Two decks with the same outside measurements can differ by a whole row because one of them has a 1/8 inch gap and the other is butted tight. That is not a trick. It is the difference between buying wood and buying air. The board calculator puts the gap in the row formula on purpose: (across + gap) ÷ (board width + gap), rounded up. There is no gap after the last board, because the rim or the fascia ends the surface.
What 1/8 inch does to a 12 foot width
A 5 1/2 inch board and a 1/8 inch gap make a pitch of 5 5/8 inches, which is 5.625 inches. Across 12 feet is 144 inches. Add the gap once, the way the formula does, and divide: (144 + 0.125) ÷ 5.625 = 25.62. Round up and you have 26 rows.
Set the gap to zero. Now it is 144 ÷ 5.5 = 26.18, which rounds up to 27 rows. You bought an extra row to fill space the gap would have given you for free. On the opening 12×16, with boards running the 16 foot way, that extra row is another 16 foot board before waste, and waste then rounds the purchase up again. The size guide keeps the 1/8 inch default so the 10×12, 12×16, and 14×20 counts stay comparable.
Wood needs a place for water and for swelling
Pressure-treated lumber and cedar move. A board that was gapped at 1/8 inch in July can be tighter in a wet week and a little wider in a dry one. A gap of zero on wood traps water on the surface and gives the boards nowhere to grow. Edges cup, fasteners tear, and the deck holds puddles. A 1/8 inch gap is the usual starting point for those species when the boards are dry enough to install. In a climate where the deck stays wet, builders often open that to 3/16 inch. Neither number is a code citation from this site. It is shop practice. If your board maker prints a different gap, use theirs.
Install wet pressure-treated boards and the gap you set today can open as the wood dries. Some people tighten the gap slightly on very wet stock and accept that it will grow. Guessing that without measuring moisture is how you get a 1/4 inch slot by August. If the boards are dripping, the better move is to wait, not to invent a negative gap.
Butt joints are a gap too
When a row is two boards end to end, the ends also need room. A 20 foot row made from two 10s, which is what the 14×20 wants when boards run the long way, should meet on a joist with a small gap between the ends. That gap is not the same as the side gap in the row formula. The row formula already assumed each piece is the full run. You still leave the end gap when you cut, which means the factory 10 footer might need a trim even though 10 plus 10 is 20. The cut plan on the calculator says the row is spliced. It does not pick the joist. You do, and the joint has to land on wood, not in the air between joists.
Composite and PVC follow the sheet, not the habit
Plastic decking moves with temperature more than with rain. A board installed on a cold morning is shorter than the same board on a hot afternoon. Manufacturers print a gap that depends on the temperature while you fasten, and hidden-fastener clips often set that gap by their own thickness. Using a wood habit of 1/8 inch on a clip system can fight the clip. Using zero because the showroom deck looked tight can buckle a run when summer hits.
Read the installation sheet for the exact product. Widths differ. A composite board is often not 5 1/2 inches. Type the real face into the calculator and type the gap from the sheet, not from this paragraph. If the sheet says the gap is different at 40 degrees than at 80, write down the temperature you are building in and use that row of the table. The cost comparison only works if both lists use their own width and gap.
Side gap is not joist spacing
Joist spacing is how far apart the framing members are. Board gap is the slot between decking. They meet at the screw or the clip: every time a board crosses a joist, something fastens it. A wider joist spacing means fewer crossings and a longer unsupported stretch of decking. Many 5/4 boards and a lot of composite are limited by their makers to 16 inches on center when the decking is perpendicular to the joists, and sometimes 12 inches when it is diagonal. That is a manufacturer limit. Your city can be stricter. The joist guide counts joists. It does not certify the span.
How to check a gap in the yard
Carry a 1/8 inch spacer, a 16d nail, or the clip you will actually use. Lay two boards on a flat spot and look. If you are using screws, the spacer is the gap. If you are using clips, the clip is the gap, and a separate spacer will lie to you. Set the calculator to that measurement before you print the list. A list printed at 1/8 inch and a deck built at 1/4 inch will not agree when you get to the last row, and the last row is where people rip a board to nothing or go back to the store.
After the deck is up, the same gap decides stain. More slot means more edge for finish to soak, which the flat-area stain calculator does not add. It also means the surface dries faster. Neither effect changes the row count after the boards are already nailed. Get the gap right when the boards go down.