How Many Deck Boards Do I Need?
Board count depends on your deck's actual footprint and which way the boards run — the table below covers common deck sizes for standard 5/4×6 decking. For your own deck's dimensions, board length, and layout, the deck board calculator computes the precise count and the stock lengths to buy.
Deck board sizes: nominal vs. actual width
A "6-inch" deck board is sold under a nominal size, not its real dimension — the actual face width of a 5/4×6 or 2×6 board is 5.5 in, the same rounding convention used across North American dimensional lumber. Home Depot, 'How to Build a Deck: Decking Guide': a 5/4x6 or 2x6 deck board measures 5-1/2 in actual face width Add the standard 0.25 in drainage gap between boards, and each board effectively covers 5.75 in of deck width once installed — that combined figure is what the board-count table below is built on.
Boards needed by deck size
Figures assume standard 5/4×6 decking at the 0.25 in gap above, boards running the shorter dimension listed, and the standard 10% waste allowance for a straight layout — the same assumptions the calculator uses by default. A diagonal layout needs a higher waste allowance; use the calculator directly for that case.
| Deck size | Boards needed | Total linear feet (before waste) |
|---|---|---|
| 8 × 10 ft (boards run the 8-ft way) | 21 | 168 ft |
| 10 × 12 ft (boards run the 10-ft way) | 26 | 260 ft |
| 12 × 16 ft (boards run the 12-ft way) | 34 | 408 ft |
| 12 × 20 ft (boards run the 12-ft way) | 42 | 504 ft |
| 16 × 24 ft (boards run the 16-ft way) | 51 | 816 ft |
Board count is a function of the deck's width (perpendicular to the boards) divided by the 5.75in each board covers, rounded up — it does not depend on the deck's run length at all, since every row needs one board (or several, spliced) rather than more boards for a longer run. That's why a 12 × 16 ft deck and a 12 × 20 ft deck in the table above need the same number of rows only when their width matches; the 12 × 20 row has more boards because its width (20 ft) is greater, not because its run is longer.
Worked example: 12 × 16 ft deck
A 12 × 16 ft deck with boards running the 12-ft way needs to cover 16 ft of width. Convert to inches: 16 × 12 = 192 in, divided by the 5.75 in each board covers = 33.4, rounded up to 34 boards — matching the table row above. Each of those 34 boards needs to span the 12-ft run, so a 12-ft stock length covers each board with no splicing; buying 34 boards at 12 ft gives 408 total linear feet, and the standard 10% waste allowance on top of that (for end-cuts and culled boards) is what the deck board calculator adds before rounding to a final board count to purchase.
Runs longer than one board
When a deck's board run is longer than the longest stock board, each row is made from two boards butted together over a joist. Standard lengths are 8, 12, 16 ft, and choosing the right pair decides how much ends up as offcut. The table shows the pairing with the least leftover for common runs:
| Board run | Boards per row | Offcut per row |
|---|---|---|
| 10 ft | 12 ft | 2 ft |
| 14 ft | 16 ft | 2 ft |
| 18 ft | 12 ft + 8 ft | 2 ft |
| 20 ft | 12 ft + 8 ft | 0 ft |
| 24 ft | 12 ft + 12 ft | 0 ft |
| 28 ft | 16 ft + 12 ft | 0 ft |
On a 20 ft run, 12 + 8 ft boards leave no offcut, where two 12s would waste 4 ft in every row. Using the same pairing in every row lines all the joints up on one joist, which looks like a seam and concentrates the load. Alternating the order (12 + 8, then 8 + 12) staggers the joints across two joists without changing the board count. Offcuts are measured before trimming the ends square, so a run that uses boards exactly, like 20 ft from 12 + 8, has no spare length to trim. Where the ends overhang the rim, or will be cut after fastening, add an inch or two to the run first.
Picture-frame borders
A picture-frame border runs one or two boards around the deck's edge, at right angles to the field boards at the ends, and hides the cut ends of the field. It changes the order in two ways. The border itself is bought by linear feet of perimeter: a single border on a 12 × 16 ft deck is 2 × (12 + 16) = 56 ft, which is two 16 ft boards for the long sides and two 12 ft boards for the short ones, before miter waste. The field inside the border then shrinks by one board width on every side, so it needs slightly fewer rows and shorter boards than the table above. The border also adds miter cuts at each corner and, usually, extra blocking under the border, so plan both before ordering.
Stair treads
Deck stairs are usually built from the same decking as the surface. Two 5/4×6 boards with a 0.25 in gap make a tread about 11.25 in deep, which is why two boards per step is the common pattern. A 4 ft wide stair with five steps needs 10 boards, each cut to 4 ft, so three 16 ft boards, cut into four pieces each, cover the treads with two pieces to spare. Add the riser boards if the stairs will be closed, and keep the stair boards on their own line of the order, since the table above covers only the flat deck surface.