Deck Calculator

How many deck boards, joists and screws you need: deck size, board width and gap, board length and joist spacing, in feet or metres.

Decking is laid in rows of boards across the joists, so the count comes from the deck's width divided by one board plus its gap, times the boards needed to cover each row's length. A 16 × 12 ft deck with 5½ in boards and 3⁄16 in gaps has 26 rows; with 16 ft boards that's 26 boards, 29 with 10% extra. Joists run the other way at 16 in centres: 13 joists 12 ft long. At two screws per board per joist, that's 676 screws. In metric, a 4.8 × 3.6 m deck of 140 mm boards with 5 mm gaps has 25 rows. Enter your deck, board size, length and joist spacing, and the calculator gives the boards, joists and screws, with a cost.

Units
Board lengths
ft
ft
in
in
in
%
Deck boards
29

16 ft lengths

Joists
13

12 ft long at 16 in

Screws
676

2 per board per joist

Deck area
192 ft²

26 rows of boards

How it was worked out
  1. Deck: 16 ft along the boards × 12 ft across = 192 ft²
  2. Rows of boards: 12 ft ÷ (5.5 in board + 0.187 in gap) = 25.32, rounded up to 26
  3. Each row needs 1 × 16 ft board: 26 × 1 = 26; plus 10% = 29 boards
  4. Joists run across the boards at 16 in centres: 13 joists of 12 ft
  5. Screws: 2 per board at each joist = 26 rows × 13 joists × 2 = 676

Joist size, joist spacing, beams and posts come from span tables, the decking maker and your local code: composite and diagonal decking often need closer joists. Leave the gap the board maker specifies.

Common use cases

  • Order deck boards and screws for a new deck
  • Replace the boards on an existing frame
  • Compare board lengths to reduce joins and waste
  • Price timber or composite decking

How to use

  1. Choose your units and country (for the usual board lengths).
  2. Enter the deck's length along the boards and its width across them.
  3. Enter the board width, gap, length and the joist spacing.
  4. Read the boards, joists and screws, and add a price per board for the cost.

FAQ

How many deck boards do I need?

Divide the deck's width by the board width plus the gap and round up for the rows; multiply by the boards needed per row; add 5 to 10% for cuts. For a 12 ft wide deck of 5½ in boards with 3⁄16 in gaps, that's 26 rows.

How far apart should deck joists be?

16 in centres is common for wood decking in the US, with 400 to 450 mm usual in the UK and Australia. Composite boards and diagonal layouts often need closer joists, so follow the board maker's span.

How many screws per deck board?

Two screws where each board crosses a joist, so rows × joists × 2. Hidden fastener systems use clips instead; check their coverage per box.

Related tools

Results are estimates for planning and buying materials, worked out from the figures you enter and typical values (weights, coverage, sizes) that vary by product and supplier. They are not engineering, building or professional advice. Check your local building code, the product maker's instructions and your supplier's figures before you buy or build, and use a qualified professional for structural, electrical and gas work. Sizes, spans, depths and reinforcement for anything structural must come from span tables, your building code or an engineer.