Deck Board Calculator

To use this deck board calculator, enter your deck's length and width in feet, then pick your board width (5.5 in is the standard 2x6) and board length. The calculator instantly returns how many deck boards you need, plus the screws and hidden fasteners to install them. Need to fine-tune? Open More options to set the gap between boards, joist spacing, a waste factor, metric units, or a price per board for a quick cost estimate. Most people only need the first four fields for an accurate answer.

The number of deck boards depends on three things: the surface area of your deck, how wide each board is, and the gap you leave between boards for drainage and expansion. A wider board covers more ground, so you buy fewer of them; a bigger gap slightly increases the count. This tool lays boards along the length of the deck, stacks rows across the width, and rounds up to whole boards — because you can't buy 3/4 of a board. It then adds a waste factor (10% by default) to cover cutoffs, mistakes, and defective boards. If you've searched *how many deck boards for a 10x20 deck* or *deck board calculator by the foot*, this is the math that answers it.

Effective board width = (board width + gap) ÷ 12 (feet)

Rows across deck = ⌈deck width ÷ effective board width⌉

Boards per row = ⌈deck length ÷ board length⌉

Total boards = ⌈rows × boards-per-row × (1 + waste%)⌉

Screws use the rule of 3.5 screws per square foot (sold ~350 to a box). Hidden fasteners equal one per board run at each joist crossing.

Example — 20 ft × 10 ft deck, 5.5 in boards, 16 ft long, 1/8 in gap:

- Effective board width = (5.5 + 0.125) ÷ 12 = 0.469 ft
- Rows = ⌈10 ÷ 0.469⌉ = 22 rows
- Boards per row = ⌈20 ÷ 16⌉ = 2
- Subtotal = 44 boards, +10% waste = 49 deck boards
- Screws = 200 sq ft × 3.5 = 700 (2 boxes of 350)

Example — same deck with 6 in boards: effective width = 0.51 ft, so rows drop to 20 and you need about 44 boards after waste. Wider boards mean fewer pieces to fasten.

- Keep the 10% waste factor for straight rectangular decks; bump it to 15% for diagonal patterns, picture-frame borders, or lots of cuts.
- Standard gap is 1/8 in (0.125) for pressure-treated lumber that will shrink, and up to 1/4 in for composite in hot climates — check the manufacturer's spec.
- Buy boards long enough to span with fewest seams. A 16 ft board across a 15 ft deck beats two shorter boards with a butt joint.
- Hidden fasteners give a clean, screw-free surface on grooved composite boards; face screws are cheaper and stronger for wood.
- Order a few extra boards from the same lot for future repairs — dye lots and weathering are hard to match later.