Deck Board Calculator – How Many Boards You Need

How the default deck works out

A 12 ft deck width is 144 inches. Each board plus its gap takes 5.75 inches of that width, so 144 divided by 5.75 is 25.04, which rounds up to 26 boards. At 12 ft per board, that is 312 linear feet, and with 10% waste it becomes 29 boards to buy, or 348 linear feet.

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How it works

Understand the result

Reading your result: boards, feet, waste, and feet to buy

The result on this page is four figures chained together, and each one answers a different question about ordering. Boards needed is the count that fits across the deck width, rounded up. Total linear feet is boards needed times board length. Boards to buy is boards needed plus the waste allowance, rounded up. Linear feet to buy is boards to buy times board length. The default case shows the whole chain: a 12 ft deck width is 144 inches, each board plus its gap takes 5.75 inches of that width, from a 5.5 in board plus a 0.25 in gap, so 144 divided by 5.75 is 25.04, which rounds up to 26 boards needed. At 12 ft per board, that is 312 linear feet. With a 10% waste allowance, 26 boards become 29 boards to buy, with 3 extra boards for waste, or 348 linear feet to buy. Every line follows from the one before it, and every line can be checked by hand from the inputs, which is the point of showing them separately: a wrong board count corrupts the feet, and a wrong waste percentage corrupts only the buying figures, and the separate lines show which one went wrong.

The widened-width row shows the count scaling with width, but not smoothly, because of rounding up. Widening from 12 ft to 16 ft takes the width from 144 inches to 192 inches, and 192 divided by 5.75 is 33.39, rounded up to 34 boards, against 25.04 rounded up to 26. The width grew by a third, the board count grew from 26 to 34, which is also roughly a third, but the rounding means the growth is not exactly proportional: a width that lands just above a whole multiple of the pitch needs a whole extra board, while a width that lands just below it does not. The waste row shows the opposite independence: setting waste to 0% leaves boards needed and total linear feet unchanged at 26 boards and 312 ft, because waste is applied only to the buying figures, and boards to buy fall from 29 to 26, exactly the boards needed, with no extra boards at all. A zero-waste order is therefore not a saving on the deck itself; it is an order with no margin for cuts, defects, or mistakes, and the page’s own guidance treats 10% as a common starting point precisely because a deck without margin is a deck that risks running short mid-build.

The gap row is the subtlest in the table, because removing the gap changes the pitch and therefore the count, in the opposite direction from what intuition suggests. With no gap, the pitch is the board width alone, 5.5 inches, and 144 divided by 5.5 is 26.18, rounded up to 27 boards, one more than the 26 needed with a 0.25 in gap, even though the boards themselves are the same width. The reason is that a gap consumes width: with a gap, each board plus its gap takes 5.75 inches, so fewer pitches fit across the width, and the count is lower, but the deck is also less covered by board and more by gap. Removing the gap fits more boards across the same width, because each pitch is narrower, and the buying figure rises to 30 boards with 10% waste, from 27 times 1.10, which is 29.7, rounded up to 30. The gap is therefore not a minor detail: it changes the count, it changes the coverage, and it exists for reasons, drainage, expansion, and buckling, that the next section explains. A count computed without the gap the manufacturer specifies is a count for a different deck, and it should not be ordered.

Two further cases from the page itself complete the picture, and they are worth quoting because they show the rounding rule at its edges. The page’s second example, a 10 ft wide deck with 8 ft boards, 5.5 in boards, and a 0.125 in gap, needs 22 boards, which is 176 linear feet, and 25 boards to buy with 10% waste, or 200 linear feet: the pitch there is 5.625 inches, 120 divided by 5.625 is 21.33, rounded up to 22, and 22 times 1.10 is 24.2, rounded up to 25. The exact-fit case, an 11.5 ft width at a 5.75 in pitch, lands exactly on 24 boards, with no rounding up needed, because 138 divided by 5.75 is exactly 24. The exact-fit case is the exception that proves the rounding rule: when the width divides exactly by the pitch, no extra board is needed beyond the exact count, and the waste allowance is then the only reason boards to buy exceed boards needed. Measure the width at several points and use the widest measurement, as the page advises, because decks are rarely perfectly square, and the widest point decides how many boards fit, not the average point.

