Pressure-treated pine
Treated 5/4×6 decking is usually stocked in 8 to 16 ft lengths. Running the boards along the 12 ft side, a 12×20 deck needs 43 rows, and every row is a single 12-ft board, so there are no butt seams. The last row is ripped to about 3-3/4". With a 10% allowance for splits, warps and mistakes, buy 48 × 12-ft boards — 576 linear feet in total.
Composite decking
Composite comes in 12, 16 and 20 ft lengths and is usually laid with a 3/16" gap. Boards along the 12 ft side give 43 rows; every row is a single 12-ft board, so there are no butt seams. The last row is ripped to about 1-1/8". Buy 48 × 12-ft boards, about 576 linear feet. Where there are seams, most manufacturers ask for doubled joists underneath.
Which composite board lengths to buy
The same 12×20 deck, priced in material rather than money. Each option keeps seams on joists; the recommended one uses the least board (each seam counted as about a foot of extra board, because of the doubled joist and extra cuts it brings).
| Buying option | Boards | Linear ft | Offcut waste | Seams |
|---|---|---|---|---|
| All 12-ft boardsRecommended | 43 | 516 | 0.0% | 0 |
| Best mix of lengths | 43 | 516 | 0.0% | 0 |
| All 16-ft boards | 43 | 688 | 25.0% | 0 |
| All 20-ft boards | 43 | 860 | 40.0% | 0 |
Counts are before the 10% allowance.
Does board direction matter for a 12×20 deck?
Yes. Running composite boards along the 12 ft side takes 576 linear feet; turning them the other way takes 580 — about 1% more board, because the rows no longer match the stock lengths as well. Boards normally run across the joists, so in practice the direction also follows how the deck is framed — usually joists run out from the house and boards run parallel to it.
Fasteners, framing and stain
- Screws: about 832 face screws (two per board at each joist, 16" on center), or about 416 hidden clips. See the screw calculator for box counts.
- Joists: about 16 joists at 16" on center for a ledger-attached deck 20 ft along the house. Sizes and spans come from your local code — the framing calculator counts beams, posts and hardware too.
- Stain: about 3 gallons for two coats on wood decking. Try the stain calculator to add stairs and railings.
How these numbers were worked out
Rows are the deck width divided by board width plus gap, rounded up. Each row is then split into the board lengths you can buy, with seams staggered on joists, and the pieces are fitted onto stock boards with offcuts reused (1/8" per saw cut). The 10% allowance is added on top. The full method is on the about page, and the board length guide explains why the mix of lengths matters.
Questions about a 12×20 deck
How many 16-ft boards do I need for a 12×20 deck?
It depends on the board direction and whether 16 ft suits the run. With composite boards and 10% extra, the best plan is 48 × 12-ft boards (48 in total). Use the calculator to force a single length if you prefer.
How many deck screws for a 12×20 deck?
About 832 screws, at two per board at every joist with joists 16" on center — or about 416 hidden clips. Add 5% for drops and strip-outs.
How much stain does a 12×20 deck need?
About 3 gallons for two coats at 200 sq ft per gallon plus 10% extra. Coverage varies by product and wood, so check the can.
How many square feet is a 12×20 deck?
A 12×20 ft deck is 12 × 20 = 240 square feet (about 22.3 m²). That's the walking surface; stairs and landings are measured separately.
Is a 12×20 deck a good size?
At 240 sq ft it fits two zones, for example a table for four to six plus a grill area, or a table plus a pair of lounge chairs. Before you settle on a size, mark the outline on the ground with stakes or a hose and set your furniture inside it.
What does a 12×20 deck look like?
The plan drawing at the top of this page shows a 12×20 deck to scale, with every board row, the joists underneath and any butt seams. Open it in the deck board calculator to switch to the 3D view, change the board colour or try the boards running the other way.
How much does a 12×20 deck cost?
Using Ergeon's 2026 installed ranges of $25 to $40 per sq ft for pressure-treated wood and $35 to $55 for mid-grade composite, a 240 sq ft deck comes to roughly $6,000 to $9,600 in treated wood or $8,400 to $13,200 in composite, before railings, extra height or demolition. Local prices vary, so get itemized quotes. The deck cost breakdown explains each line.
How much does labor cost to build a 12×20 deck?
HomeGuide's 2026 figures put deck labor at about $11 to $22 per sq ft, which is roughly $2,640 to $5,280 for a 12×20 deck. Raised decks, complex shapes and difficult access cost more.
How many posts do I need for a 12×20 deck?
For a deck attached to the house with one beam along the 20 ft side and posts no more than 8 ft apart, you need 4 posts (4 footings). A freestanding deck needs a second beam, which doubles that. If the 12 ft joist span is beyond your code's span table, a mid-span beam adds another row of posts. The deck framing calculator works this out for your layout.
How much material do I need to build a 12×20 deck?
For the walking surface: 48 × 12-ft pressure-treated boards (48 total) or 48 × 12-ft composite boards (48), plus about 832 deck screws. Underneath, an attached deck needs about 16 joists at 16" on center, a ledger, rim joists, 1 beam and 4 posts. The framing calculator lists the hardware, and the stair and railing calculators cover the rest.
How do you build a 12×20 deck?
In outline: check your local code and get a permit; lay out the 12×20 outline and square it with the 3-4-5 method; dig 4 footings below your frost line and set the posts; fit the ledger to the house with flashing (or a second beam for a freestanding deck) and the beam on the posts; hang about 16 joists at 16" on center; then lay the deck boards with even gaps and two fasteners per board at every joist, and finish with stairs and railings where needed. The deck planning guide, footings guide and framing calculator go through each stage.
How long does it take to build a 12×20 deck?
It depends on who builds it, the height, the site and the weather, so ask builders for a schedule with their quote. Plan around the stages rather than a single number: the permit and any inspections, digging and pouring footings (the concrete must cure for the time on the bag before it carries load), framing, decking, then stairs and railings. Raised decks, difficult access and extra inspections add time.
How many joists for a 12×20 deck?
With joists at 16" on center running across the 12 ft projection, a 20 ft wide deck needs about 16 joists including the two ends. Joist size depends on span and must come from your local code tables.