Square footage starts a deck order and makes a bad final answer. A 12 by 16 foot deck is 192 square feet. Divide 192 by the face of a 5 1/2 inch board and you are already wrong twice: you ignored the gap, and you ignored that boards are sold in lengths that may not match the run. The board calculator exists so those two mistakes show up as different shopping lists.
Every count in this guide uses the defaults the calculator opens with. The face is 5 1/2 inches, which is a 5/4×6 or a 2×6. The gap is 1/8 inch. Waste is 10 percent. Stock lengths of 8, 10, 12, and 16 feet are all available. On the calculator, length is along the house and width is out from the house. Boards run along the house unless you flip the direction.
The row formula
Rows stack across the deck, perpendicular to the board run. There is a gap between boards and no gap after the last board. The count is (across + gap) ÷ (board width + gap), rounded up. For a 12 foot width, across is 144 inches. Board plus gap is 5.625 inches. (144 + 0.125) ÷ 5.625 is 25.62, which rounds up to 26 rows.
Butt the same boards tight, with a zero gap, and 144 ÷ 5.5 is 26.18, which rounds up to 27 rows. The gap did not add a board. It replaced a sliver of wood with air, and the last row dropped off. The gap guide is about when 1/8 inch is the wrong number for the material you bought.
Which way the boards run
On an attached deck, joists usually run out from the house. Decking runs the other way, along the house, so each board crosses joists. Turn the boards and you are usually turning the joists too. The shopping list is not a smaller copy of the first one.
A 12×16 with boards on the 16 foot side wants 16 foot stock. The same deck with boards on the 12 foot side wants 12 foot stock, more rows, and a different screw count, because the joist count changed with the run. Build the direction you are framing. A row that fits on one stock board avoids a butt joint in the field.
A 10×12
Put 12 feet along the house and 10 feet out from it. Across is 10 feet, so you need 22 rows. With 10 percent waste you buy 25 boards at 12 feet. That is 300 linear feet to purchase and 264 linear feet on the deck, which is 22 rows times 12 feet.
Turn it. Boards along the 10 foot side, stacked across 12 feet: 26 rows, and you buy 29 boards at 10 feet. That is 290 linear feet purchased and 260 on the deck. Same rectangle, two lists. Copying “25 boards” onto the second layout shorts the deck by a row pattern you cannot stretch.
A 12×16
This is the deck the calculator opens on: 16 feet along the house, 12 feet out, boards along the house. Twenty-six rows cover it. Waste lifts the plan to 29 rows. Sixteen-foot boards cover those rows one for one, so you buy 29 boards at 16 feet. Four hundred sixteen linear feet sit on the deck. Four hundred sixty-four linear feet are on the receipt.
Joists at 16 inches on center, with a joist at both ends, come to 13 on this 16 foot run. Two screws at every crossing, then the same 10 percent, is 744 screws. Those framing numbers belong to this direction. They are not a bonus that travels with the square footage. The joist guide shows why the end joists are already in the 13.
Run the boards the 12 foot way instead. The run is 12 feet and the rows stack across 16 feet. You need 35 rows. Buy 39 boards at 12 feet: 468 linear feet purchased, 420 on the deck. Joists are now counted along 12 feet, which is 10 joists at 16 inches on center, and the screw line becomes 770 after waste. If someone says a 12×16 takes about 30 boards, ask which way the boards run before you treat that as an order.
A 14×20
Boards along the 20 foot side, stacked across 14 feet: 30 rows. No single 8, 10, 12, or 16 foot board covers a 20 foot row, so each row is two 10 foot boards. With 10 percent waste you buy 66 boards at 10 feet. Six hundred linear feet are on the deck. Six hundred sixty are purchased. The butt joint has to land on a joist when you build. The list counts lumber. It does not mark which joist gets the joint.
Boards along the 14 foot side, stacked across 20 feet: 43 rows. A 16 foot board covers a 14 foot row and leaves about 2 feet. You buy 48 of those 16 foot boards. Six hundred two linear feet are on the deck. Seven hundred sixty-eight linear feet are purchased, because the offcut is real waste on top of the 10 percent. Do not plan to stitch those 2 foot offcuts into another row. They are shorter than a row, and the calculator does not reshuffle scraps across rows.
Waste and the length the yard does not have
Ten percent is extra rows. It is not a fudge factor that uses offcuts for you. The calculator adds it to the row count, then buys whole boards for those rows. Zero percent buys the cover count only. On the opening 12×16 that is 26 boards of 16 feet, not 29. Fifteen percent is a fair allowance when you are new to cutting, the boards are crowned, or you are adding a picture frame the rectangle did not include. Five percent fits a run that matches the stock and a cutter who does not ruin pieces.
Uncheck a length the yard does not sell. Drop 16 foot boards on a 16 foot run and each row becomes two 8s. The list gets longer in pieces even though the deck did not grow. A 16 foot board cut down to 12 feet leaves 4 feet. That remnant does not count as half of the next row. The lumberyard guide is about how to say this at the counter.
What the count still does not buy
Picture-frame borders, breaker boards, diagonal decking, and a curved edge change the count. Stairs are their own treads. Fascia is a face board, not a decking row. Joist hangers, post bases, flashing, and ledger fasteners are not in the board total. The footing calculator counts bags for a round tube you already sized. It does not choose the hole. Use the board list as the decking line of a larger order, then add the hardware the frame actually needs.
Open the calculator, leave the defaults, and read the worked steps under the list. Change one thing at a time: the gap, the waste, the direction. If your hand tally matches the steps, the shopping list is that tally with waste added. If it does not, the usual cause is nominal width. A 5/4×6 and a 2×6 are both about 5 1/2 inches on the face. A 2×4 face is about 3 1/2 inches. Measure a board at the yard when the sticker and the tape disagree.