decking and many composite boards need closer spacing, often 12 in. Check the decking manufacturer's instructions.
How many screws per deck board?
Two at every joist. A 16 ft board over joists at 16 in on center crosses 13 joists, so it takes 26 screws. Add two extra screws at every butt joint.
What gap should I leave between deck boards?
About 1/8–3/16 in for dry wood. Wet pressure-treated boards are often installed tight because they shrink. Composite and PVC boards need the side and end gaps in the manufacturer's instructions.
Do I need a permit to build a deck?
Usually. The model IRC exempts only small, low, freestanding decks: not over 200 sq ft, not more than 30 in above grade, not attached to the house and not serving a required exit door. Local rules can be stricter, so ask your building department.