How Many Deck Boards for a 12x20 Deck?

A 12 × 20 ft deck (240 ft²) needs about 48 deck boards of 12 ft (or 36 of 16 ft) with 10% waste, using 5½ in boards with 1/8 in gaps. Underneath it needs 10 joists at 16 in spacing.

Results

Deck boards to buy

48 boards

At your chosen board length, with waste

Rows of decking across the width: 43 rows
Total decking length (with waste): 568 linear ft
Deck area: 240 ft²

Framing underneath

JoistsEach as long as the deck width: 10 joists

Fasteners

Screws (2 per board at each joist): 860 screws

If your boards are shorter than the deck length, plan for butt joints to land on a joist (use a double joist there), and add boards for the extra cuts.

Boards for a 12 × 20 ft deck, by board length

A 12 × 20 ft deck spans most of a house wall, with space for several zones. With 5½ in boards and 1/8 in gaps the deck has 43 rows, which is 516 ft of decking before waste. These counts include 10% waste:

Board length	Boards to buy	Total length bought
8 ft boards	71	568 ft
10 ft boards	57	570 ft
12 ft boards	48	576 ft
16 ft boards	36	576 ft
20 ft boards	29	580 ft

Waste and layout

Layout	Waste	12 ft boards	16 ft boards
Straight, careful installer	5%	46	34
Straight (typical)	10%	48	36
Diagonal or picture-frame border	15%	50	38

Joists and screws

Joists run the 20 ft width and are spaced along the 12 ft length.

Joist spacing	Joists	Screws
12 in on center	13	1,118
16 in on center	10	860
24 in on center	7	602

That is just the decking surface, for a 240 ft² deck. Posts, beams, footings, ledger, railings and stairs are extra, and a permit is usually required.

Want to change anything else? The calculator above is the full Deck Board Calculator: edit any number and the results update instantly.

Tools & materials for this project

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Deck screws (exterior)Composite decking Joist tape Hidden deck fasteners Deck stain / sealer

Frequently asked questions

How many deck boards for a 12x20 deck?

With 5½ in boards, 1/8 in gaps and 10% waste, you need 48 boards of 12 ft, or 36 boards of 16 ft. That is 43 rows of decking across the 20 ft width.

How many joists does a 12x20 deck need?

At 16 in on center, 10 joists running the 20 ft width. At 12 in it is 13, and at 24 in 7. Joist size and span must follow your local building code.

How many screws for a 12x20 deck?

About 860 screws: two at every joist for each board. Buy a few extra, since they strip and get dropped.

Related sizes and guides

Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.

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