DIYCalcLab

Topic · Decking estimating

Deck boards: actual width, gaps and waste

By DIYCalcLab Editorial Team · Updated

This page is the method for deck boards, not a catalog of outdoor structures and not a structural design. Rows come from the actual board width plus the gap. The joist count uses a spacing you already have from a plan or a prescriptive table. This site does not choose that spacing.

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Core calculators

Start with the tool that matches your job. Each page shows the formula, defaults, shopping list and sources.

Rows, boards and a joist count you did not design here

  • Rows = ceil(deck width ÷ (actual board width + gap))
  • Boards = ceil(rows × deck length × (1 + waste %) ÷ stock length)
  • Joists = ceil(deck length ÷ on-center spacing) + 1, using the spacing you type
  • Face screws = rows × joists × 2. Boxes = ceil(screws ÷ screws per box). The 350-count default is not a brand

Variables

Actual width
Trex describes several 1×6-class boards as 5.5 in wide. A nominal 6 in board is not 6 in of coverage
Waste
This calculator defaults to 10%. Trex’s board article calls 5% a general rule of thumb. They are different allowances. The worked example uses 10%
Spacing
An input. AWC DCA 6 tables choose an allowable span from species, size and spacing. This page does not read those tables
Lengths the board calculator already uses
FromTo
5.5 in140 mm, approximately
3/16 in gapabout 5 mm
16 in on center406 mm
12, 16 and 20 ft boardsthe lengths Trex lists, and the imperial choices on the calculator

Worked examples

Each example uses the same inputs as the linked calculator. The link loads those inputs; the page URL without parameters stays the canonical.

16 × 12 ft deck, 5.5 in boards, 3/16 in gap, 16 ft stock, 16 in spacing supplied, 10% waste

Imperial · length 16 ft (board direction) · width 12 ft · board width 5.5 in · gap 0.1875 in · board 16 ft · spacing 16 in · waste 10% · 350 screws per box (scenario default)

  1. Rows = 144 ÷ (5.5 + 0.1875) = 25.32 → 26
  2. Linear = 26 × 16 = 416 ft. × 1.10 ÷ 16 = 28.6 → 29 boards
  3. Joists = 16 ft ÷ 16 in + 1 = 13. This uses the 16 in spacing as a given
  4. Screws = 26 × 13 × 2 = 676 → 2 boxes at 350 (700 screws, 24 spare)

Result: 29 boards and 13 joists. Purchase is 29 boards and 2 screw boxes. The joist count is not a span approval.

Open this example in the calculator →

Board order versus framing

  1. Buy boards from rows, stock length and the waste percent you accept.
  2. Count joists only from the spacing already on your plan. Do not treat that count as proof the joists can span.
  3. Buy screw boxes from the printed count on the box. Stain is a separate calculator after the deck is built.

10% here is not Trex’s 5%

  • The deck-board calculator defaults to 10% and its help text also mentions 15% for diagonal layouts or a picture-frame border.
  • Trex’s public board article says a general rule of thumb is 5% extra. If you follow that article, type 5 into the waste field. The example on this page does not silently switch to 5%.
  • Screw boxes round up to the count you type. The board waste percent is not applied again to the screws.

Limits

  • Joist size, beam size, ledger attachment and footings are structural. Bring a spacing and a footing size from an engineered plan or a code table. This page will not recommend one.
  • Pergolas, roof structures and stairs are outside this topic. There is no calculator here for them.
  • A tube-footing concrete count belongs on the sonotube calculator only after diameter and depth are already decided.

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Guides

Quick questions

Why is a 6 inch board about 5.5 inches wide?

The name size is not the coverage. Trex lists several of its boards as 5.5 in wide. The calculator asks for the actual width because rows are deck width divided by width plus gap.

Why does this site default to 10% when another article says 5%?

They are different publishers’ allowances. Trex describes 5% as a general rule of thumb. This calculator’s default is 10%, and the field says 15% for diagonal layouts or picture-frame borders. Type the percent you intend. The worked example keeps 10% so it matches the calculator default.

Does the joist count choose a safe span?

No. The count is ceil(length ÷ spacing) + 1 for a spacing you supply. Allowable span depends on species, grade, loads and wet service. The American Wood Council publishes span tools for that decision. This page does not.

Sources

  1. Trex – How many deck boards do I need? — accessed : “The 1” Transcend® and Enhance® boards are .94” x 5.5”. Trex boards come in 12’, 16’ and 20’ lengths. The general rule of thumb is that you need 5% extra decking material to account for waste and mistakes.”
  2. American Wood Council – online span calculator help — accessed : “Maximum horizontal joist and rafter spans are taken as the smallest span (L) calculated from bending strength, deflection and shear.”
  3. American Wood Council – DCA 6 (2015) deck guide — accessed : “Use Table 2 to determine allowable joist span, LJ, based on lumber size and joist spacing.”

Prefer manufacturer data sheets and published factors. Coverage, yield and waste vary by product — confirm the label you buy.

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