How many deck boards by deck size

Find your size in the table. A 12x16 deck takes 26 boards at 16 ft.

That is 26 rows across the 12 ft span, 416 linear feet, or 458 with the usual 10% for cuts and culls.

Every deck size

Deck	Sq ft	Rows	Boards	Length	Linear ft	+10%	Joists	Footings
8x8	64	18	18	8 ft	144	159	7	4
8x10	80	18	18	10 ft	180	199	8	6
8x12	96	18	18	12 ft	216	238	10	6
10x10	100	22	22	10 ft	220	243	8	9
10x12	120	22	22	12 ft	264	291	10	9
10x16	160	22	22	16 ft	352	388	13	9
12x12	144	26	26	12 ft	312	344	10	9
12x16	192	26	26	16 ft	416	458	13	9
12x20	240	26	26	20 ft	520	572	16	12
16x16	256	35	35	16 ft	560	616	13	9
16x20	320	35	35	20 ft	700	771	16	12
20x20	400	43	43	20 ft	860	947	16	16

Boards run the long way, so the SHORT dimension decides how many rows you need and the long one decides the board length. Turning the decking 90 degrees swaps those two numbers and usually changes the order.

How the count is worked out

A 5/4x6 or 2x6 deck board is 5.5 in wide once milled, not 6 in, and each row also carries a 0.125 in drainage gap — so a row covers 5.625 in of deck, not 6 in. That 6% difference is why decks sized off the nominal width come up short.

Short span in inches ÷ 5.625 = rows, rounded up
Rows × the long dimension = linear feet
Linear feet ÷ the stock length you can buy = boards
Add 10% for cuts and culls

Joist and footing counts assume joists at 16 in on centre and a beam every 8 ft. The four mistakes that make this arithmetic wrong before you pick up a tape.

Common questions

How many deck boards for a 12x16 deck?: 26 boards at 16 ft — 26 rows, 416 linear feet, or 458 with waste.
How many for a 10x10?: 22 boards, 220 linear feet. A 10 ft run also divides into a 20 ft board exactly twice, so long stock halves the pieces you handle.
How many for a 16x20 or 20x20?: 35 and 43 boards. Both are past 200 sq ft, which is the area the IRC uses in its no-permit exemption, so both usually need a permit whatever else is true.
Does a 2x4 deck board change the count?: Considerably — a 2x4 is 3.5 in wide, so a row covers 3.625 in and you need roughly 55% more rows. Every figure in the table assumes a 5.5 in board.
What about the gap between boards?: Included: 1/8 in, which is standard for kiln-dried lumber. Green pressure treated shrinks as it dries, so it is often laid tight and opens to about the same gap on its own.
Do I need to add for a picture frame border?: Yes, and for diagonal decking. A border eats the boards you would otherwise run to the edge, and a 45 degree layout cuts every board at both ends — allow 15% rather than 10%.

Deck calculator for a size not in the table.