How many deck boards do I need for a 12-foot span?

With 5½-inch boards and a ¼-inch gap: floor((144 + 0.25) ÷ 5.75) = 25 boards. The leftover space spreads to a 0.27-inch gap between each.

What is the standard gap between deck boards?

About 1/4 inch (0.25 in) for wood, for drainage and seasonal movement. Composite decking usually specifies 3/16 inch, and always ends-to-ends spacing too — check the manufacturer's instructions.

Why is the adjusted gap different from the gap I entered?

Only whole boards fit, so the space left over after all the boards are placed gets divided equally among the joints. That adjusted value is what you set your spacers to for a truly even layout.

Should I account for board shrinkage?

Yes for wet pressure-treated lumber — it can shrink up to 1/8 inch across the face as it dries, so many builders butt or nearly butt those boards knowing gaps will open. Kiln-dried and composite boards need the full gap from day one.

What if the adjusted gap comes out ugly?

Change the board count by one and recheck. Adding a board tightens every gap; dropping one widens them. Or rip the two outer boards to equal narrower widths so the field gaps stay exactly on target.

Does this include the picture-frame border?

No. Calculate the field first, then subtract the border board widths (plus their gaps) from the deck width and run the field boards through the remaining space.

Deck Board Spacing Calculator

Even gaps across a deck look simple until you find that whole boards never divide cleanly into the frame width. Too wide and it reads as sloppy; too tight and it traps debris and moisture against the board edges. This calculator does the layout so every gap comes out identical and you don’t have to rip the last board to a sliver.

The deck board spacing formulas

is the deck width, is the board width, and is the target gap. whole boards leave gaps between them, and the leftover space is shared evenly across those gaps.

Worked example

The calculator’s defaults are a 144-inch (12-foot) width, 5.5-inch boards, and a 0.25-inch target gap.

So use a 0.27-inch spacer, not a 0.25 — the difference is what keeps the last board full width.

Deck Board Spacing Calculator worked example diagram — A visual read of the worked example above — the same numbers the calculator returns for its default inputs.

What Your Results Mean

Boards Needed is the number of full-width boards to purchase for that span. In our example, 25 standard 5.5-inch deck boards cover a 12-foot (144-inch) frame width.

Adjusted Gap is the actual spacer width you should use during installation. Here, 0.2708 inches is just slightly wider than the 0.25-inch target, meaning a standard 1/4-inch spacer will leave you a hair tight, but well within acceptable tolerance for wood decking.

Total Gap Space shows the cumulative opening across all joints. At 6.5 inches total, roughly 4.5 percent of the deck surface is open, which is ideal for water drainage.

Tips for a Clean Layout

Start from one edge and work across, placing a spacer between every board. If the adjusted gap rounds nicely to a common spacer size (1/8, 3/16, or 1/4 inch), use that off-the-shelf spacer. If it falls between sizes, cut a plywood shim to the exact dimension or use an adjustable deck-spacing tool.

For long runs over 16 feet, check your lumber for crown and bow before committing to a fixed gap, as warped boards can eat into your spacing budget on one end while opening it on the other.

Choosing between two board counts

The formula always rounds the board count down, which makes the real gap a little wider than your target. Sometimes one more board gives a closer fit. For a 192-inch (16-foot) width with 5.5-inch boards and a 3/16-inch target gap:

33 boards: total gap in, so each gap is in.
34 boards: total gap in, so each gap is in.

Neither hits 3/16 inch (0.1875) exactly. The 34-board layout is closer and tighter; the 33-board layout drains better. A third option is to keep the target gap and rip the last board to width, which works best against the house wall, where a slightly narrow board is hidden.

Wet lumber and gap size

Pressure-treated boards often arrive wet and shrink as they dry, sometimes by an eighth of an inch or more across the width. Many builders lay wet boards tight or with a very small gap and let the drying open it up. Kiln-dried and composite boards don’t shrink much, so space those to the final gap. Check the manufacturer’s instructions for composite boards: some also need end gaps for expansion with temperature.

Common mistakes

Using the nominal width. A “6-inch” deck board is about 5.5 inches wide. Use the measured width.
Measuring the wrong span. Measure where the boards will actually sit, from the house-side edge to the outside face of the rim or the inside of any picture-frame border.
Mixing batches. Boards from different bundles can differ slightly in width. Measure a few and use the average.
Forgetting the gap at the house. Leave a small gap along the wall so water can drain and air can circulate.

About the formula: The calculator finds how many whole boards fit within the deck width at the desired gap, then redistributes the remaining space evenly across all gaps.