How Many Deck Boards Do I Need?

The count comes from the width you're spanning, not the area — and the gap between boards changes it. The arithmetic, and the mistakes it prevents.

It isn't an area calculation

This is the thing that trips people up. Almost every other material on a build is area ÷ coverage. Deck boards aren't.

Boards run in one direction. So what decides how many you need is how many board-widths fit across the other direction. The length of the deck tells you how long each board has to be, not how many there are.

Get those two the wrong way round and the answer is wildly off.

Your details

Deck length(ft)

The direction the boards RUN along.

Deck width(ft)

The direction the boards are laid ACROSS.

Board width(in)

A nominal 6-inch board is usually 5.5 in actual.

Gap between boards(in)

For drainage and movement. 1/8 to 1/4 in is typical.

Joist spacing(in)

On centre. 16 in is the most common.

Waste allowance(%)

Result

Boards to buy

Full-length boards, waste included.

Boards to cover the deckBefore any allowance for cuts and rejects.: 26
Linear feet to orderBoards multiplied by their length - how timber is often priced.: 448
Screws neededTwo per board at every joist crossing.: 728
Deck areaSquare feet of deck surface.: 192

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The gap is part of the board

Here's the second thing people miss. Every board takes up its own width plus the gap beside it, because each board carries one gap along with it.

A nominal "6 inch" board is actually 5.5 inches wide. With a quarter-inch gap, each board occupies 5.75 inches of deck.

That quarter inch sounds trivial. Across a 12-foot deck it takes you from 27 boards down to 26 — the gap is deck you don't have to fill with timber.

The arithmetic

A deck 16 feet long by 12 feet wide, 5.5 inch boards, quarter-inch gaps.

What each board occupies:

5.5 + 0.25 = 5.75 inches 5.75 ÷ 12 = 0.479 feet

How many fit across the width:

12 ÷ 0.479 = 25.04, so 26 boards to cover it

Add waste at 10%: 26 × 1.1 → 28 boards

Linear feet, which is how timber is often priced:

28 × 16 = 448 linear feet

Screws. Joists at 16 inches on centre across a 16-foot run gives 13 crossings, two screws per board at each:

28 × 13 × 2 = 728 screws

Which measurement is the length?

Worth being explicit, because the tool asks and the answer changes completely:

Length = the direction the boards run along. This sets how long each board needs to be.
Width = the direction you're laying them across. This is what the board count comes from.

If your boards run parallel to the house, the length is the dimension parallel to the house.

What gap should you leave?

An eighth to a quarter of an inch, and the right answer depends on how wet the timber is:

Pressure-treated timber straight from the yard is usually still wet. It will shrink as it dries, so it's laid tighter — sometimes almost touching — and the gap appears on its own.
Dry timber and composite don't shrink much, so lay them at the full gap you want to end up with.

The gap isn't cosmetic. It's drainage and it's room for the board to move with the weather. A deck laid tight with dry boards will buckle the first wet season.

Lay them out dry first

The single most useful ten minutes on a deck build.

Set all the boards across the joists without fixing anything, then measure what's left at the far edge. If you're going to end up with a two-inch sliver against the wall, you find out now — and you can spread or close the gaps by a fraction each to absorb it.

A hundred boards each a sixteenth wider swallows six inches. Discovering the problem at the last board means ripping a board down its length, which never looks right.

What this doesn't cover

Picture-frame borders — a border round the edge adds boards and a lot of mitred corners.
Diagonal layouts — every board meets the edge at an angle, so offcuts pile up. Budget around 15% more, not 10%.
The frame underneath — joists, beams, posts and footings are a separate calculation entirely. That one is structural rather than cosmetic, and it's governed by code: the deck joist span calculator looks up the published AWC DCA 6 table, and your building department has the final say on it. Boards are a surface; the frame is what holds people up.
Hidden fasteners — the screw figure here is face screws. Hidden systems are sold by coverage and have their own figures on the box.

Where people get it wrong

Treating it as area. Deck area ÷ board area gets you close by accident and ignores the gaps entirely.
Using the nominal width. A "6 inch" board is 5.5 inches. Over 26 boards that's a foot of deck you didn't account for.
Forgetting the gap. Small per board, whole boards across a deck.
Swapping length and width. Changes both the board count and the length you need to buy.
Buying no spares. Timber comes bowed, split and occasionally unusable. You'll reject some at the yard and more at home.
Not sealing cut ends. Every cut through treated timber exposes untreated wood. Seal them or the deck rots from the cuts outward.

The short version

Deck width ÷ (board width + gap), converted to feet, rounded up, plus about 10%. Length tells you how long the boards are, not how many. A "6 inch" board is 5.5 inches. Lay them out dry before you fix anything, and seal every cut end.