How are offcuts counted?

A run that is not a whole number of boards leaves a short piece at the end of each row. If the deck is 20 ft long and the boards are 16 ft, each row is one whole board plus a 4 ft piece. With offcut reuse the calculator cuts the 4 ft pieces from shared boards: a 16 ft board makes four of them, so 26 rows need ceil(26 ÷ 4) = 7 boards for the short pieces, 33 boards in all. Without reuse each row takes a second board, 52 in all. The reuse figure assumes you plan the cuts so that the pieces are the right length and end on a joist; the percentage of waste on top covers kerf and the boards you cull. Where the short piece is over half a board, nothing is shared and the two methods give the same count.

What gap should I leave between deck boards?

Follow the decking manufacturer’s gap spec for your board. 3/16 in is a common starting point and composite is often 3/16 to 1/4 in, with a wider gap for boards that shrink or swell. Pressure-treated wood laid wet is often installed tighter because it shrinks as it dries. A wider gap means fewer rows to cover the deck and fewer boards to buy, so a gap that is off from the spec by 1/16 in can change the count by one or two rows on a wide deck.

How far apart should deck joists be?

The decking’s spec decides: 16 in on center is common for 5/4 boards laid straight across the joists, and 12 in on center is commonly required for diagonal decking and for some composite boards. The IRC lists maximum joist spacing for each decking type and thickness, and decking maker’s guides can be stricter, so check yours. A 20 ft run needs 16 joists at 16 in and 21 at 12 in.

How many screws do I need for deck boards?

Count the places each board crosses a joist and put 2 screws in each: the IRC asks for at least two fasteners per board at each joist. A 20 × 12 ft deck has 26 rows crossing 16 joists, 416 crossings, so 832 screws. This is worked from the geometry rather than a rule of thumb, but butt joints and the boards at the edges take a few more, so buy 5 to 10% extra. Hidden fastener systems usually use one clip per board per joist; check your system’s spec.

How much extra do I need for a diagonal deck?

Boards laid at 45° waste more at the edges, where each board is cut to a point. 15% extra is the usual advice, against 10% for a straight lay, and it is an estimate: a small or broken-up deck wastes more. This calculator counts a diagonal deck from the area: a 14 × 10 ft deck of 5.5 in boards with 3/16 in gaps needs 295 ft of board, 29 boards of 12 ft with 15% extra. Diagonal decking commonly needs joists 12 in on center (15 joists for a 14 ft run); check your decking’s spec.

What length of deck board should I buy?

Pick the length that leaves the least in short pieces for your run, and that ends cuts on joists. A 12 ft deck takes whole 12 ft boards with nothing left over; a 16 ft deck takes 16 ft boards. The table of boards by run and board length shows how the count changes. Longer boards are heavier to carry, and an odd length may cost more per foot than two shorter ones, so check the price by the foot.

Is a 2 × 6 the same width as 5/4 × 6 decking?

Yes, both are 5.5 in wide; the 5/4 board is about 1 in thick and the 2 × 6 about 1.5 in. A 2 × 4 is 3.5 in and a 2 × 8 is 7.25 in. Pick Custom width for any other board, such as a composite board that is not 5.5 in, and enter the actual width.

Deck Board Calculator: How Many Deck Boards Do I Need?

A 20 × 12 ft deck of 5.5 in boards (5/4 × 6) with 3/16 in gaps, boards running along the 20 ft side, needs 37 boards of 16 ft: 26 rows, each a whole 16 ft board plus a 4 ft piece, and one board makes four of the 4 ft pieces, so 33 boards plus 10% extra. Under them: 16 joists at 16 in on center (check your decking’s spec; 12 in is common for diagonal and some composite), and about 832 screws at 2 per board per joist, or 416 hidden clips. Rows = deck width in inches ÷ (board width + gap), rounded up.

