How do I calculate how many deck boards I need?

Divide the deck width by the board width plus the gap, round up, then multiply by the number of board rows needed to cover the deck length. A 12×12 ft deck with 5.5-in boards needs about 26 boards per row.

What spacing should deck boards have?

About 1/8 inch between composite boards (they expand), and 1/8–1/4 inch for wood to allow airflow and drainage. Check the manufacturer's spec for your specific decking.

How far apart should deck joists be?

Typically 16 inches on center for residential decks, or 12 inches for heavier loads and some composite boards. Check the decking manufacturer's joist-spacing table.

How much decking do I need for a 12×12 deck?

For a 12×12 ft deck with 5.5-in boards laid across a 12-ft width, you need about 26 boards per row; if the deck length is covered in one row of 12-ft boards, that's roughly 26 boards (about 312 board feet).

What is a board foot?

A board foot is 1 ft long × 1 ft wide × 1 in thick (144 cubic inches). For decking, people often use it loosely to mean a 1-in × 1-ft length of a standard board. Price decking by the linear foot or board to keep it simple.

Deck Calculator — Boards, Board Feet & Cost

Published: July 15, 2026

Enter your deck's dimensions to get the number of boards, joists, and total cost. No sign-up, no data stored.

Quick answer: Boards per row = deck width ÷ (board width + gap), rounded up. A 12×12 ft deck with 5.5-in boards needs about 26 boards per row.

Lay boards across joists with a small gap between them. Board count = width ÷ (board width + gap).

Deck length (ft)

Deck width (ft)

Board width (in)

Gap (in)

Board length (ft)

Price per board ($) — optional

Decking needed

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Per row

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Rows

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Joists

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Cost

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How decking is calculated

The math is simple and exact — no assumptions to adjust for:

Boards per row = deck width ÷ (board width + gap), rounded up.
Rows = deck length ÷ board length, rounded up.
Total boards = boards per row × rows.
Board feet = total boards × board length (as a linear-foot count).
Joists = (deck length ÷ joist spacing) + 1 (16-in spacing on center).
Cost = total boards × price per board (only if you enter a price).

Boards usually run the length of the deck, so you lay enough side by side to cover the width. Add a little extra for cuts, knots, and waste at the ends — buying a spare board or two is cheap insurance.

Worked example: A 12×12 ft deck with 5.5-in boards and a 1/8-in gap needs 144 ÷ 5.625 = 26 boards per row (rounded up). One row of 12-ft boards covers the 12-ft length, so you need about 26 boards — roughly 312 board feet. With 16-in joists, that's about 10 joists.

Deck material chart

Quick reference for common deck sizes (5.5-in boards, 1/8-in gap, 12-ft board length).

Deck size	Boards	Board feet	Joists @16"
10×10 ft	22	~264	9
12×12 ft	26	~312	10
12×16 ft	26	~416	13
16×20 ft	35	~560	16

Last updated: July 2026 · Board counts assume straight runs with 5.5-in boards and 1/8-in gaps. Add 5–10% for cuts, waste, and any diagonal layout.

Deck calculator FAQ

How many deck boards do I need for a 12×16 deck?

Laying 5.5-in boards across a 12-ft width, you need 26 boards per row; a 16-ft length needs two rows of 8-ft boards, so about 52 boards total (minus anything you can cover with full 16-ft boards).

What's the standard gap between deck boards?

1/8 inch is common for composite; wood often uses 1/8–1/4 inch. Follow the decking manufacturer's spec — expansion and drainage matter.

How many joists do I need?

At 16-in on-center spacing, divide the deck length in inches by 16 and add 1. A 12-ft deck needs about 10 joists; a 16-ft deck about 13.

Do deck boards run lengthwise or across?

They usually run the length of the deck, laid on joists that run the other way. That's why you count boards side by side to cover the width, then multiply by the number of length-rows.

Should I buy extra decking?

Yes — plan for about 5–10% waste on cuts, knots, and board ends. Ordering one or two extra boards is much cheaper than a second delivery.

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