How many deck boards do I need?

Deck boards are counted in rows, not in square feet, because the gaps between them mean the surface is not solid. Each row needs enough boards to span the deck length, and the number of rows depends on the board width plus the gap.

Deck length (ft)

The direction the boards run.

Deck width (ft)

Board width (in)

A nominal 5/4x6 deck board is 5.5 in wide.

Gap between boards (in)

1/8 in is typical; composite boards often need 3/16 in.

Board length 8 ft10 ft12 ft16 ft

Boards needed

26boards

Rows 26 rowsLinear feet 416 lin ftDeck area 192 sq ft

Assumes a rectangular deck with boards parallel to one edge. Diagonal, herringbone and picture-frame layouts waste more, and a complex shape should be broken into rectangles.

The gap matters more than people expect. On a 12 ft wide deck, 5/4 boards (5.5 in) with a 1/8 in gap cover 5.625 in per row, so 144 in ÷ 5.625 = 25.6 rows, rounded up to 26. Narrow the gap to 1/16 in and you need 25 rows; widen it to 1/4 in and it is 24. Composite boards usually specify a wider gap than wood because they expand more in heat.

Board length decides how many pieces per row. A 16 ft deck with 16 ft boards is one board per row — 26 boards, no butt joints. The same deck built from 8 ft boards needs two per row and wastes the offcut from every second one, which is why longer boards are worth the handling.

How the calculation works

Rows = ceil(deck width in inches ÷ (board width + gap)). Boards per row = ceil(deck length ÷ board length). Total boards = rows × boards per row. Linear feet = total boards × board length.

• Boards run along the length and the deck is a rectangle; a diagonal or picture-frame layout adds 10-15% in cuts and should use a higher board count.
• Board width is the actual face width, not the nominal size — a 5/4x6 board is 5.5 in, not 6 in.
• No allowance is made for board ends overhanging the rim joist, which is a design choice, not waste.
• Fascia, stair treads, railings and the substructure are separate materials and are not counted.

Sources: Nominal vs actual lumber: a 5/4x6 deck board is 1 in thick and 5.5 in wide; Typical gap: 1/8 in wood, 3/16 in composite (manufacturer guidance)

Worked examples

Questions

How do I calculate how many deck boards I need?

Divide the deck width in inches by the board width plus the gap to get the number of rows, then multiply by the number of boards needed to span the length. Round both up.

How many deck boards are in 100 square feet?

About 24 with a 1/8 in gap, depending on board width and whether you count offcuts. A 10 × 10 deck is roughly 22 to 26 boards.

What gap should I leave between deck boards?

1/8 in for kiln-dried wood, 3/16 in for composite, and 1/4 in if the deck will see a lot of rain and debris. Gaps that are too tight trap water and swell shut.

Should I use 16 ft boards or 8 ft?

16 ft boards mean no butt joints on a 16 ft deck, which is faster and cleaner, but they are heavy and need two people. 8 ft boards are easier to handle and cheaper per piece.

How many screws do I need?

Two per joist per board. A 16 × 12 deck with 16 in joist spacing has about 10 joists and 26 rows, so roughly 520 screws — buy by the 5 lb box.

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Written and maintained by Sam Winters. Last updated 2026-11-04. Every figure is computed in your browser; nothing you enter is sent anywhere. Found an error? Tell me — accuracy reports get fixed first.