A standard 12 ft deck board, 5-1/2” wide, covers 5.5 sq ft of face. Once it’s laid with a gap beside it, it covers a little more: 5.63 sq ft with a 1/8” gap and 5.69 sq ft with a 3/16” gap. So how many square feet is a 12 ft deck board? About 5.5 to 5.7, depending on how you count the gap. Use that number as a rough check only, because dividing your deck area by it usually comes up a few boards short.
Below are the numbers for every common length, plus a worked example showing where the square-foot shortcut goes wrong.
The math: face coverage vs coverage with the gap
A deck board’s coverage is its length times its actual width. A “6-inch” deck board isn’t 6” wide. Both 5/4×6 and 2×6 wood decking measure about 5-1/2” wide, and most composite boards are about 5-1/2” too (the deck board sizes guide lists the exceptions).
Face coverage is the board alone:
- 12 ft × 5.5” ÷ 12 = 5.5 sq ft
Coverage with the gap adds the board’s share of the space next to it. Each row of boards takes up its own width plus one gap, so that’s the width you multiply by:
- 1/8” gap: 12 ft × (5.5 + 0.125)” ÷ 12 = 5.625 sq ft
- 3/16” gap: 12 ft × (5.5 + 0.1875)” ÷ 12 = 5.6875 sq ft
The gap figure is the honest one for estimating, because the gaps are part of the deck surface. A 12 × 12 ft deck is 144 sq ft whether the boards touch or not. The difference is small, about 2–3%, but it adds up over a big deck.
Which gap? TimberTech’s installation guide calls for side-to-side gaps of 1/8” minimum to 3/16” maximum for its boards (TimberTech installation guide), and 1/8” is a common choice for dry wood. Wet pressure-treated boards are often laid tight and shrink open as they dry. The deck board spacing calculator helps you pick one.
Square feet per deck board: 8 to 20 ft
Here’s the coverage for every common stock length. The first three columns match the short table on our deck board calculator page. The last column is for narrow 3-1/2” boards (2×4 decking), which some people use for a finer look.
| Board length | 5-1/2” face only | 5-1/2” + 1/8” gap | 5-1/2” + 3/16” gap | 3-1/2” + 1/8” gap |
|---|---|---|---|---|
| 8 ft | 3.67 sq ft | 3.75 sq ft | 3.79 sq ft | 2.42 sq ft |
| 10 ft | 4.58 sq ft | 4.69 sq ft | 4.74 sq ft | 3.02 sq ft |
| 12 ft | 5.50 sq ft | 5.63 sq ft | 5.69 sq ft | 3.63 sq ft |
| 16 ft | 7.33 sq ft | 7.50 sq ft | 7.58 sq ft | 4.83 sq ft |
| 20 ft | 9.17 sq ft | 9.38 sq ft | 9.48 sq ft | 6.04 sq ft |
The 5-1/2” columns apply to 5/4×6 wood, 2×6 wood and most 5-1/2” composite boards alike. Thickness doesn’t change coverage. If your composite board is narrower, rerun the math with its spec-sheet width. A 5.36” board with a 3/16” gap, for example, covers about 5.55 sq ft per 12 ft.
The same thing in linear feet
Lumberyards often price by the linear foot, so it helps to flip the numbers around. For every 100 sq ft of deck you need roughly:
- 211 linear ft of 5-1/2” board with a 3/16” gap
- 213 linear ft with a 1/8” gap
- 331 linear ft of 3-1/2” board with a 1/8” gap
That’s before any waste. In 12 ft boards, 100 sq ft works out to about 17.6 boards with a 3/16” gap. Narrow boards need about 55% more length, plus more screws and more labor.
What the manufacturers say
TimberTech publishes this shortcut in its install guide: multiply length by width, divide by 5.5 for 12 ft boards, 7.3 for 16 ft boards or 9.1 for 20 ft boards, round up, then add roughly 10% for scrap (TimberTech installation guide). Those divisors are face coverage rounded down, which builds in a little extra. They’re a fair starting point. The trouble is what the formula can’t see.
Why square-foot counts run short
Dividing area by coverage assumes every inch of every board ends up on the deck. On a real deck, four things get in the way.
1. The last row is a whole board. Rows rarely divide evenly into the deck width. On a deck 12 ft across, 5-1/2” boards with a 3/16” gap give 25 full rows and a 26th row only about 1-13/16” wide. With a 1/8” gap the last row is about 3-3/8”. Either way, you rip it from a full board, so you buy 26 boards for that width, not 25.3.
2. Cut-offs. If the run isn’t a multiple of the board length, each row leaves an offcut. Cut a 10 ft run from 12 ft boards and 2 ft of every board, about 17%, falls off. Some offcuts make good stair treads or blocking, but most won’t fit back into the deck.
