Deck Board Calculator

Decking boards, linear feet and screws for your deck frame.

How the deck estimate works

Boards run one direction across the joists. The calculator divides the deck's cross-direction width by each board's effective width (a 5½-inch "6-inch" deck board plus a ⅛-inch gap is about 5.63 inches of run) to get the row count, multiplies rows by the board length for linear feet, and counts screws at two per board per joist.

Rows = ceil( width x 12 / (board width + gap) )
Screws = rows x joists x 2

Board length planning

Buy board lengths that span the full deck depth where possible — a 12-foot deck wants 12-foot boards, not spliced 8s. Butted end joints on the same joist are where decks first show wear, so stagger any butt joints across at least two joists. Composite brands publish their own gap requirements (thermal expansion), so their effective width differs from wood — check the installation sheet.

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Use cases

Ground-level patio deck - parallel boards, picture-frame border adds ~10% board footage.
Second-story deck - same math for boards; framing is a separate (bigger) calculation.
Diagonal boards - add 15% to the board count for angled cuts and waste.
Board replacement - measure only the damaged run; keep one spare board for future swaps.

Frequently asked questions

How many deck boards for a 12x12 deck?

With 5½-inch boards and ⅛-inch gaps running one way, 12 feet is 144 inches ÷ 5.63 ≈ 26 rows. At 12-foot board lengths that is 26 boards, or 312 linear feet — buy 10% extra for cuts.

What gap should I leave between deck boards?

⅛ inch for pressure-treated wood (it shrinks wider) and per the manufacturer's spec for composite (it expands). The gap lets water drain and boards dry, which is what keeps a deck alive.

Estimates only. Joist direction, borders and diagonal layouts change counts; frame the deck first, then measure it.

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Planning the deck surface

How to use this calculator

Confirm the deck frame's outside dimensions — length (joist direction) and width (board direction).
Enter board width and gap; 5½ in + ⅛ in fits most 6-inch pressure-treated decking.
Pick a board length that spans the deck in one piece where possible.
Add 10% waste — 15% for diagonal or picture-frame layouts.
Order fasteners from the calculated screw count; use coated or stainless screws for treated lumber.

Quick reference

Board size (nominal)	Actual width	Typical use
2x6	5½ in	Standard deck boards
2x4	3½ in	Tight-gap premium look, more boards
5/4 x 6	5½ in	Rounded-edge decking, radius corners
Composite 1x6	~5½ in	Check brand gap spec (expands)

Pro tips

Acclimate composite boards flat on the deck site for a day or two before fastening — they ship with a curve.
Two screws per joist crossing, ¾ inch from each board edge, prevents the cupping that single-center screws cause.
Hidden fastener systems eat ~2 extra inches per board side; if using clips, re-check the count with the system's spacing.

Frequently asked questions

Should deck boards run parallel or perpendicular to the house?

Perpendicular to the house (boards running away from it) sheds water toward the outer edge and matches how joists are framed. Running boards parallel to the house is possible but needs careful slope planning.

How many screws per deck board?

Two at every joist crossing — one about ¾ inch from each edge. At 16-inch joist spacing, a 12-foot board crossing 10 joists takes 20 screws.

Does deck board direction affect drainage?

Yes. Boards act like a roof over the frame; running them across the short direction with proper gaps drains fastest. Under-deck drainage systems invert this math if you plan a dry space below.