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Deck Board Calculator

Work out how many deck boards, joists and screws a deck needs, with the layout direction stated.

Deck length (boards run this way)Deck widthJoist spacing (centre to centre)Board widthGapClose-up
Key Boards run the length of the deck, across the joists.
Units
ft
ft
Typical board. A 5/4 x 6 deck board is about 5.5 in wide. Use the width of the board you are buying. Editorial rule of thumb.
in
in
Set lengths. Boards are sold in set lengths, so check your supplier’s list. The starting figure just matches the deck.
ft
in

Follow your board maker’s guide.

%
Board rows across the deck26 rows of 1 board
Boards, no waste26
Boards to buy, with waste29 boards
Joists13 × 12 ft
Screws676

Boards run along the 16 ft length, across joists that run along the 12 ft width. Rows: (12 ft + gap) ÷ (5.5 in board + gap) = 25.62, rounded up to 26. Joists: 16 ÷ 16 in spacing, rounded up, plus one = 13. Screws: 26 × 13 × 2 = 676.

Check the joist spacing against your board manufacturer’s guide and local rules.

Fig. 1 Boards, joists and screws for your deck.

Every formula is tested against worked examples. How we check

Joist spacingJoistsScrews
12 in17884
16 in13676
18 in12624
24 in9468

Fig. 2 The same deck at different joist spacings.

Last checked 11/10/2026. Sources: Olathe, Kansas: deck guidelines reproducing the IRC decking table (1¼ in boards 16 in on centre across joists).

Guide

How the amount is worked out

Boards run along the deck’s length, across the joists. The number of board rows = (deck width + gap) ÷ (board width + gap), rounded up. Each row is as many boards as it takes to span the length. Joists run across the width, one every joist spacing along the length, plus one at the end. Screws = rows × joists × screws per crossing.

Joist spacing

The IRC decking table, as reproduced in a Kansas city deck guide, allows 1¼ inch thick wood decking 16 inches on centre when laid across the joists (12 inches when laid diagonally) and 2 inch decking 24 inches. Composite boards often need closer spacing. The tool starts at 16 in; follow your board’s instructions and the code in your area.

Common mistakes

  • Forgetting the gap. Boards need a small gap for drainage and movement, so board width alone undercounts rows.
  • Ignoring the extra joist at the end.
  • Buying boards in lengths that leave short offcuts. Joints should land on a joist, so choose board lengths to suit the deck.

Safety and rules

This is not building or structural advice. Footings, ledger attachment, posts, beams, guard rails and stair details are all outside this calculator. Most areas require a permit for decks above a certain height or attached to a house. Before you dig, call 811 (free) so utilities can mark buried lines. Local rules, permits and height limits vary, so check with your city or county.

Common questions

How many deck boards for a 12x16 deck?

With 5-1/2 in boards, a 1/8 in gap and the boards running along the 16 ft length in 16 ft lengths, a 12 ft wide deck needs 26 rows, so 26 boards before waste.

How far apart should deck joists be?

16 inches on centre is common for 1-1/4 inch decking laid across the joists. Check your board manufacturer’s guide and local code.

How many screws per board?

Two screws at each joist crossing is common. Edge-board crossings sometimes get two as well.

Should I add waste?

5 to 10% covers cut-offs and the odd bad board. Diagonal layouts need more.

Planning a related job?

Worked examples

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