How Much Decking for a 10x10, 12x12, 12x16 or 16x20 Deck

With 5.5 inch wide boards laid straight across the joists, a 10x10 deck takes 21 boards at 12 ft or 16 at 16 ft, a 12x12 takes 30 or 22, a 12x16 takes 39 or 30, and a 16x20 takes 65 or 49, each count carrying 10 percent for waste. Lay the same four decks diagonally on 12 ft stock and they take 22, 32, 41 and 68, on joists pulled in from 16 to 12 inches.

Deck length (ft)

Deck width (ft)

Board width (in)

Board length (ft)

Board layout pattern

Perpendicular (standard)Diagonal (45°)Herringbone / picture frame

16" on-center joist spacing, 10% waste allowance for this layout.

Decking boards

Deck area

192 sq ft

Linear feet needed

419 ft

Boards needed (incl. waste)

30

27 boards before waste, plus a 10% waste allowance for this layout.

Joists

Joist spacing

16" on center

Joists needed

10

Based on a 12 ft deck width at 16" on-center spacing.

Every common size at 12 ft and 16 ft, side by side

The counts below start from the deck’s area and the 5.5 inch face of a nominal 5/4x6 or 2x6 board, the width nearly all residential decking shares once it is dressed. Area divided by that width gives linear feet of board. Linear feet divided by the stock length, rounded up to a whole board, is the raw count, and 10 percent on top of that, rounded up again, is what goes on the order.

Deck (ft)	Area (sq ft)	Linear feet of board	12 ft boards, raw	12 ft boards, with 10%	16 ft boards, raw	16 ft boards, with 10%
8x10	80	174.5	15	17	11	13
10x10	100	218.2	19	21	14	16
10x12	120	261.8	22	25	17	19
12x12	144	314.2	27	30	20	22
12x14	168	366.5	31	35	23	26
12x16	192	418.9	35	39	27	30
14x16	224	488.7	41	46	31	35
16x16	256	558.5	47	52	35	39
16x20	320	698.2	59	65	44	49
20x20	400	872.7	73	81	55	61

Two things the table deliberately ignores. It treats the board as covering its full 5.5 inches, so the 1/8 to 3/16 inch drainage gap between boards is not credited; across a 12 ft deck those gaps add up to about one board, and the 10 percent absorbs it. And it does not know which way your boards run. It divides linear feet by board length and nothing more, which is why the 16 ft column is always smaller: fewer, longer pieces for the same footage. Whether those longer pieces actually reduce waste depends on the run, which is the next section. The deck calculator above runs the same arithmetic for any size and any board width, so a deck built from 2x4 decking at 3.5 inches, or a composite at 5.4 inches, gets its own count rather than a scaled one.

Match the board length to the run, not to the price per foot

A run is one line of decking from one edge of the deck to the other, and the run length is the deck’s dimension in the direction the boards travel. The cheapest order is the one where a stock length divides the run with nothing left over, because every piece then lands with both ends on a joist and nothing goes in the scrap pile but the trimmed ends.

On a 12 ft run, 12 ft boards do that: one board, one run, 27 runs across a 12 ft deck, which is the 27 raw boards in the 12x12 row. On a 16 ft run, 16 ft boards do the same, and the 12x16 drops from 39 boards of 12 ft stock to 30 of 16 ft stock. The linear feet are nearly identical, 468 against 480, so the saving is not lumber. It is joints. With 12 ft stock on a 16 ft run every run carries a butt joint, and each joint has to sit centered on a joist, which means the pieces either side of it have to be multiples of the joist spacing. At 16 inches on center that is any multiple of 16 inches, which in whole feet means 4, 8, 12 and 16. A 12 ft board cut to 4 and 8 works. An offcut of 6 ft does not land anywhere, and it becomes a 5 ft 4 in piece plus a scrap.

That is also why 10 ft runs are awkward. Twelve-foot stock loses 2 ft from every board, 17 percent of the lumber before any other waste, and a 2 ft offcut has almost no use on a deck. Sixteen-foot stock cut to 10 leaves 6, which puts its joint between joists. The clean answer for a 10x10 is 10 ft lumber where the yard carries it, or 20 ft boards cut in half: two runs, no joints, no offcut. The 14 ft run is the same problem one size up, and 14 ft pressure-treated decking exists for exactly that reason, though not every yard stocks it.