The gap: drainage, movement, and the manufacturer’s word

The gap between deck boards is not empty space wasted, and it is not a rounding convenience: it lets water drain, lets boards expand and shrink with weather, and keeps the deck from buckling, as the page’s own guidance states. A deck with no gap, or with gaps too small for the board material and the climate, traps water on its surface, forces boards against each other as they expand in heat and moisture, and can buckle, cup, or push fixings out, while a deck with gaps too large for its boards wastes coverage and can feel uneven underfoot. The common gap on this page is 0.25 in, but the page states the governing rule alongside it: always follow your board manufacturer’s guidance, because different board materials, composite, hardwood, and treated softwood among them, move differently with moisture and temperature, and the manufacturer’s gap is specified for the material you are actually buying, not for decks in general. A count computed with a gap the manufacturer does not specify is a count for a different deck, and it should not be ordered, however neatly its arithmetic works out.

Gap also interacts with board width in the pitch, which is board width plus gap, and the pitch is the unit that divides the deck width. A wider board with the same gap has a larger pitch, so fewer boards fit across a given width, but each board covers more of it, and the total board area is roughly conserved apart from rounding and gap area. A narrower board with the same gap has a smaller pitch, so more boards fit, but a larger share of the deck width is gap rather than board, because each pitch carries the same gap on a smaller board. The probed gap cases show the pitch effect directly: at a 5.75 in pitch the default deck needs 26 boards, while at a 5.5 in pitch, with the gap removed, it needs 27, because narrower pitches fit more often across the same width. When comparing board widths, compare the pitch and the gap share together, not the board width alone, because a board that looks economical by width alone can be wasteful by pitch once its required gap is added, and a board that looks expensive by width can be economical by pitch if its gap is small.

Measure the width across the boards, at right angles to how the boards run, as the page states, and measure it at several points, using the widest measurement. Boards run in one direction, so only the width across them decides the count, and board length never changes the count, only the linear feet, as the page’s own guidance states. That separation is the source of a common error: entering the deck’s long dimension as the width, or entering the board length as if it affected the count, produces a count for a deck rotated ninety degrees from the one being built. Check the orientation before entering anything: stand on the deck, face along the boards, and measure across them, from edge to edge, at several points along their length, because a deck that is wider at one end than the other needs the widest width’s count, and a count based on the narrow end will leave the wide end short. The widest point decides, not the average, and not the point nearest the house, because the last board must fit at the widest point or the deck will be incomplete there.

Waste: what it covers, and when 10% is not enough

Waste covers cuts at the ends, boards rejected for defects, and mistakes, as the page states, and 10% is a common starting point, not a universal allowance. End cuts alone consume a predictable share: every board that must be cut to length, or cut around a feature, produces an offcut, and boards that arrive in fixed lengths produce offcuts whenever the deck’s board length does not divide evenly into the supplied lengths. Defects consume an unpredictable share: a board with a split, a warp, a stain, or a damaged edge cannot be used where it shows, and it must be replaced from the waste allowance or from a spare board, not by hoping the defect will not be noticed. Mistakes consume the remainder: a board cut to the wrong length, a board damaged during fixing, or a board laid in the wrong place and then moved all draw on the same allowance. A 10% allowance on 26 boards gives 3 extra boards, which is enough for ordinary end cuts and a small number of defects on a simple rectangular deck, and it is the page’s default for that reason.

