Decking and roofing

Deck Board Calculator

Enter your deck size, board width, gap and board length to see how many deck boards to buy, how many rows you need and how wide to cut the last board.

Units
ft
Measured in the direction the boards run.
ft
Measured side to side, across the boards.
in
AWC DCA 6: about 1/8 in (0.125) for wood decking. Composite: follow the maker.
ft
Lumber is sold from 6 to 24 ft long (SPIB).
%
For bad boards and cutting mistakes. 0 gives the exact count.

Enter your deck size and board details, then press Calculate.

How we calculate

We work out how many rows of boards fit across the deck, then how many boards each row needs.

Rows = (deck width + gap) / (board width + gap), rounded up Boards per row = deck length / board length, rounded up Boards = rows x boards per row, plus any extra %, rounded up

Why add one gap to the deck width? A row of n boards has only n – 1 gaps between them, so this counts exactly enough boards to cover the width. If the last board sticks out, we tell you how wide to cut it.

Actual vs nominal size: lumber is sold by nominal size. The Southern Pine Inspection Bureau explains that “the 2×4’s and 2×6’s are 1/2″ less than nominal in width”, following the American Softwood Lumber Standard (PS 20). So a 2×6 is 5.5 in wide and a 2×4 is 3.5 in wide. Source: SPIB, Nominal vs actual lumber sizes.

Gap: the American Wood Council deck guide says to “space decking boards approximately 1/8″ apart” and that “each segment of decking must bear on a minimum of 3 joists (or 3 supports)”. It also calls for decking of dimension lumber (2 in nominal thickness) or span-rated decking products. For composite boards, use the gap in the product instructions. Source: American Wood Council, DCA 6 Prescriptive Residential Wood Deck Construction Guide (PDF).

We assume boards run straight across the joists, whole boards in each row, and no reuse of offcuts. DCA 6 allows decking laid at up to 45 degrees to the joists; diagonal decking needs more boards than this count.

Worked example

A 16 x 12 ft deck with 2×6 boards, a 1/8 in gap and 16 ft boards:

  1. Deck width: 12 x 12 = 144 in
  2. Rows: (144 + 0.125) / (5.5 + 0.125) = 25.6, rounded up to 26 rows
  3. Boards per row: 16 / 16 = 1
  4. Boards: 26 x 1 = 26 boards (416 linear ft)

26 boards and 25 gaps cover 146.125 in, 2.125 in more than the deck, so the last board is cut to about 3.38 in. With 8 ft boards the same deck needs 2 per row: 52 boards, with joints planned over joists.

Top view of deck boards in rows with small gaps between themBoard length (along)Width (across)Gap about 1/8 in5 rows shown
Rows run across the deck width. Each row is one board, or more if the deck is longer than a board.

What this calculator does not cover

It counts deck boards only. It does not size joists, beams, posts, footings, stairs or railings. For those, see the American Wood Council DCA 6 deck guide and 2024 span tables for joists and rafters, and check with your local building department: decks often need a permit.

DCA 6 also covers fastening: it says to attach decking to each joist with 2 threaded nails (8d) or 2 #8 screws.

Materials checklist

  • Deck boards, using the count above
  • Fasteners: 2 per board at every joist (per DCA 6), or the hidden fasteners your product requires
  • A spacer for even gaps (a nail or a purpose-made spacer)
  • Saw, drill or driver, chalk line, tape measure and square
  • Gloves and eye protection

FAQ

How many deck boards do I need for a 12×16 deck?

With 2×6 boards (5.5 in wide), a 1/8 in gap and 16 ft boards running the 16 ft way: 26 boards. With 8 ft boards it is 52.

What gap should I leave between deck boards?

The American Wood Council deck guide (DCA 6) says approximately 1/8 in for wood decking. Composite and PVC boards have their own gap rules: follow the product instructions.

Why is a 2×6 only 5.5 inches wide?

Lumber is named by nominal size. Under the American Softwood Lumber Standard, 2x4s and 2x6s are 1/2 in narrower than nominal, so a 2×6 is 5.5 in wide.

Does this include joists or railings?

No. It counts deck boards only. Use the AWC span tables and DCA 6 guide for framing, and check local code.

Can I use metric units?

Yes. Switch to Metric and enter meters for the deck and board length and centimeters for the board width and gap.

Part of our outdoor-career toolkit: From office to outdoors.

Estimates only. Results are estimates based on the measurements you enter. Check local building codes and your product labels, confirm quantities with your supplier, and buy a little extra material. This is not professional or structural advice. Decks can need a permit, and framing, footings and railings are set by your plans and local code, not by this calculator. Last checked: September 27, 2026.

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