Figures based on the sources listed belowUpdated 2026-10-03How we calculate

Deck length

Deck width

Extra boards

Standard: cuts, defects and bad ends (10%)Diagonal lay: more offcuts (15%)Exact (0%)Custom

Board directionA diagonal lay is counted from the deck area, so treat it as an estimate and pick 15% extra. Diagonal decking commonly needs joists 12 in on center.

Board widthActual width, not nominal. Many composite boards are also 5.5 in; check the one you are buying.

Gap between boards (in)

Follow the decking maker’s gap spec. 3/16 in (0.1875) is a common default; composite is often 3/16 to 1/4 in.

Board lengthThe length you will buy. Pick one that lands cuts on joists with little left over; the table of boards by length shows the difference.

OffcutsA run that is not a whole number of boards ends in a short piece. With reuse, one board makes as many short pieces as fit in it (a 4 ft piece: four from a 16 ft board). Straight lay only.

Joist spacingCheck your decking’s spec: 12 in is commonly required for diagonal decking and for some composites. It sets the joist and fastener counts.

Price per board (optional)

Buy 37 boards

240 sq ft + 10% waste = 264 sq ft

Rows of boards: 26 rows (each 20 ft), 1 board plus a 4 ft piece per row
Board length laid: 520 ft (buying 592 ft)
Joists (16 in on center): 16 joists (each 12 ft), check your decking’s spec
Face screws (2 per board per joist): 832 screws, buy 5 to 10% extra
Hidden clips (1 per board per joist): 416 clips, check the system’s spec

How this number is calculated

240 sq ft + 10% waste = 264 sq ft ÷ 7.14 sq ft per board = 37 boards

Shopping list

Deck screws (#10 × 2-1/2 in, exterior coated)
Hidden deck fastener clips
Deck board spacing tool
Joist tape
Chalk line
Circular saw blade for decking

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Deck boards per 100 sq ft by board width and gap

Linear ft of board per 100 sq ft = 1,200 ÷ (board width + gap), and the 8 ft and 16 ft boards that makes, worked by this calculator with the area method and no waste. Real decks take more: each row ends in a cut, so add 10%.
Board width	Gap	Linear ft per 100 sq ft	8 ft boards	16 ft boards
3.5 in (2 × 4)	1/8 in	331	41.4	20.7
3.5 in (2 × 4)	3/16 in	325	40.7	20.3
3.5 in (2 × 4)	1/4 in	320	40	20
5.5 in (5/4 × 6, 2 × 6)	1/8 in	213	26.7	13.3
5.5 in (5/4 × 6, 2 × 6)	3/16 in	211	26.4	13.2
5.5 in (5/4 × 6, 2 × 6)	1/4 in	209	26.1	13
7.25 in (2 × 8)	1/8 in	163	20.3	10.2
7.25 in (2 × 8)	3/16 in	161	20.2	10.1
7.25 in (2 × 8)	1/4 in	160	20	10

Joists by deck width

Joists = deck width in inches ÷ spacing, rounded up, plus one for the end. "Width" here is the side the joists are spaced along, which is the side the boards run along in a straight lay. Worked by this calculator.
Deck width	Joists at 16 in on center	Joists at 12 in on center
8 ft	7	9
10 ft	9	11
12 ft	10	13
14 ft	12	15
16 ft	13	17
20 ft	16	21
24 ft	19	25

Boards for a 12 ft wide deck by run and board length

5.5 in boards with 3/16 in gaps (26 rows), offcuts reused, no waste, worked by this calculator. The run is the side the boards run along; add the waste percentage on top.
Run	8 ft boards	10 ft boards	12 ft boards	16 ft boards	20 ft boards
8 ft	26	26	26	13	13
10 ft	33	26	26	26	13
12 ft	39	32	26	26	26
14 ft	52	39	31	26	26
16 ft	52	52	35	26	26
20 ft	65	52	52	33	26
24 ft	78	65	52	39	32