When the run is 12 ft and the stock is 16 ft, the 4 ft offcuts are not scrap. They are stair treads. A tread on a 36 to 48 inch wide stair takes two boards cut to the stair width, so a five-tread flight down from a raised deck uses ten of them. The stair calculator gives the riser and tread count for your drop, and the stair code requirements guide covers the rise and run limits and the guard that becomes mandatory once the deck surface is more than 30 inches above grade.

Diagonal boards: 15 percent waste and joists at 12 inches

Turn the boards 45 degrees and two numbers move at once. The waste allowance goes from 10 to 15 percent, because every board end is now a miter and the triangle cut off each end is scrap, and the joists close up from 16 inches on center to 12.

Deck (ft)	12 ft boards, raw	Diagonal, with 15%	Straight, with 10%	Joists at 12 in	Joists at 16 in
8x10	15	18	17	11	9
10x10	19	22	21	11	9
10x12	22	26	25	13	10
12x12	27	32	30	13	10
12x14	31	36	35	15	12
12x16	35	41	39	17	13
14x16	41	48	46	17	13
16x16	47	55	52	17	13
16x20	59	68	65	21	16
20x20	73	84	81	21	16

The joist change is geometry, not caution. A board crossing joists at 45 degrees spans the diagonal of the joist bay, and the diagonal of a 16 inch bay is 16 times 1.414, or 22.6 inches. Most 5/4 decking and nearly every composite is rated for 16 inches between joists laid straight, and few if any are rated for 22.6. Closing the joists to 12 inches brings the diagonal span down to 17 inches, which is what the installation sheets that allow 16 inches straight typically call for on a diagonal. Composite manufacturers print the spacing in the installation sheet, and inspectors read the sheet.

The extra board count is small, two or three on the mid-size decks, because 5 percent of a modest number is a modest number. The extra joists are where the real cost sits: a 12x16 goes from 13 joists to 17, and each one needs a hanger at the ledger end and often another at the rim. Herringbone and other patterned fields sit on the same 12 inch frame and waste more again, 19 percent in the deck calculator’s allowance, the middle of the usual 18 to 20 percent range, since each board gets two angled cuts instead of one. There is no herringbone table on this page because the count depends on the pattern’s module; set the calculator to the herringbone layout for a figure.

What the joist column counts, and what it leaves out

The joist count is the deck width in inches divided by the spacing, plus one for the joist at the far edge, rounded up. That is why 14x16 and 16x16 share 13 joists: both are 16 ft across, and the count comes from the width alone. Frame the deck with the joists running the other way and the count comes from the length instead.

It counts joists and nothing else. Not in the column: the rim or header boards that cap the joist ends, the ledger bolted to the house, the beam or beams the joists bear on, the posts under the beams, the footings under the posts, the hangers at every joist end, and the blocking between joists that keeps a long run from rolling and that a diagonal field relies on for stiffness. Whether and where blocking is required is a question for your building department. The footings are their own estimate, in bags or sonotube volume rather than boards, and the fence posts and footings guide carries those figures for 8 to 16 inch tubes at frost depth.

Nor does the column say how big the joists are. A 2x8 at 16 inches on center spans roughly 10 to 12 ft depending on species and grade, a 2x10 two or three feet more, and the beam that catches them is sized separately. Those spans come from the deck framing tables in Section R507 of the 2021 IRC, or whatever edition your jurisdiction has adopted, and the ledger connection and footing depth are the two details a plan reviewer will look at first. Use the joist count here to price lumber, then size it with your building department.

Composite decking in 12, 16 and 20 ft lengths

Composite boards are sold in 12, 16 and 20 ft lengths, usually at a 5.5 inch face, and the arithmetic does not change: the 12x16 row still wants 30 boards at 16 ft. What changes is which length is right. The 20 ft board makes the 10 ft run clean, two runs from one board with no joint, and it makes a 20x20 deck a single board per run with no joint anywhere in the field, a count the calculator gives directly once the board length is set to 20. Composite also removes one waste reason and adds another. There is no crown to cull and no checked end to cut back, but a grooved-edge board shows an open groove wherever it is cut, so the cut ends along the house and the open edges get covered with a square-edge picture frame, and the frame boards are an order line of their own.