3. Seams have to land on joists. When a row needs two boards, the joint can’t fall anywhere you like. TimberTech says board ends must fall on a joist and to always double joists at butt seams (TimberTech installation guide). With joists at 16” on center, seams can only sit at 16” steps, and staggering them between rows means some pieces get cut shorter than they could be. The board lengths guide covers seam planning in detail.
4. Defects and mistakes. Bowed boards, a split end, a miscut. That’s what the usual 10% allowance is for, and it sits on top of the layout waste above, not instead of it.
Worked example: a 12 × 16 ft deck
Take a 12 × 16 ft deck (192 sq ft) in 5-1/2” composite with a 3/16” gap, joists at 16” on center and a 10% allowance. Here’s the square-foot shortcut next to what the deck board calculator actually tells you to buy.
The shortcut, using 5.69 sq ft per 12 ft board: 192 ÷ 5.6875 = 33.8, round up to 34, add 10% → 38 boards. TimberTech’s divisor (192 ÷ 5.5 = 34.9 → 35, plus 10%) gives 39 boards.
The calculator, for three ways of building the same deck:
| Layout | Rows | Boards before allowance | Offcut waste | Butt seams | Boards to buy |
|---|---|---|---|---|---|
| 12 ft boards running the 12 ft way | 34 | 34 | 0% | 0 | 38 × 12 ft |
| 16 ft boards running the 16 ft way | 26 | 26 | 0% | 0 | 29 × 16 ft |
| 12 ft boards running the 16 ft way | 26 | 39 | 11.1% | 26 | 43 × 12 ft |
What this shows:
- When the run matches the board, the shortcut works. Boards running across the 12 ft width are one 12 ft board per row. The 34 rows end in a ripped last row about 4-5/16” wide, so the calculator’s 38 matches the shortcut. That’s also the composite layout on the 12 × 16 deck page.
- When it doesn’t, the shortcut is short. If the boards have to run the 16 ft way and your store only stocks 12 ft composite, every row needs a 12 ft board plus a 4 ft piece. Because the seams have to land on joists and stagger between rows, you end up buying 43 boards, 5 more than the shortcut’s 38. That’s about 516 linear ft bought for 416 ft installed. You’d also need doubled joists along two seam lines.
- Longer boards fix it. The same direction with 16 ft boards takes 26 boards with no offcuts or seams, and 29 with the allowance.
Even the “good” case hides a detail: 192 sq ft of deck is only 405 linear ft by the area math, but 416 ft goes onto the frame, because the last row is a full board ripped down.
An awkward run makes it worse. A 14 × 12 ft deck with 16 ft boards running the 14 ft way: area math says 168 ÷ 7.58 = 22.2 boards. The calculator needs 26 boards before allowance (29 to buy), since every row leaves a 2 ft offcut.
How to use square-foot coverage properly
Coverage per board is still useful, as long as you use it for the right jobs:
- Ballpark budgets. Area ÷ coverage, plus 10–15%, gets you in the right range for a price check before you’ve settled the layout.
- Comparing board widths. The table shows quickly how much more material narrow boards take.
- Checking a quote. If a supplier’s count is far below area ÷ coverage, ask why.
For the order itself, count rows and runs instead. Measure the deck with the deck square footage calculator if it has an odd shape, then enter the dimensions in the deck board calculator. It counts rows, flags thin last rows, places seams on joists, compares 12, 16 and 20 ft boards, and gives you a cut list.
Frequently asked questions
How many square feet does a 12 ft 5/4×6 deck board cover?
A 5/4×6 is about 5-1/2” wide, so a 12 ft board covers 5.5 sq ft of face. Laid with a 1/8” gap it accounts for about 5.63 sq ft of deck, and with a 3/16” gap about 5.69 sq ft. A 12 ft 2×6 covers the same, because it’s the same width.
How many square feet is a 16 ft deck board?
A 5-1/2” board 16 ft long covers 7.33 sq ft of face, or about 7.5 sq ft with a 1/8” gap and 7.58 sq ft with a 3/16” gap. A 20 ft board covers 9.17 sq ft of face and about 9.4–9.5 sq ft with the gap.
How many 12 ft deck boards do I need for 100 square feet?
Roughly 17.6 boards with 5-1/2” boards and a 3/16” gap, or about 211 linear feet, before waste. That’s a ballpark only. The real count depends on your deck’s width and run, so a 100 sq ft deck can need noticeably more once you allow for the last row, offcuts and a 10% allowance.
Should I use square footage to order deck boards?
Use it for a budget estimate, not the order. Area ÷ coverage assumes no offcuts, no ripped last row and no seams, so it runs short whenever the deck length isn’t a multiple of the board length. Counting rows and runs gives the real number.
Get the exact count for your deck, with board lengths and a cut list, from the deck board calculator.