Deck Calculator

How this calculator works

Boards run along the length of the deck and stack in rows across its width. Each row takes up the board width plus one gap, so the calculator works out the number of rows, multiplies by the boards needed per row, and adds wastage:

rows   = ceil( deck width ÷ (board width + gap) )
boards = ceil( rows × ceil(deck length ÷ board length) × wastage )
joists = floor( deck length ÷ joist spacing ) + 1

It also reports the total linear run of board - handy when decking is priced by the metre or foot rather than per board.

Worked example

This example follows the unit system you pick in the calculator above.

A 20 ft × 13 ft deck, 12 ft boards 5½ in wide with a ¼ in gap, 10% wastage, joists at 16 in:

1. Rows across the width: 13 ft = 156 in; 156 ÷ (5.5 + 0.25) = 156 ÷ 5.75 ≈ 27.1 → 28 rows.
2. Boards per row: 20 ÷ 12 = 1.67 → 2 boards.
3. Base boards: 28 × 2 = 56 → with 10%: 62 boards.
4. Total board run: 28 × 20 = 560 ft.
5. Joists: 20 ft = 240 in; 240 ÷ 16 = 15 → 15 + 1 = 16 joists.

Deck shape and edge treatment

Most decks are rectangular, but you can set any section to a right-angled triangle - handy for a wedge that follows a fence line or a corner cut-off. The board run tapers row by row, so the calculator measures each row’s length separately and adds them up; the area is ½ × leg A × leg B. Build an L-shape or angled deck by adding several sections.

The edge treatment controls what happens at the perimeter:

• Flush - field boards are trimmed level with the outer frame. The simplest finish and the baseline count.
• Overhang - boards run a little past the frame on every side and are trimmed straight after fixing. Set the amount in the calculator’s overhang field; the extra length and width can pull in another board or row, so it is counted in.
• Picture frame - a mitred border course (one to three boards wide) wraps the perimeter and the field boards are inset to butt against it. The calculator insets the field, adds the border boards needed to cover the perimeter run, and reports them separately. Pair this with the 15% wastage allowance for the angled mitre cuts.

Board and gap guidance

This table follows the unit system you pick in the calculator above.

Board type	Typical width	Suggested gap
Softwood (wet treated)	4¾–5¾ in	1/8 in (closes as it dries)
Kiln-dried hardwood	3½–5½ in	3/16–1/4 in
Composite	5¼–6 in	Per maker (often 3/16–5/16 in)

Tips for ordering

• Run boards in the direction that needs the fewest joins - usually the longer dimension.
• Stagger end joints over joists so no two adjacent rows join on the same joist.
• Order hidden fixings or screws by the board count: roughly two fixings per board per joist crossing.
• Composite boards expand more with heat - always follow the maker’s gap, which can be larger than for timber.

Board sizes and standards where you are

In the United States, deck boards are usually sized in inches - nominal 5/4 × 6 and 2 × 6 boards, about 5.5 in (140 mm) wide. Enter the board width you are actually buying in the calculator above so the row and board counts match what you order.

The counts here cover the deck surface. For joist, beam and post spans that meet local code, see the American Wood Council's DCA 6 Prescriptive Residential Wood Deck Construction Guide.