Deck Board Count Calculator

Decking boards run perpendicular to the joists, and the total count depends on three things: how wide the deck is in the direction the boards span, how wide each board face is, and how big a gap you leave between boards.

The gap matters more than people expect.
On a 16-foot span with 5.5-inch boards you need 35 boards laid tight, or 34 with a 1/4-inch gap.
The 33 gaps between those 34 boards swallow 8 1/4 inches of span, which is a board and a half of wood you did not have to buy.
Most builders use 3/16 inch or 1/4 inch.
Wet treated lumber goes down with no gap at all, because it shrinks as it dries and opens its own.
Composite is different: it barely shrinks, so the manufacturer’s spacing is what you get, usually 3/16 inch.

The formula:

boards_across = deck_span ÷ (board_face_width + gap) total_linear_feet = boards_across × board_run_length × 1.10

The 10% waste factor covers end cuts, bad boards, and the occasional split.
On a complex deck with angles or multiple sections, increase the waste factor to 15%.

Common board face widths:

• 5/4×6 lumber, the most popular decking: 5.5 inch face
• 2×6 dimensional lumber: 5.5 inch face
• 2×4 dimensional lumber: 3.5 inch face
• Composite “6 inch” boards: anywhere from 5.25 to 5.5 inches depending on brand, and grooved boards for hidden fasteners are narrower again

That composite range is not a rounding detail. Measure the actual board in front of you before ordering, because a quarter inch of face width across 34 boards is a whole extra board on a 16-foot span.

For ordering, the lumber yard sells boards in 8, 10, 12, 16, or 20-foot lengths.
This calculator works out the piece count for each stock length so you can compare the offcut waste before ordering.
Buying longer boards reduces end joints but costs more per piece.

One practical note: always orient the grain crown-side down on flat decks.
If installed crown-up, boards cup over time and hold water in the channels, which accelerates rot.