How the deck calculator works
A deck is counted in three layers: the boards you walk on, the joists underneath, and the screws that tie them together. The calculator assumes the boards run along the deck length and the joists run across it at the spacing you enter.
Rows = ⌈(width + gap) ÷ (board width + gap)⌉ · Boards = ⌈rows × length × 1.10 ÷ board length⌉ · Joists = ⌈length ÷ spacing⌉ + 1 · Joints per row = ⌈length ÷ board length⌉ − 1 · Screws = rows × (joists + joints per row) × 2The 10% extra on boards pays for offcuts, staggered butt joints and the twisted or split boards you reject at the yard. The rows formula adds one gap to the width because n boards have only n − 1 gaps between them. Each board gets two screws at every joist it crosses, the usual face-fixing pattern for boards up to about 150 mm (6 in) wide; where two boards butt together on a joist, both ends need their own pair, so every butt joint adds two screws per row. Joists are counted with one at each end, each as long as the deck width.
Worked example
A 4.8 m × 3.6 m deck (17.3 m²) with 145 mm boards and 5 mm gaps: 24 boards cover only 24 × 145 + 23 × 5 = 3,595 mm, 5 mm short, so the formula gives (3,600 + 5) ÷ 150 = 24.03, rounded up to 25 rows. In practice many builders close that 5 mm by letting the edge board overhang the rim joist, but the calculator counts the full row. At 4.8 m per row that is 120 m of board, 132 m with 10% extra, or 28 boards of 4.8 m. Joists at 40 cm centres give 4.8 ÷ 0.4 = 12 spaces, so 13 joists of 3.6 m. Full-length boards have no butt joints, so screws are 25 × 13 × 2 = 650; buy about 700 to cover dropped and snapped ones. With 2.4 m boards instead, every row has one joint and needs 25 × (13 + 1) × 2 = 700 screws. The same deck in feet, 16 × 12 ft with 5.5 in boards, 3/16 in gaps and joists at 16 in, needs 26 rows, 29 boards of 16 ft, 13 joists and 676 screws.
Board sizes and joist spacing
| Decking | Actual size | Usual joist spacing (square to joists) |
|---|---|---|
| Softwood boards, Europe | 28 × 120 or 28 × 145 mm, 2.4–4.8 m long | 40 cm |
| Hardwood boards | 21–25 × 90–145 mm | 40–50 cm, as the supplier states |
| 5/4×6 treated pine, US | 1 × 5.5 in, 8–20 ft long | 16 in |
| 2×6 treated pine, US | 1.5 × 5.5 in | 24 in (16 in on a diagonal) |
| Composite boards | about 25 × 140 mm (1 × 5.5 in) | 40 cm / 16 in, 30 cm / 12 in on a diagonal |
Boards laid at 45° span further between joists, so the spacing must drop by about a quarter. Diagonal and herringbone patterns also waste more timber: allow 15% instead of 10%.
Tips before you order
- Gap for the material. Wet pressure-treated pine shrinks as it dries, so it is often laid almost tight. Kiln-dried hardwood needs 5–6 mm. Composite boards expand lengthwise with heat, so leave the end gaps the maker specifies.
- Land every joint on support. Butt joints must sit on a joist, ideally a doubled joist or blocking, and should be staggered row to row.
- Add picture-frame boards separately. A border around the edge adds the deck perimeter to the board length; add it before dividing by board length.
- Screw choice. Use stainless or coated deck screws rated for treated timber, about 2.5 times the board thickness: 60–65 mm for 28 mm boards, 2½ in for 5/4 boards. Hidden clip systems need one clip per board per joist instead of two screws.
- Structure is not a guess. Joist size, beams, footings and ledger fixings depend on span, height and local code. Raised decks often need a permit, so check with your building department or an engineer.
Frequently asked questions
How many deck boards do I need for a 12x12 deck?
With 5.5 in boards and 3/16 in gaps a 12 ft deck has 26 rows. Using 12 ft boards plus 10% extra, that is 29 boards.
How many screws per square foot of decking?
About 3.5 screws per ft² with 5.5 in boards on joists at 16 in, or about 38 per m² with 145 mm boards on joists at 40 cm, fixing two screws per joist. Every butt joint adds two more per row.
What is the standard joist spacing for a deck?
16 in (40 cm) on centre is the most common. Use 12 in (30 cm) for diagonal boards or where the composite maker requires it; 24 in only suits thick 2× decking.
How much gap should I leave between deck boards?
Dry boards: 3–6 mm (1/8–1/4 in). Wet pressure-treated boards can go almost tight because they shrink as they dry.
How long should deck screws be?
About 2.5 times the board thickness: 60–65 mm for 28 mm boards and 2½ in for 1 in (5/4) boards, so at least an inch or 25 mm bites into the joist.
Last reviewed: 2026-09-27 · How we calculate