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Decking Calculator

Estimates the number of deck boards (rows of boards across the width, board lengths along the length, plus waste), the joists at a given spacing and the screws at two per board per joist for a rectangular deck.

Unit system (optional)Unit system of the length inputs (metric: m / cm / mm; imperial: ft / in). Outputs are reported in both systems. Deck length (board direction) (m or ft)Deck dimension parallel to the boards (m or ft). Joists run across this direction. Deck width (m or ft)Deck dimension across the boards (m or ft); equals the joist length. Board width (mm or in) (optional)Actual face width of one board. Default 140 mm (metric) or 5.5 in (imperial, a nominal 2 × 6 or 5/4 × 6). Gap between boards (mm or in) (optional)Drainage gap between boards. Default 5 mm (metric) or 3/16 in = 0.1875 in (imperial). Board length (m or ft) (optional)Length of the boards you will buy. Default 3.6 m (metric) or 12 ft (imperial). Joist spacing (mm or in) (optional)Centre-to-centre joist spacing. Default 400 mm (metric) or 16 in (imperial); composite boards laid diagonally usually need 300 mm / 12 in. Waste allowance (%) (optional)Extra material for cuts, breakage and offcuts, applied as a multiplier (1 + waste_percent/100). 5 % for straight decking; 10–15 % for diagonal or picture-frame patterns.

When to use

You need a material list for a rectangular timber or composite deck: boards, joists and fasteners.

Do not use when: You need structural sizing of joists and beams (span tables depend on species, grade and load; see the AWC DCA 6 guide) or stairs to the deck (use stairs).

Formula

rows = ceil(deck_width / (board_width + board_gap)); linear = rows × deck_length; boards = ceil(linear × (1 + waste/100) / board_length); joists = floor(deck_length / joist_spacing) + 1; screws = rows × joists × 2.

Assumes boards run parallel to deck_length and joists perpendicular to them at the given spacing, with a joist at each end. Ledger, beams, posts, blocking and stair framing are not included.

Inputs

Parameter	Type	Unit	Required	Description
`unit_system`	enum: metric \| imperial		default metric	Unit system of the length inputs (metric: m / cm / mm; imperial: ft / in). Outputs are reported in both systems.
`deck_length`	number	m or ft	yes	Deck dimension parallel to the boards (m or ft). Joists run across this direction. Range: > 0, ≤ 1000
`deck_width`	number	m or ft	yes	Deck dimension across the boards (m or ft); equals the joist length. Range: > 0, ≤ 1000
`board_width`	number	mm or in	no	Actual face width of one board. Default 140 mm (metric) or 5.5 in (imperial, a nominal 2 × 6 or 5/4 × 6). Range: > 0, ≤ 1000
`board_gap`	number	mm or in	no	Drainage gap between boards. Default 5 mm (metric) or 3/16 in = 0.1875 in (imperial). Range: ≥ 0, ≤ 100
`board_length`	number	m or ft	no	Length of the boards you will buy. Default 3.6 m (metric) or 12 ft (imperial). Range: > 0, ≤ 100
`joist_spacing`	number	mm or in	no	Centre-to-centre joist spacing. Default 400 mm (metric) or 16 in (imperial); composite boards laid diagonally usually need 300 mm / 12 in. Range: > 0, ≤ 2000
`waste_percent`	number	%	default 5	Extra material for cuts, breakage and offcuts, applied as a multiplier (1 + waste_percent/100). 5 % for straight decking; 10–15 % for diagonal or picture-frame patterns. Range: ≥ 0, ≤ 50

Outputs

Output	Type	Unit	Description
`area_m2`	number	m²	deck_length × deck_width.
`area_ft2`	number	ft²	Deck area in square feet.
`board_rows`	integer		ceil(deck_width / (board_width + board_gap)).
`linear_length_m`	number	m	board_rows × deck_length, before waste.
`linear_length_ft`	number	ft	Same in feet.
`boards_needed`	integer		ceil(linear_length × (1 + waste_percent/100) / board_length).
`joists`	integer		floor(deck_length / joist_spacing) + 1, each as long as deck_width (rim joists included in the count, beams/posts not).
`screws`	integer		board_rows × joists × 2.

Example

4 m × 3 m deck, 140 mm boards, 5 mm gap, 3.6 m boards, 400 mm joists, 5 % waste: {"unit_system":"metric","deck_length":4,"deck_width":3,"board_width":140,"board_gap":5,"board_length":3.6,"joist_spacing":400,"waste_percent":5} → {"area_m2":12,"area_ft2":129.17,"board_rows":21,"linear_length_m":84,"boards_needed":25,"joists":11,"screws":462}

16 ft × 12 ft deck, 5.5 in boards, 3/16 in gap, 16 ft boards, 16 in joists, 5 % waste: {"unit_system":"imperial","deck_length":16,"deck_width":12,"board_width":5.5,"board_gap":0.1875,"board_length":16,"joist_spacing":16,"waste_percent":5} → {"area_ft2":192,"area_m2":17.84,"board_rows":26,"linear_length_ft":416,"boards_needed":28,"joists":13,"screws":676}

GET https://tttkmbb.com/api/v1/calculate/decking?unit_system=metric&deck_length=4&deck_width=3&board_width=140&board_gap=5&board_length=3.6&joist_spacing=400&waste_percent=5

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/decking (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/decking · Markdown: https://tttkmbb.com/construction/decking.md · JSON definition: https://tttkmbb.com/construction/decking.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="decking"
OpenAPI operationId: estimate_decking_materials
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

FAQ

Which way should the boards run?

Boards are usually laid perpendicular to the joists and parallel to the longest deck side to minimise cuts; enter that side as deck_length and buy boards close to it or to a multiple of it.

Why two screws per joist?

Face-fixing with two screws (or one hidden clip) at every joist crossing is the standard pattern in manufacturer guides; hidden fastener systems use about one clip per board per joist instead.

Related calculators

Fence Calculator — Posts, rails and pickets for the fence around the deck.
Stair Calculator — Steps from the deck to the ground.
Board Feet Calculator — Convert lumber dimensions to board feet for pricing.