Deck Board Count Calculator
Calculate how many deck boards you need for a given area with your chosen board width and gap spacing.
Includes waste factor and linear footage total.
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.
A 1/4-inch gap on a 20-foot deck run adds up to over 3 inches of wasted span across 15 boards.
Most builders use 3/16 inch or 1/4 inch gaps.
Treated lumber should be installed with no gap — it will shrink as it dries and self-gap.
Composite decking usually specifies 3/16 to 1/4 inch per the manufacturer.
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 (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: 5.4 to 5.5 inch face depending on brand
For ordering, the lumber yard sells boards in 8, 10, 12, 16, or 20-foot lengths.
This calculator gives you total linear feet — divide by your chosen board length to get a piece count.
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.