Complex layouts need more, and the page states the rule: for a deck with stairs, angles, or a border, raise the waste allowance before ordering, rather than discovering the shortfall mid-build. Stairs add short boards and angled cuts, each with its own offcut, and a staircase that looks small on a plan can consume several boards’ worth of offcuts. Angles, especially diagonal boards or a deck that is not square, produce triangular offcuts at every cut, and the offcut from an angled board is larger, proportionally, than the offcut from a square cut. A picture-frame border, where boards frame the deck’s edge, adds a second orientation of boards with its own cuts and its own waste, and the border boards themselves must be counted in the width or the length as they fall, not assumed to come free from the main field’s allowance. Joining short boards adds seams and offcuts, as the page warns, because a deck built from boards shorter than its board length needs joints, and each joint is a cut and a potential defect point. If your boards come in fixed lengths, check that the length you enter matches what you will actually buy, because a length mismatch turns a linear-feet figure into a fiction: the feet may be right in total, but the boards that make them up will not be the boards you receive.

Linear feet, board length, and ordering in fixed lengths

Total linear feet is boards needed times board length, and linear feet to buy is boards to buy times board length, and both figures exist because boards are bought by length as well as by count. The default case shows both: 26 boards at 12 ft each are 312 linear feet, and 29 boards to buy at 12 ft each are 348 linear feet. The distinction matters when boards are sold in fixed lengths, because the linear feet figure alone does not tell you how many boards of what length to order: 348 linear feet could be 29 boards at 12 ft, or 43.5 boards at 8 ft, or any other combination that multiplies to the same total, and only the boards-to-buy figure, combined with the board length entered, gives the order. If the boards you will actually buy come in a different length from the length entered, the boards-to-buy figure is still correct as a count across the width, because board length never changes the count, but the linear feet to buy will be wrong for the boards you receive, and the order must be recomputed with the actual length. Enter the length you will buy, not the length you wish you could buy, and check it against the supplier’s stock before ordering.

Seams deserve a paragraph of their own, because they are where linear feet and board count interact most unhappily. A deck whose board length, the direction boards run, is longer than the boards supplied needs joints: two shorter boards butt-joined to span the length, with each joint adding a cut, a fixing, and a potential weakness, and with the offcuts from cutting supplied boards to joint positions consuming part of the waste allowance. A deck whose board length matches the supplied length avoids joints entirely, and its linear feet figure is exact in a way a jointed deck’s figure is not, because no length is lost to joint cuts beyond the ordinary end cuts. The page’s own warning covers this: joining short boards adds seams and offcuts, so if your boards come in fixed lengths, check that the length you enter matches what you will actually buy. A plan that orders by linear feet alone, without checking length, will arrive at the site with the right total length in the wrong pieces, and the deck will be built from joints that were never planned.

Measure the deck yourself, confirm board dimensions and gap with your board manufacturer, and allow extra material for complex layouts, steps, and borders, as the page’s own disclaimer advises. That sentence is the whole ordering discipline in one line: the width is yours to measure, at its widest, across the boards; the board width and gap are the manufacturer’s to specify, for the material you are buying; and the waste allowance is yours to raise for the complexity of your layout, before ordering, not after discovering a shortfall. A deck order that follows that sentence will have the right count, the right feet, and the right margin, and a deck order that skips any clause of it, a width guessed, a gap assumed, or waste left at default for a complex deck, will have at least one figure wrong, and the wrong figure will be discovered at the point where it costs most to fix, mid-build, with the deck half laid.

Scope note, and a checklist before ordering

Scope note, stated plainly: this calculator is for planning and ordering estimates, as its own disclaimer states. It does not design the deck, it does not check structural adequacy, fixings, joists, or footings, and it does not substitute for the board manufacturer’s guidance on gap, fixing, or material suitability. Every figure in the new sections above comes from this page’s own calculator, quoted exactly as displayed for the cases probed, or from the page’s own worked examples, with the inputs stated alongside. No board, material, or manufacturer is recommended here.