Methodology & assumptions

Rows = deck width (the side the boards stack across) in inches ÷ (board width + gap), rounded up. This is a little generous: the first board needs no gap beside it, so the count can be one gap more than strictly needed. Boards per row come from the run (the side the boards run along) and the board length. A run that is not a whole number of boards ends each row in a short piece: with the run of 20 ft and 16 ft boards that is one whole board and a 4 ft piece. With offcut reuse (the default) one board is cut into as many short pieces as fit in it, floor(16 ÷ 4) = 4, so the short pieces take ceil(rows ÷ 4) boards; offcuts are only reused whole, so when the short piece is over half a board nothing is shared, and an offcut is never spliced to another. Without reuse, every row starts a new board. Then the waste percentage is added and the total rounded up. Saw kerf, defects and the rule that end joints land on a joist and are staggered between rows are not modeled; the extra percentage is what pays for them, and a short piece with its end off a joist needs the board moved or a cut changed. The diagonal option is an estimate: boards at 45° have no fixed row length, so the board length laid is the deck area ÷ (board width + gap), divided by the board length, with the edge offcuts left to the 15% extra. Cutouts reduce the count in proportion to the area they take. Joists run across the boards and are spaced along the boards’ run: ceil(run in inches ÷ spacing) + 1, the extra one at the end. Fasteners are counted from the geometry: one fastening point where each board crosses each joist, rows × joists, which takes 2 face screws (the least the IRC asks for) or 1 hidden clip. A diagonal board crosses a joist every spacing ÷ cos 45° along its length. Butt joints put extra screws in the joist they land on, so buy 5 to 10% more. Many decking makers want a particular gap and a joist spacing for their board and for diagonal lay, usually 12 in on center for a diagonal lay and for some composites; follow the installation guide for the boards you buy. The metric toggle shows lengths in m, widths and gaps in cm.

International Residential Code (IRC), Section R507 Exterior decks: the decking provisions, with the table of maximum joist spacing for decking and the minimum two fasteners per board at each joist (section numbers differ between editions)
Decking manufacturers’ installation guides (for example Trex and TimberTech): board gap, joist spacing for straight and diagonal lay, and fastener spacing for the board you buy
Nominal and actual lumber sizes (American Softwood Lumber Standard, PS 20): a 5/4 × 6 and a 2 × 6 deck board are 5.5 in wide, a 2 × 4 is 3.5 in, a 2 × 8 is 7.25 in

How to calculate deck boards

Measure the deck. Measure the length and width, and decide which side the boards will run along. Subtract any cutouts, such as stairs or a hot tub.
Count the rows. Divide the side the boards stack across, in inches, by the board width plus the gap and round up.
Count the boards in each row. Divide the run by the board length. Each whole board counts as a board; the short piece left over takes a board shared with other short pieces if it fits, or a board of its own.
Add waste. Add about 10% for cuts, defects and mistakes, 15% for a diagonal lay, and round up.
Count joists and fasteners. Joists = the run in inches ÷ spacing, rounded up, plus one. Fasteners = 2 screws, or one clip, for every place a board crosses a joist.

Worked examples

Project	Result
20 × 12 ft deck, 5.5 in boards, 3/16 in gaps, 16 ft boards along the length, offcuts reused	240 sq ft + 10% → 37 boards (16 ft), 26 rows, 16 joists, 832 screws
The same deck with every row starting a new board (no offcut reuse)	240 sq ft + 10% → 58 boards (16 ft), 26 rows, 16 joists, 832 screws
16 × 12 ft deck, 5.5 in boards, 3/16 in gaps, 16 ft boards (no short pieces)	192 sq ft + 10% → 29 boards (16 ft), 26 rows, 13 joists, 676 screws
12 × 10 ft deck of 2 × 4 boards (3.5 in) with 1/4 in gaps, 12 ft boards along the length, joists at 16 in	120 sq ft + 10% → 36 boards (12 ft), 32 rows, 10 joists, 640 screws
14 × 10 ft deck laid diagonally, 5.5 in boards, 3/16 in gaps, 12 ft boards, 15% extra, joists at 12 in (estimate)	140 sq ft + 15% → 29 boards (12 ft), about 295 ft laid, 15 joists, 418 screws

