Deck Board Calculator

How to use this calculator

Enter the deck dimension measured across the boards — the direction in which rows are added — and the dimension along the boards. Board width defaults to 5½ inches, the actual width of nominal 6-inch boards; the gap defaults to ⅛ inch. Composite manufacturers often specify 3/16–¼ inch, so check the installation guide.

The calculator returns the number of rows, the total linear footage (the figure many yards price by) and the number of boards of your chosen length, with a 10% allowance for cut-offs and rejected boards. If the deck is longer than your boards, the extra length is covered by joining boards over a joist; the waste allowance absorbs the offcuts.

The formula

n = W / (b + g)

LF = n × L

N = LF × (1 + w%) / ℓ

W: deck width across the boards
L: deck length along the boards
b: board width (actual)
g: gap between boards
n: board rows
LF: total board length (linear)
ℓ: length of one board
w%: waste allowance
N: boards needed

Worked example

A 12 × 16 ft deck is decked with 5/4 × 6 boards (5½ in actual width) running the 16-ft direction, with ⅛-inch gaps.

Rows across the deck = 144 in ÷ (5.5 + 0.125) = 25.6, so 26 rows.
Linear feet = 25.6 × 16 = 409.6 ft.
Boards = 409.6 × 1.10 ÷ 16 = 28.2, so 29 boards of 16 ft.

Buy 29 sixteen-foot boards. Matching the board length to the deck length avoids butt joints entirely.

Actual widths of common decking

Board	Actual width
5/4 × 6 wood decking	5½ in
2 × 6 lumber	5½ in
2 × 4 lumber	3½ in
Composite, standard	5½ in (check the brand)
Hardwood (ipe), 1 × 4	3½ in

Assumptions and limits

Rectangular deck; boards run parallel to one side. For diagonal decking, add 10–15% more allowance.
Wet pressure-treated lumber shrinks as it dries; installers often butt it tight and let the gap open up.
Joists, beams, fasteners and railings are not included.

Frequently asked questions

How many deck boards do I need for a 12 × 12 deck?

With 5½-inch boards and ⅛-inch gaps, 144 ÷ 5.625 = 25.6 rows of 12-ft boards: 26 boards, or 29 with a 10% allowance.

What gap should I leave between deck boards?

About ⅛ in for kiln-dried wood and the manufacturer’s figure for composite (often 3/16 in). Wet pressure-treated boards can be laid tight.

Which way should deck boards run?

Usually perpendicular to the joists, and often parallel to the house. Joist spacing must suit the board: 16 in on center is typical for wood and most composites.