Deck Calculator
Find how many deck boards to buy for a wood or composite deck, and the total length of decking. Enter each rectangle of the deck and the width of your boards, then add the board length to count the boards, and the joist spacing to count the joists and screws.
Add another rectangle for an L-shaped deck or another level.
- Choose US or Metric. You can still change the unit of any single field.
- Enter the deck as rectangles. Length (along the boards) is the direction the boards run, and Width (across the boards) is the direction you lay one row after another. Use Add section for each part of an L-shaped deck or each level.
- For a deck with angled corners or a curved edge, enter the rectangle around it, so the estimate comes out a little high. Don't take out an opening for a tree or a hot tub: the boards are cut at its edge, and the joists around it need extra framing.
- Enter the actual Board width and the Gap between boards from your decking's installation instructions.
- Enter a Board length to see how many boards to buy. Leave it blank to get only the total board length, for decking sold by the foot or by the meter.
- Enter the Joist spacing to count the joists and screws, and Screws per box to count the boxes.
- Optionally, enter a price for the boards, the joists, and the boxes of screws to estimate the cost.
Length fields accept whole numbers, decimals, fractions, and mixed
numbers, such as 16, 10.5, 1/8, or
5 1/2. Choose ft + in as the unit to enter a
length such as 12 ft 6 in in two fields.
How to calculate deck boards
Boards run along the length of each section, side by side, with a gap between them and none at the edges:
Rows = (width across the boards + gap) ÷ (board width + gap), rounded up
Total board length = rows × length along the boards × (1 + waste % ÷ 100)
Boards to buy = total board length ÷ board length, rounded up
A last row that you rip to fit counts as a full row. Trex's guide How Many Deck Boards Do I Need? divides the deck's depth by the board width and rounds up in the same way. The calculator also adds the gap to the width each row covers.
The board count assumes you use the cut-off ends in other rows. Each piece of decking must rest on at least three joists, according to AWC's DCA 6 deck guide and Trex's installation guide, so ends shorter than that are left over. The waste allowance covers them. If you want each row in one piece, buy one board per row, at least as long as the section: the breakdown shows the rows. TimberTech's guide How to Calculate How Much Decking You Need notes that two 12 ft boards fit a 24 ft deck better than 20 ft boards.
Board width and length
Enter the actual width, not the nominal size. Trex's guide gives 5.5 in for most 1x6 and 2x6 boards. In the UK, Travis Perkins lists a 35mm x 148mm board with a finished size of 32mm x 144mm, and Meyer Timber in Australia gives common widths of 70 to 140 mm. A nominal width gives too few rows.
Trex says boards usually come in lengths from 8 to 20 ft, and both Trex and TimberTech sell 12, 16, and 20 ft boards. Travis Perkins lists timber decking from 1.8 to 4.8 m, and Millboard's boards are 3,600 mm. These are examples only, so check the product you buy.
Gap between boards
The 1/8 in default follows AWC's DCA 6 and TimberTech's minimum gap. The 3 mm Metric default is the low end of Meyer Timber's guide, which gives 3 to 4 mm for 90 mm boards and 5 to 6 mm for 140 mm boards. A narrow gap gives more rows, which is why TimberTech suggests estimating with the smallest gap you might use. Many products need more: Trex's installation guide and TimberTech's composite boards call for 3/16 in (5 mm), Millboard's guide 4 mm, and the UK Timber Decking & Cladding Association 5 to 8 mm. Enter the gap in your decking's instructions.
Waste allowance
The 10% default is the low end of TimberTech's advice to add 10 to 15%; Trex's guide suggests 5%. Add more for a deck with many sections, or where the section lengths are far from a multiple of the board length, because more of the cut-off ends are too short to use. The waste allowance applies to the boards only.
Joists and screws
The calculator places joists across the length of each section, one at each end and the rest no farther apart than the joist spacing:
Joists = length along the boards ÷ joist spacing, rounded up, + 1
Screws = rows × joists × screws per board at each joist
Boxes of screws = screws ÷ screws per box, rounded up
The joist count includes the joists at both ends of each section. It doesn't include rim joists, the ledger, blocking, or extra joists under board joints. Each section is counted on its own, so where two sections share a joist, it is counted twice.
The calculator uses the spacing you enter and doesn't check it. The largest spacing depends on the type of decking, its thickness, and the direction you lay it, so check your decking's installation instructions and the code where you build. For reference, the US IRC 2024 (Table R507.7) allows wood decking laid square to the joists on joists up to 16 in apart for 1 1/4 in boards that span three or more joists (12 in when a board rests on only two), and 24 in for 2 in boards, and leaves composite decking to the maker's instructions. Trex recommends 16 in for a stiffer deck, Millboard recommends 400 mm for homes, and Meyer Timber says timber decking is normally laid on joists 450 mm apart.
The default of two screws per board at each joist follows the IRC (R507.7), AWC's DCA 6, Trex, Millboard, and Meyer Timber. At a joint, each board end is screwed on its own, and Millboard uses three screws at the end of a board, so buy some extra. For hidden fasteners, follow the maker's count instead. Simpson Strong-Tie's 350-count pack covers about 100 ft² of 6 in nominal boards on joists 16 in apart. A 20 × 20 ft deck in this calculator needs 1,376 screws, about 344 per 100 ft². Smaller decks need more per 100 ft² because of the joists at the ends.
For screws sold by weight, such as a 5 lb box, enter the count printed on the box, or the count per lb times the weight.
Cost
The estimated cost covers the boards, joists, and boxes of screws you enter a price for. It doesn't include beams, posts, footings, railings, or labor. Price the boards per foot, per meter, or per board. If joists are priced by length, enter the price for one joist: the price per foot times the joist's length.
Examples
A 16 × 12 ft deck
With 5 1/2 in boards and 1/8 in gaps, the 144 in width takes (144 + 1/8) ÷ 5 5/8 = 25.62, rounded up to 26 rows. That is 26 × 16 = 416 ft of boards, or 457.6 ft with 10% added, so 29 boards of 16 ft. With joists 16 in apart, 192 ÷ 16 = 12 spaces need 13 joists, and two screws where each row crosses a joist make 26 × 13 × 2 = 676 screws, or 2 boxes of 350.
An L-shaped deck
Adding an 8 × 6 ft section to the deck above adds 13 rows of 8 ft, or 104 ft, and 7 joists. The deck then needs 572 ft of boards with 10% added, which is 36 boards of 16 ft, plus 20 joists and 858 screws.
A Metric deck
A 4.8 × 3.6 m deck of 140 mm boards with 3 mm gaps takes 3,603 ÷ 143 = 25.2, rounded up to 26 rows, or 124.8 m of boards. With 10% added, that is 137.28 m, so 29 boards of 4.8 m. With joists 450 mm apart, 4,800 ÷ 450 = 10.67 rounds up to 11 spaces, so 12 joists and 624 screws.
FAQ
About 29 boards of 16 ft for a 16 × 12 ft deck of 5 1/2 in boards with 1/8 in gaps. Divide the width across the boards by the board width plus the gap and round up, multiply by the length along the boards, add 10%, and divide by the board length.
Often 16 in apart, center to center, or 400 to 450 mm. The largest spacing depends on your decking and how you lay it, so check its installation instructions and your local code.
About 350 per 100 ft² for 5 1/2 in boards on joists 16 in apart, with two screws per board at each joist. A 16 × 12 ft deck needs about 676, plus a few extra for the board ends at joints.
About 1/8 in (3 mm) for many boards, but many composite boards need 3/16 in (5 mm), and UK guidance for timber decking gives 5 to 8 mm. Use the gap in your decking's installation instructions.
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