Decking board calculator

What a deck board calculator does

A deck board calculator works out how many rows of decking fit across a deck, how many linear feet of board are installed and how many stock-length boards to buy. Enter the deck size, the actual face width of the board, the gap between boards and the length you buy boards in, and it returns the rows, the installed length, the boards per row and a stock order with your waste allowance.

The count is driven by coverage width, not nominal size. A 5/4x6 deck board is about 5-1/2 in wide, and with a 1/4 in gap each row covers 5-3/4 in. That is why a 12 ft wide deck needs 26 rows, not 24. The calculator adds the gap to every row and rounds up, so the last row is a full board that you rip to fit.

Boards run along whichever deck dimension you choose. If the deck is longer than your stock boards, the calculator counts how many boards each row needs and assumes offcuts are not reused, which gives a safe order for a straight-lay rectangular deck, with an optional picture-frame border. It counts joists at your spacing and two fasteners per board–joist crossing, but it does not size joists, beams or footings, and it does not check spans or permits.

How to use the decking board calculator

1. Choose Imperial or Metric. Imperial is the default, and your choice carries across the free tools.
2. Enter the deck Length and Width, then set Board direction to run the boards along either one; rows are counted across the other. For a deck against a house with boards parallel to the wall, the run is usually the wall length.
3. Pick a Board preset (timber 5.5 in with ¼ in gaps, composite 5.5 in with 3/16 in gaps, or 140 mm and 90 mm timber with 5 mm gaps) or choose Custom and enter the Board face width as the actual width, not the nominal size (5.5 in for a 5/4x6 or a 2x6, 3.5 in for a 2x4), and the Gap between boards.
4. Set the Stock board length you plan to buy (12, 16 and 20 ft are common), switch on a Picture-frame border for perimeter boards, enter the Joist spacing (16 in by default) and a Waste allowance percentage. Add an optional price per stock board to get a cost.
5. Read the board rows, installed and purchased linear length, stock boards per row and with waste, border boards, joist count and total length, and the fastener count. Download the PDF, CSV or the SVG layout diagram, or copy the result into a quote or order.

Worked example

Imperial: A 12 ft wide deck with 5.5 in boards and ¼ in gaps needs 26 rows.

Metric: A 3 m wide deck with 140 mm boards and 5 mm gaps needs 21 rows.

Nominal vs actual deck board sizes

Lumber is sold by nominal size and delivered smaller. Under the American Softwood Lumber Standard (PS 20), a dry 2x6 is dressed to 1-1/2 × 5-1/2 in and a 2x4 to 1-1/2 × 3-1/2 in. Radius-edge 5/4 decking is typically about 1 in thick and 5-1/2 in wide. Composite and PVC boards are sold by actual size, and many are close to 5-1/2 in wide, but check the product data because some are narrower.

Only the face width matters for the row count. Measure a board or read the spec sheet, and enter that figure in the calculator. Thickness matters for joist spacing, which the board manufacturer or a span table sets, not this tool.

How many linear feet of decking per square foot?

Linear feet per square foot is 12 divided by the board width plus the gap, in inches. For 5-1/2 in boards with a 1/4 in gap, that is 12 ÷ 5.75 = 2.087 linear feet of board for every square foot of deck. In metric, divide 1,000 by the width plus gap in millimetres: 140 mm boards with 5 mm gaps need 6.90 m of board per m².

This ratio is a good quick check, but it treats the last row as a fraction of a board. The calculator is more conservative because it rounds rows up and counts whole stock boards in every row.

Deck board spacing: how big should the gap be?

Side gaps let water drain and debris fall through, and they give boards room to move with moisture and temperature. For dry wood decking, a gap of about 1/8 in to 1/4 in is common practice. Pressure-treated lumber delivered wet is often laid tight or with a small gap, because it shrinks across its width as it dries; the gap you see a few months later is larger than the one you set.

Composite and PVC boards move more along their length with temperature, and manufacturers publish side-gap and end-gap requirements that can depend on the temperature at installation. Hidden-fastener clips usually set the side gap for you. Enter the gap your fastening system produces, because the manufacturer's installation guide governs the warranty.

The gap changes the quantity more than people expect. On a 14 ft wide deck with 5-1/2 in boards, 1/8 in and 1/4 in gaps both give 30 rows, but on a 16 ft deck the tighter gap adds a row (35 against 34).

Worked example: a 20 × 14 ft deck and stock length

Take a deck 20 ft long and 14 ft wide, with 5-1/2 in composite boards running the 20 ft length, a 3/16 in gap and 10% waste. The row count is (168 + 0.1875) ÷ (5.5 + 0.1875) = 29.57, rounded up to 30 rows, and the installed length is 30 × 20 = 600 linear feet.

What you buy depends on the stock length, because the calculator counts whole boards per row and does not reuse offcuts. Full-length 20 ft boards buy 660 linear feet with no butt joints. 16 ft boards need two per row and leave a 12 ft offcut in each row, so the same deck takes 1,056 linear feet unless you plan a cutting list that reuses the offcuts. Every butt joint must land on framing, detailed the way the board manufacturer requires.

Who it is for and when to use it

Method, formulas & assumptions

Rows = ceil((width across the boards + gap)/(board width + gap)). Boards per row = ceil(run ÷ stock length); a picture-frame border adds perimeter boards and shrinks the field. Joists = floor(run ÷ spacing) + 1; fasteners = 2 per board–joist crossing. Stock count includes waste, assuming no offcut reuse.

Imperial inputs are converted with exact factors (1 ft = 0.3048 m, 1 lb = 0.45359237 kg, 1 US gal = 3.785411784 L) before the calculation runs in SI, then results are converted back.

Common mistakes to avoid

• Entering the nominal size. A 2x6 is 5-1/2 in wide, and entering 6 in undercounts a 12 ft wide deck with 1/4 in gaps by two rows (24 instead of 26).
• Swapping length and width. Length is the direction the boards run; width is the direction you count rows across.
• Ordering stock lengths that leave large offcuts. On a 20 ft deck, 16 ft boards cost 60% more footage than 10 ft or 20 ft boards when offcuts are not reused.
• Forgetting picture-frame borders, steps and fascia. Borders run the other way and stair treads are separate; calculate each as its own small rectangle.
• Choosing a board without checking the joist spacing. The manufacturer's maximum joist span applies, and diagonal layouts usually need closer framing.
• Setting gaps on wet pressure-treated lumber as if it were dry. It shrinks as it dries and the gap grows.

Scope & limits

Rectangular deck with boards along either dimension; board presets are typical sizes and gaps. No optimization for staggered joints or offcut reuse. Joist counts are geometric only: no span, beam, connection or consent checks.

Frequently asked questions

Further reading

• Deck, pergola and shade structure template
• Building permit process explained
• All materials & quantities tools
• All free architecture & construction tools
• Harth resource library

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