Composite is heavier, often half again to twice the weight of pressure-treated 5/4 depending on the product, which matters for whoever is carrying 61 boards up a slope, and it is priced per piece. Hardwood decking such as ipe is the one that is not: it is often sold by the linear foot or the board foot, and a count of 39 boards at 12 ft has to be converted before it can be priced. The board foot calculator does that for a 1x6 or 5/4x6 at any length.

Screws, hidden clips, and how many of each

Face-screwed decking takes two screws per board at every joist it crosses. On a 12 ft wide field over joists at 16 inches, that is 27 runs times 13 joists times 2, about 700 screws for a 12x16 deck, which works out near 350 per 100 square feet. Treat that as a typical figure, not a spec. It barely moves on a diagonal deck, which surprises people: there are a third more joists, but a board crossing them at 45 degrees meets one every 17 inches instead of every 16, and each joist meets fewer boards, so the crossings and the screws come out close to the straight count. Screws come by count or by weight depending on the box, so read the label rather than assuming one box is a deck’s worth.

Hidden fasteners work the other way. A clip sits in the groove at every joist and holds the edge of two boards at once, so the count is one clip per board per joist, about half the screw count, around 350 clips on the same 12x16. They are sold in boxes rated by square feet of coverage, typically at several times the cost of screws for the same area, and they only work with grooved boards, or with solid boards after a slot cutter has been run along every edge. The first board against the house and the last board at the edge still get face-screwed, as does the whole picture frame, so a hidden-fastener deck still needs a box of color-matched screws, or a plug kit to hide them.

Where the 10 percent actually goes

Decking waste is not the random breakage of tile or the over-dig of concrete. It has three specific sources, and a lumber order that understands them can sometimes trim the allowance, and sometimes needs more than it.

End cuts. Treated boards arrive with checked, split or crushed ends, and the fix is to cut an inch or two off each end before the board goes down. Two inches from both ends of a 12 ft board is 3 percent gone before anything else happens. Stock often comes a little over length, but not two inches over, so a run framed to exactly 12 ft leaves no room for that trim.

Crowning and culling. Some share of every lift of 5/4 decking is crowned, twisted or wane-edged. A crowned board can go down crown up and be pulled flat by the screws, up to a point; a twisted one cannot, and a wane edge shows in the gap. As a working allowance, figure on culling a board or two in every twenty, more on the lower grades, and the culls are the boards that get cut into stair treads, blocking and short runs rather than sent back.

Picture-frame borders. A border board running around the perimeter is the most common upgrade on a plain rectangle, and it changes the count in three ways. The frame itself is a linear-feet order, 56 ft on a 12x16 for four boards of 16 ft or five of 12 ft mitered at the corners. Every field board then gets cut to end against the frame rather than running wild, so the clean one-board-per-run saving disappears and each field board is 11 inches shorter than the run. And the two frame boards that run parallel to the joists need blocking between the joists under their full length to screw into, which is framing lumber the joist count does not include. On a composite deck the frame is close to mandatory, since it is what hides the cut ends.

Butt joints are the fourth source, and the only one you control at the order stage: every joint that fails to land on a joist is a piece cut again, shorter. The material waste factors guide sets out why the allowance for a cut-dominated material like decking behaves differently from the one for concrete or gravel, and why neither should be skipped.

Checking the count against your own deck

Three things to confirm before ordering off the table. First, that the board width is 5.5 inches: 2x4 decking at 3.5 inches needs 57 percent more pieces, and some composites run a little narrower, around 5.25 to 5.4. Second, which way the boards run, since the run length picks the stock length and the deck width picks the joist count, and on a deck that is not square the two are different numbers. Third, whether anything other than the floor is being built from the same boards. A privacy screen along one side is a fence count, not a deck count, pickets per linear foot with posts at a fixed spacing, and the fence calculator handles that run separately so it does not get folded into the decking waste.