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Generator Hut

Deck Board Calculator

Deck boards, joists and screws from your deck dimensions.

Your measurements

Measured along the direction the boards run.

A nominal 2x6 measures 5.5 in, a 5/4x6 also 5.5 in.

An eighth of an inch is typical for pressure-treated timber that will shrink.

Covers cuts, crooked boards and the odd split end.

Decking required

29boards

29 boards at 16 ft

Deck area16 ft × 12 ft
192 sq ft
Rows of deckingat 5.5 in wide plus a 0.125 in gap
26
Linear feet of board
416 ft
With waste allowance+10%
457.6 ft
Boards to buy
29 × 16 ft
Joistsat 16 in centres
13
Deck screwstwo per board per joist
≈ 676

The gap matters. Pressure-treated timber arrives wet and shrinks as it dries, so boards laid tight can open to a quarter inch within a season — while kiln-dried hardwood barely moves and needs a gap built in from the start.

Joists, beams, posts and footings are structural and are not calculated here beyond a joist count. Span tables depend on timber species, grade and load, and getting them wrong is not a cosmetic problem.

Buy boards a little longer than the deck rather than joining mid-span. A butt joint over a joist is a weak point and a visible line.

Enter the deck size and the width of the boards you are buying, and it works out how many rows you need, how much linear footage that is, and how many boards of a given length to order. The gap between boards is an input because it changes the row count more than people expect.

Why use this tool?

The gap is accounted for

A board and its gap are the repeating unit, not the board alone. Over a 12 foot width an eighth-inch gap changes the row count.

Answers in boards, not square feet

Decking is sold in lengths, so linear footage is converted to a count of boards at the length you are actually buying.

Joists and screws too

Joist count at your chosen spacing, and a screw estimate at two per board per joist.

Any board width

Nominal 2x6 and 5/4x6 both measure 5.5 inches. Hardwood and composite vary — enter what you are buying.

How this deck board calculator works

The repeating unit across the deck is one board plus one gap. Dividing the deck width in inches by that figure and rounding up gives the number of rows, which multiplied by the deck length gives the linear footage of decking.

That linear figure is uplifted by the waste allowance and divided by the length of board you are buying. Buying longer boards than the deck run avoids butt joints entirely, which is worth doing where the span allows.

Joists are counted at your chosen spacing along the deck length, plus one for the closing joist. Screws assume two per board at every joist crossing, which is the standard fixing pattern for face-screwed decking.

How to use it

  1. Step 1: Measure the deck

    Length along the direction the boards will run, and width across it. If the boards run the short way, swap the two figures.

  2. Step 2: Enter the board width

    The actual measurement, not the nominal size. A 2x6 is 5.5 inches wide, not 6.

  3. Step 3: Set the gap

    An eighth of an inch for pressure-treated timber that arrives wet and will shrink. Closer to a quarter for kiln-dried hardwood or composite that will not.

  4. Step 4: Pick a board length

    Choose a length that spans the deck without a joint if you can. Otherwise stagger the joints over joists.

Example usage

A 16 by 12 deck
At 5.5 inch boards with an eighth-inch gap, 12 feet of width needs 26 rows. That is 416 linear feet of decking, 458 with waste, or 29 sixteen-foot boards — plus 13 joists at 16 inch centres and around 676 screws.
The same deck in narrower boards
Switching to 3.5 inch boards raises the row count from 26 to 40, and the linear footage from 416 to 640. Narrow boards look finer and cost substantially more in both material and labour.
A deck wider than the boards sold
If the deck runs 20 feet but boards come in 16, you either accept a staggered butt joint line or run the boards the other way. Calculate both orientations and compare — the row count changes and so does the price.

Frequently asked questions

How much gap should I leave between deck boards?

About an eighth of an inch for pressure-treated timber, which arrives wet and shrinks as it dries — boards laid tight can open to a quarter inch within a season. Kiln-dried hardwood and composite barely move, so build the gap in from the start, usually a quarter inch.

How many screws per deck board?

Two at every joist crossing is standard for face-screwed decking. This calculator uses that figure. Hidden fastener systems use a different count entirely and follow the manufacturer's spacing.

Does this calculate the frame?

Only the joist count at your chosen spacing. Beam sizing, post spacing, footing depth and span tables depend on timber species, grade and load, and getting them wrong is a structural problem rather than a cosmetic one.

Should boards run along the length or the width?

Either works structurally as long as the joists run perpendicular to them. Run them the long way for fewer joints, or the short way if the board lengths sold suit it better. Calculate both and compare the board count.

What waste allowance should I use for decking?

Ten per cent covers cuts and the occasional crooked or split board. Push it higher for an angled or picture-frame layout, where perimeter cuts are mitred and the offcuts are rarely reusable.

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