Frequently asked questions

How many deck boards do I need?: Find how many rows fit across the deck: the width in inches ÷ (board width + gap), rounded up. A 12 ft (144 in) wide deck of 5.5 in boards with 3/16 in gaps is 144 ÷ 5.6875 = 25.3, so 26 rows. Multiply by the boards in each row (the run ÷ the board length), then add about 10% for cuts and bad boards. A 16 × 12 ft deck with 16 ft boards is 26 rows × 1 board = 26, or 29 with 10% extra.
How are offcuts counted?: A run that is not a whole number of boards leaves a short piece at the end of each row. If the deck is 20 ft long and the boards are 16 ft, each row is one whole board plus a 4 ft piece. With offcut reuse the calculator cuts the 4 ft pieces from shared boards: a 16 ft board makes four of them, so 26 rows need ceil(26 ÷ 4) = 7 boards for the short pieces, 33 boards in all. Without reuse each row takes a second board, 52 in all. The reuse figure assumes you plan the cuts so that the pieces are the right length and end on a joist; the percentage of waste on top covers kerf and the boards you cull. Where the short piece is over half a board, nothing is shared and the two methods give the same count.
What gap should I leave between deck boards?: Follow the decking manufacturer’s gap spec for your board. 3/16 in is a common starting point and composite is often 3/16 to 1/4 in, with a wider gap for boards that shrink or swell. Pressure-treated wood laid wet is often installed tighter because it shrinks as it dries. A wider gap means fewer rows to cover the deck and fewer boards to buy, so a gap that is off from the spec by 1/16 in can change the count by one or two rows on a wide deck.
How far apart should deck joists be?: The decking’s spec decides: 16 in on center is common for 5/4 boards laid straight across the joists, and 12 in on center is commonly required for diagonal decking and for some composite boards. The IRC lists maximum joist spacing for each decking type and thickness, and decking maker’s guides can be stricter, so check yours. A 20 ft run needs 16 joists at 16 in and 21 at 12 in.
How many screws do I need for deck boards?: Count the places each board crosses a joist and put 2 screws in each: the IRC asks for at least two fasteners per board at each joist. A 20 × 12 ft deck has 26 rows crossing 16 joists, 416 crossings, so 832 screws. This is worked from the geometry rather than a rule of thumb, but butt joints and the boards at the edges take a few more, so buy 5 to 10% extra. Hidden fastener systems usually use one clip per board per joist; check your system’s spec.
How much extra do I need for a diagonal deck?: Boards laid at 45° waste more at the edges, where each board is cut to a point. 15% extra is the usual advice, against 10% for a straight lay, and it is an estimate: a small or broken-up deck wastes more. This calculator counts a diagonal deck from the area: a 14 × 10 ft deck of 5.5 in boards with 3/16 in gaps needs 295 ft of board, 29 boards of 12 ft with 15% extra. Diagonal decking commonly needs joists 12 in on center (15 joists for a 14 ft run); check your decking’s spec.
What length of deck board should I buy?: Pick the length that leaves the least in short pieces for your run, and that ends cuts on joists. A 12 ft deck takes whole 12 ft boards with nothing left over; a 16 ft deck takes 16 ft boards. The table of boards by run and board length shows how the count changes. Longer boards are heavier to carry, and an odd length may cost more per foot than two shorter ones, so check the price by the foot.
Is a 2 × 6 the same width as 5/4 × 6 decking?: Yes, both are 5.5 in wide; the 5/4 board is about 1 in thick and the 2 × 6 about 1.5 in. A 2 × 4 is 3.5 in and a 2 × 8 is 7.25 in. Pick Custom width for any other board, such as a composite board that is not 5.5 in, and enter the actual width.

Handy tools

Board Foot Calculator: Price lumber decking by the board foot, or compare a 2 × 6 cost with a 5/4 × 6.
Stair Calculator: Work out the rise, run and stringers for the steps down from the deck.