Deck Board & Joist Calculator
Estimate deck boards, lineal metres or feet of decking, the number of joists, and deck screws for a timber or composite deck from its size, board width, gap, and joist spacing.
Deck area 20.0 m²; 28 board courses across the width. Screw estimate is 2 per board-course / joist crossing — add extra for butt joints and buy screws by the box.
How this works
Boards run along the deck length and are laid in courses across the width. Joists run across the width and are spaced along the length:
coverage = board width + gap
courses = ceil(deck width / coverage)
boards/course = ceil(deck length / board length)
boards = courses × boards/course × (1 + waste)
joists = floor(deck length / spacing) + 1
screws = courses × joists × 2Lineal length of decking = courses × deck length. Suppliers often price timber and composite by the lineal metre or foot, so that figure is shown too.
Worked example
A 5 m × 4 m deck with 140 mm boards, a 5 mm gap, 3.6 m boards, and joists at 406 mm centres, 10% waste:
- coverage =
145 mm; courses =ceil(4 / 0.145)= 28 - boards/course =
ceil(5 / 3.6)= 2; boards =28 × 2 × 1.10≈ 62 - joists =
floor(5 / 0.406) + 1= 13; screws =28 × 13 × 2= 728
Sources
- Board gap and spacing guidance follows common timber and composite decking fixing guides; always confirm against the manufacturer's span table.
FAQ
How do I calculate how many deck boards I need?
Divide the deck width by the board coverage (board face width plus the gap between boards) to get the number of courses, then divide the deck length by the length of the boards you buy to find how many boards make up each course. Multiply the two and add a waste allowance for cuts, offcuts, and defective boards. This calculator does all of that and also reports the total lineal length of decking, which is how many suppliers price timber and composite decking.
What gap should I leave between deck boards?
For pressure-treated or hardwood timber that is installed wet or green, a 3 to 5 mm (about 1/8 to 3/16 in) gap lets the boards drain and allows for shrinkage as they dry. Kiln-dried timber can be spaced wider because it will expand. Most composite decking is installed with a 5 to 6 mm (roughly 1/4 in) gap along the length and a slightly larger gap at board ends — always follow the composite manufacturer's fixing guide, because it affects the warranty.
How far apart should deck joists be?
Joist spacing depends on the decking material and the direction the boards run. Timber decking laid at 90 degrees to the joists is commonly supported at 400 to 450 mm (16 to 18 in) centres; boards laid diagonally need closer spacing, around 300 mm (12 in). Composite decking often requires 300 to 400 mm centres depending on the brand. The joist size and span are structural decisions — check the decking manufacturer's span table and your local building code.
How many screws do I need for a deck?
As a rule of thumb, use two screws at every point where a board crosses a joist. This calculator estimates screws as the number of board courses multiplied by the number of joists, times two. Add extra for butt joints, where two board ends meet on a shared joist and each end needs its own pair of screws. Buy screws by the box and keep a spare box — running out mid-deck is a common frustration.
Does the board run direction change the quantities?
Yes. Boards run along the length you enter, and joists run across the width and are spaced along the length. If you rotate the deck boards 90 degrees, swap the length and width values, because the number of courses, the joist count, and the offcut pattern all change. Running boards along the longer dimension usually means fewer butt joints but can mean more joist material.
Should I include the substructure, fascia, and railing?
No. This calculator covers the deck boards, joists, and deck screws only. A complete deck also needs bearers or beams, posts and footings, ledger fixings, joist hangers, fascia or edge boards, and often a balustrade or railing. Size the posts, beams, and footings from an engineer or an approved span table, and add fascia, stairs, and railing materials to your order separately.