How many deck boards do I need?

1. The formula, in plain English

A deck surface is a rectangle covered by long, narrow boards, all running the same way — down the length of the deck. You lay one row, leave a small drainage gap, lay the next, and repeat across the width. That makes the board count a grid problem with two rounding rules. The first is the number of rows: divide the deck width by the width of one board plus the gap, and round up, because the last row against the far edge takes a full or ripped-down board, never a neat gap. The second is the boards in each row: divide the deck length by the length of one board and round up, because a board that is a little short still means buying a whole next board to finish the run. Multiply rows × boards-per-row and you have the bare board count.

The gap is easy to forget but it matters. Every board after the first uses up its own width plus the gap of deck width, so on a wide deck the gaps add up to a whole board or two. The deck calculator folds the gap into the row count automatically, then adds your waste allowance on top — which a quick “deck area ÷ board area” estimate never does, which is why the rough method under-orders.

2. A worked example

Say you are building a 16 ft long by 12 ft wide deck and running the boards down the 16 ft length. Using a standard 5.5 in wide board (a nominal “6 inch” deck board) with a 1/4 in gap, each row is allotted 5.75 in of width. The rows are 12 ft ÷ 5.75 in = 144 ÷ 5.75 = 25.04, rounded up to 26 rows. If you buy 16 ft boards to match the deck length, that is one board per row, so the bare count is 26 × 1 = 26 boards. Add a 10% waste allowance for cuts and the odd bad board and you are ordering about 29 boards. That is 26 rows × 16 ft = 416 lineal feet of decking covering a 192 sq ft deck.

The metric version works the same way. A 5 m long by 4 m wide deck in 140 mm boards with a 5 mm gap allots 145 mm per row, so the rows are 4000 mm ÷ 145 mm = 27.6, rounded up to 28 rows; with 5 m boards that is 28 boards, or about 31 with 10% waste, over a 20 m² deck. Notice that if your boards are shorter than the deck — say 12 ft boards on a 16 ft deck — each row needs two boards butt-joined over a joist, which doubles the board count and adds a line of end-joints, so it is usually worth buying boards that match the longer deck dimension.

3. Deck-board chart for common sizes

Here are the bare board counts (before waste) for common deck sizes, using a 5.5 in board with a 1/4 in gap (5.75 in per row) and full-length boards run down the length — so each count is just the number of rows across the width. For 140 mm metric boards with a 5 mm gap (145 mm per row) the same idea applies to the deck measured in metres.

The board width changes everything: a narrower 3.5 in board needs far more rows than a 5.5 in board for the same deck, while a wide composite board needs fewer. Always run the numbers for the board you will actually buy, add your waste allowance, and remember these counts assume boards as long as the deck — shorter boards mean butt-joints and more pieces.

4. Screws, clips and fasteners

Boards are only part of the order — you also need the fasteners. The standard for face-screwed decking is two screws per board at every joist the board crosses. The joists run across the deck, perpendicular to the boards, spaced along the length, so a board crosses (deck length ÷ joist spacing) + 1 joist lines. Multiply by two screws and by the number of rows and you have the total: screws = 2 × rows × joist lines. For the 16 × 12 ft deck above, framed at 16 in on-centre, that is about 13 joist lines × 26 rows × 2 ≈ 676 screws.

Hidden-fastener systems — the clips that sit in a groove on the board edge so no screw shows on top — use roughly one clip per board per joist instead, plus a screw each, so you order about half the count but a clip for each intersection. Buy stainless or coated screws rated for exterior use and, for composite or hardwood boards, the fastener the board maker specifies. The deck calculator estimates the face-screw count for you once you set the joist spacing.

5. Board direction, layout and waste

Which way the boards run is both a look and a cost decision. Running boards the long way gives the fewest joints and the least waste, which is what the chart above assumes. A diagonal layout looks good and stiffens the deck, but every board meets the frame at 45°, so both ends are angle-cuts and waste jumps — add 15% rather than 10%. A picture-frame border (a band of boards around the perimeter) and herringbone or parquet patterns add still more cuts and usually a doubled-up joist or blocking under every joint, so budget the higher waste figure and a few extra boards.

Whatever the layout, order the whole job — boards plus the waste margin — in one delivery so the colour and batch match, and keep a few spare boards stored flat and dry for future repairs. Timber decking can bow or twist, so over-ordering slightly beats running one board short at the end of the job when the new batch no longer matches.

6. What this does not cover: the frame

This guide and the calculator size the decking — the boards you walk on, the lineal length and the fasteners. They do not size the frame underneath. How big the joists and beams must be, how far each can span, how many footings you need and how deep they go are structural questions answered by span tables and set by the load, the span, and the timber species and grade — not by a covering calculation. Getting the frame right is what keeps the deck safe, so it is worth doing properly.

Design the frame from the span tables in your local building code, and confirm the joist and beam sizes, footing depth and any permit with your local building authority before you build. For the rough framing member count you can start with the framing calculator, and for concrete footings the concrete calculator works out the volume of each pier. More estimating tools live on the carpentry hub.

Common questions

How do you calculate how many deck boards you need?

Find the number of rows by dividing the deck width by (board width + gap) and rounding up; find the boards per row by dividing the deck length by the board length and rounding up; then multiply the two and add about 10% for waste. A 16 × 12 ft deck in 6 in boards with a 1/4 in gap is 26 rows of full-length 16 ft boards = 26, or about 29 with a 10% allowance.

How many deck boards do I need for a 12x12 deck?

For a 12 × 12 ft deck using a 5.5 in board with a 1/4 in gap, each row is allotted 5.75 in of width, so 144 in ÷ 5.75 in = 26 rows. If you run 12 ft boards the length of the deck that is one board per row = 26 boards, or about 29 with a 10% waste allowance. A wider gap or a narrower board changes the row count.

How many screws per deck board?

Two face-screws per board at every joist it crosses is the standard. So total screws = 2 × rows × joist lines, and joist lines = deck length ÷ joist spacing + 1. A 16 × 12 ft deck framed at 16 in on-centre is about 26 rows × 13 joist lines × 2 ≈ 676 screws. Hidden-clip fasteners use roughly one clip per board per joist instead.

How much extra decking should I buy for waste?

About 10% for a standard straight layout, and 15% for a diagonal, herringbone or picture-frame layout with more cuts. The allowance covers cutting boards to length, off-cuts at the ends, and the occasional bowed or split board. Buy the whole job in one delivery so the batch and colour match.