Checklist, in order, before ordering: measure the width across the boards at several points and use the widest; confirm the board width and gap with the manufacturer for the material being bought; enter the board length you will actually buy, in the fixed lengths supplied; read boards needed and total linear feet, and check the pitch, board width plus gap, by hand; set the waste allowance for the layout’s complexity, raising it above 10% for stairs, angles, or borders; read boards to buy and linear feet to buy; and order those boards, in those lengths, with the waste boards included, before starting. A checklist followed in that order produces an order the page’s arithmetic supports; an order placed without it is a guess with arithmetic attached.

When the count surprises you: diagnosing a wrong-looking answer

A board count that looks wrong usually has one of four causes, and the result panel’s separate lines identify which one. First, the width: check that it was measured across the boards, at the widest point, in feet, and not along the boards or at the narrow end. Second, the pitch: check that board width and gap are both in inches, that the gap is the manufacturer’s gap for the material, and that the pitch shown, board width plus gap, matches the two inputs added by hand. Third, the length: check that the board length entered matches the length to be bought, because a length error does not change the count but changes both linear-feet figures, and a feet figure that looks wrong usually means the length, not the count, was wrong. Fourth, the waste: check that the waste percentage matches the layout’s complexity, because a count that looks low usually means waste was left at default for a complex deck, and a count that looks high usually means waste was set high for a simple rectangle. Work through those four lines in order, and the wrong-looking count will explain itself; change the line that is wrong, not the count, because the count is never wrong on its own.

Rounding up deserves a final word, because it is the rule most often questioned and the rule least worth questioning. Rounding down would leave a strip of deck uncovered at the far edge, as the page states, and the last board is bought whole and cut down to the remaining width, which is why the count always rounds up. The only case where rounding up adds nothing is the exact fit, where the width divides exactly by the pitch, as in the 11.5 ft example at a 5.75 in pitch, which lands exactly on 24 boards. Every other width needs one more board than its exact division suggests, and that extra board is not waste: it is the board that covers the final partial pitch, cut to fit. Waste is applied after rounding up, to the rounded count, so the two roundings compound: the count rounds up to a whole board, and then waste rounds up again to whole boards to buy. A buyer who tries to save the rounding board, or the waste boards, by ordering the exact division figure will arrive short by at least one board, and usually by more, once cuts and defects are counted. Order the rounded figures, and keep the offcuts: the final partial board’s offcut, and the waste boards’ spares, are the material that repairs the deck later.

Common deck sizes: board counts at the default pitch

Boards run in one direction, so the width across them decides the count. At the default 5.5 in board and 0.25 in gap, the pitch is 5.75 in, and the table reads the width in inches divided by 5.75, rounded up.

The table counts boards only, before waste: add the waste allowance on top, exactly as the calculator does, and read boards to buy from the result rather than from the table. Board length never appears in the table, because it never changes the count — it changes the linear feet, which is the count multiplied by the board length. A 12 ft wide deck needs 26 boards whether the boards are 8 ft, 12 ft, or 16 ft long; what changes is how many linear feet those 26 boards represent, and how many joints fall along the length. Where the deck length exceeds the longest board available, boards must be butted end to end, and each butt joint is a cut, a fastener pair, and a place where water can sit, so longer boards, where the deck and the supplier allow them, mean fewer joints and a cleaner surface, at the cost of handling longer, heavier stock.

Widths that fall between table rows are read the same way: convert the width to inches, divide by the pitch, and round up, because a part board must still be bought whole and cut. A width of 13 ft is 156 inches, and 156 divided by 5.75 is 27.13, which rounds up to 28 boards. Measure the width at several points and use the widest, as the scope note above insists, because a deck that is 13 ft at one end and 13 ft 4 in at the other needs the count for 13 ft 4 in, and the narrower measurement leaves the wide end short. For decks with angles, curves, steps, or picture-frame borders, the table is a starting point, not an order: the waste section above explains when 10% is not enough, and the companion guide works a 12 by 16 deck and other sizes end to end, including the diagonal-layout allowance.

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