How to use the deck board calculator
- Enter the deck size. Length runs along the house (the direction the boards run); width is how far the deck projects out (the direction the joists run).
- Pick the board. Most decking — 5/4×6 wood, 2×6 and composite — is 5.5 in wide. The gap between boards is usually ⅛–¼ in; check your composite maker’s install guide.
- Choose the board length you’ll buy. The calculator works out how many full boards each row needs, or how many rows you can cut from one long board.
- Set joist spacing and fastening. 16 in on center is standard for 5/4 and composite boards laid straight; diagonal 5/4 decking needs 12 in (IRC Table R507.7), and composites follow their maker’s spec.
Deck board formula with spacing
A row of decking covers its own width plus one gap. So the number of rows is the deck width divided by (board width + gap), rounded up:
Boards = rows × boards per row (or rows ÷ rows per board)
Joists = deck length (in) ÷ spacing + 1 → round up
Screws = rows × joists × 2
Each board crosses every joist and gets two face screws at each crossing — the standard pattern for wood and most capped composites. Hidden fastener systems use one clip per crossing (the calculator switches to that count).
Why spacing matters
On a 12 ft wide deck with 5.5 in boards, ignoring a ³⁄₁₆ in gap gives 26.2 rows (27 to buy); including it gives 25.4 (26 to buy). That one-row difference is a full deck length of board, and it grows on wider decks or with ¼ in gaps.
Worked examples
16 × 12 ft deck
- Rows: (144 + 0.1875) ÷ (5.5 + 0.1875) = 25.4 → 26 rows of 16 ft boards
- Boards: 26 + 10% waste = 29 boards (416 linear ft)
- Joists at 16 in o.c.: 192 ÷ 16 + 1 = 13 joists, each about 12 ft
- Screws: 26 × 13 × 2 = 676, plus 5% = 710 screws
20 × 14 ft composite deck, 12 ft boards
The rows need 20 ft of length but the boards are 12 ft, so each row takes two boards with a butt joint on a joist (stagger the joints row to row). 30 rows × 2 = 60 boards, plus 10% = 66 boards. At 12 in on center you need 21 joists and about 1,323 screws.
Butt joints need a doubled joist or blocking so both board ends land on solid wood. Plan board lengths to minimize joints — buying 20 ft boards for a 20 ft deck often costs less than the extra framing.
Waste factor and buying tips
- 10% waste covers boards with defects, end trims and cut-offs on a straight-laid deck. Use 15% for diagonal decking, a picture-frame border or lots of posts to notch around.
- Hand-pick boards if you can. Reject boards with crook, cupping, loose knots or splits at the ends.
- Pressure-treated lumber shrinks. Wet PT decking is often installed tight (no gap) and opens up as it dries; kiln-dried and composite boards need the gap at install.
- Buy the right screws. Use coated or stainless deck screws rated for treated lumber, or the color-matched screws specified for your composite. Ordinary drywall screws corrode and snap.
- Joists are code items. Joist size and maximum span depend on species, grade, spacing and load. Most US jurisdictions follow IRC section R507 for decks — check with your building department before you buy framing.
Setting the footings? The concrete bag calculator handles post holes. Building a roof over it? See the roofing calculator.
Deck board quick reference
| Deck size | Rows (5.5 in, ³⁄₁₆ in gap) | Joists @ 16 in | Screws |
|---|---|---|---|
| 10 × 10 ft | 22 | 9 | ≈ 416 |
| 12 × 12 ft | 26 | 10 | ≈ 546 |
| 16 × 12 ft | 26 | 13 | ≈ 710 |
| 20 × 16 ft | 34 | 16 | ≈ 1,143 |
Sources & assumptions
How this calculator gets its numbers. Full method: our methodology.
- Boards run parallel to the house; rows = deck depth ÷ (board width + gap). Nominal 5/4×6 and 2×6 boards are 5.5 in wide.
- Default joist spacing is 16 in on center, the common maximum for 5/4 wood decking laid perpendicular to joists; composite boards follow the maker's evaluation report.
- Default waste is 10%; boards and screws are rounded up. The tool does not size joists, beams or footings.
Sources
- International Code Council, 2021 International Residential Code, Chapter 5 (Section R507, Exterior Decks). Deck joist spacing for decking (Table R507.7) and prescriptive deck framing rules.
- American Wood Council, DCA 6, Prescriptive Residential Wood Deck Construction Guide. Free guide to code-based deck framing, ledgers and footings.
Last reviewed . Product figures vary by brand; the label on what you buy wins.
Frequently asked questions
How many deck boards do I need for a 12x12 deck?
With 5.5-inch boards and a 3/16-inch gap, a 12 × 12 ft deck needs 26 rows of 12 ft boards. Adding 10 percent for waste, buy 29 boards. You also need 10 joists at 16 inches on center and about 550 deck screws.
How do you calculate deck boards with spacing?
Divide the deck width in inches, plus one gap, by the board width plus the gap, and round up. That gives the number of rows. Multiply rows by the number of boards each row needs (based on board length) and add 10 percent for waste.
What is the standard gap between deck boards?
Typically 1/8 to 1/4 inch, with 3/16 inch common. Composite manufacturers specify their own gaps, and wet pressure-treated lumber is often laid tight because it shrinks as it dries.
How many screws do I need per deck board?
Two screws at every joist the board crosses. With joists at 16 inches on center, a 16 ft board crosses 13 joists and needs 26 screws. Hidden clip systems use one clip per crossing.
Should deck joists be 12 or 16 inches on center?
16 inches on center is standard for 5/4 wood and most composite decking laid straight. The IRC (Table R507.7) limits 5/4 wood decking laid diagonally to 12 inches on center; 2-inch decking can span 24 inches straight or 16 inches diagonally. Composite boards follow the manufacturer's spacing, often 12 inches for diagonal layouts. Always check your local code for joist size and span.