How Many Deck Boards Do I Need? (Chart + Calculator)

May 16, 2026·7 min read

*Quick answer: Divide your deck's square footage by the square footage of one board, then multiply by your waste factor. A 12×16 deck using standard 5.5-inch × 16-foot boards needs about 30 boards (with 10% waste). Use our deck board calculator for exact counts.

I ordered exactly 28 boards for my first deck. Measured twice, did the math three times, felt great about it. Then reality showed up: 3 boards were bowed so badly they wouldn't sit flat, 1 split when I tried to force it, and I was short by 4 boards. That meant a second trip to the lumberyard mid-project, and of course they only had a slightly different shade of pressure-treated pine left. You can still see the color mismatch if you know where to look.

The formula itself is dead simple. The part people get wrong is the waste factor.

The Quick Formula

``` Boards needed = (Deck sq ft ÷ Board sq ft) × Waste factor ```

Worked example for a 12×16 deck:

Deck area: 12 × 16 = 192 sq ft

Board area (5.5" × 16'): 0.458 ft × 16 ft = 7.33 sq ft per board

Boards without waste: 192 ÷ 7.33 = 26.2 boards

With 10% waste: 26.2 × 1.10 = 28.8 → round up to 30 boards

Deck Size	Sq Ft	5.5" × 12' Boards	5.5" × 16' Boards	3.5" × 16' Boards
8×10	80	18	13	20
10×10	100	22	16	25
10×12	120	26	20	30
12×12	144	31	24	36
12×16	192	42	30	48
16×16	256	56	40	64
16×20	320	70	50	80
20×20	400	87	63	100

Always round up. Nobody sells half a board.
One thing the formula doesn't tell you: board width in lumber terms isn't actual width. A "6-inch" deck board (a 2×6) is actually 5.5 inches wide. A "4-inch" board (a 2×4) is 3.5 inches. Use the real dimensions or your math will be off by 10–15%.

Deck Board Count by Size (Chart)

I did the math for the most common deck sizes so you don't have to. Every number below includes 10% waste, rounded up.

Deck Size Sq Ft 5.5" × 12' Boards 5.5" × 16' Boards 3.5" × 16' Boards
8×10 80 18 13 20
10×10 100 22 16 25
10×12 120 26 20 30
12×12 144 31 24 36
12×16 192 42 30 48
16×16 256 56 40 64
16×20 320 70 50 80
20×20 400 87 63 100
Notice how much board length matters. A 12×16 deck takes 42 boards at 12-foot lengths but only 30 at 16-foot lengths. Fewer boards means fewer seams, less cutting, and a cleaner look. If your deck dimension matches a standard board length, buy that length — it saves real time and material.
5.5-Inch vs 3.5-Inch Deck Boards

This choice affects your board count more than most people expect.

For a 12×16 deck: 30 boards at 5.5 inches wide vs 48 boards at 3.5 inches wide. That's 60% more boards for the narrower option.

5.5-inch boards (2×6) — my recommendation for most decks:

Fewer boards to buy, carry, and screw down

Fewer gaps between boards means less debris falling through

Faster installation — you're covering more surface per board

Standard for residential decks; easiest to find in stock

3.5-inch boards (2×4) — specific use cases:
Curved decks where narrower boards bend more easily

Smaller accent sections or bench surfaces

Personal aesthetic preference (some people prefer the narrower plank look)

Price-wise, 2×6 boards typically run $8–14 per 16-foot board at a big box store (pressure-treated pine). A 12×16 deck at 30 boards costs $240–420 in lumber alone. Go with 3.5-inch boards and you're buying 48 of them — that's $384–672 for the same deck surface, plus more screws and more labor hours.

For a straightforward rectangular deck, 5.5-inch boards win on cost, labor, and speed. I've built with both, and the 3.5-inch boards took noticeably longer to lay. More boards = more measuring, more screws, more chances for misalignment.

How Much Extra Should You Buy? (Waste Factor)

This is where my first deck went sideways. Here's what I use now after four builds:

Straight, simple layout — 5 to 10% waste: Rectangular deck, boards running parallel, no angles. This is the minimum. Even with a perfect plan, you'll get a warped board or two.

First-time builder — 10 to 15% waste: If this is your first deck, buy 15% extra. You'll miscut at least a couple, and having spares beats making a second trip. I speak from painful experience.

Diagonal pattern — 15 to 20% waste: Running boards at 45 degrees looks great, but every board that meets the edge needs an angle cut. That leftover triangle piece is usually too short to use elsewhere. Budget accordingly.

Complex layout (herringbone, picture frame, multiple angles) — 20 to 30% waste: Intricate patterns eat material. Herringbone in particular generates a lot of unusable offcuts. If you're doing this, you already know it'll cost more — just make sure your board count reflects it.

My rule of thumb:* take whatever percentage you think you need and add 5%. Lumber you don't use can go back (most big box stores accept returns within 90 days with a receipt) or be saved for future repairs. Lumber you don't have at 4 PM on a Saturday cannot be wished into existence. On my second build I kept 4 extra boards and used 2 of them three years later when a section started cupping — perfect color match because they aged alongside the deck in my garage.

Does Joist Spacing Matter for Board Count?

Short answer: no, joist spacing doesn't change how many deck boards you need. The surface area is the same regardless of what's underneath.

But joist spacing absolutely matters for which boards you can use. Standard 16-inch on-center (OC) joist spacing works with any common deck board — 5/4×6, 2×6, 2×4, composite, whatever. If your joists are 24 inches OC, thinner boards like 5/4×6 may sag between joists over time, especially in heat. Stick to full 2× lumber or check the manufacturer's span rating for composite.

For most residential decks, 16-inch OC is the default and what building codes typically require. If you're inheriting an existing joist structure, measure the spacing before ordering boards. Wrong board thickness on wide spacing leads to a bouncy, flexing deck surface — I've walked on one, and it felt like the floor might give out.

FAQ

How many deck boards for a 12×12 deck?

24 boards using 5.5-inch × 16-foot lumber (includes 10% waste). If you use 12-foot boards instead, you'll need 31. See the full chart above for other sizes.

How many screws per deck board?

Two screws per joist crossing. A 16-foot board over joists at 16-inch OC crosses about 12 joists, so that's 24 screws per board. For a 12×16 deck with 30 boards, budget around 720 screws — buy a 5-pound box to be safe.

Should I buy extra boards for future repairs?

Yes. Set aside 2–3 boards from the same batch. Deck boards weather and fade over time, so a replacement board bought two years later won't match. Store them flat, off the ground, in a dry spot.

Can I mix board lengths on one deck?

You can, and sometimes it makes sense. If your deck is 14 feet wide, you might use 16-foot boards and trim them, or stagger 8-foot and 6-foot boards with a seam over a joist. Staggered seams are structurally fine as long as every board end lands on a joist. Just avoid lining up all seams in one row — it looks bad and creates a weak line.

How do I account for the gap between deck boards?

Standard gap is 1/8 inch to 3/16 inch between boards for drainage and expansion. Over a full deck, those gaps add up to very little — roughly 2–3% of the surface area on a typical build. The 10% waste factor in the chart already covers this and then some. Don't overthink it.

Next Steps

Use the deck board calculator to get an exact count for custom dimensions, board sizes, and patterns.
Already built the deck? Check the paint calculator to figure out stain or sealant coverage.