Why use this calculator
Decking is sold by the board or the linear metre, but people plan decks in square metres, and the conversion is where estimates slip. The gap between boards changes how many rows fit, the stock length decides how many joins you get and how much ends up as offcuts, and the joist spacing drives both the frame and the screw count.
Rough rules such as boards per square metre or screws per hundred square feet can be out by 20 per cent either way. This calculator works from the actual geometry: rows of board plus gap across the width, linear length along the deck, stock boards with your wastage, joists at your spacing including both ends, and two screws per board at every joist crossing.
That gives an order you can take to the timber yard. The frame itself, bearers, footings and joist sizes, must come from span tables such as AS 1684 in Australia or the IRC deck provisions in the US.
How to use it
- Choose metric or imperial units.
- Enter the deck length in the direction the boards run, which is usually along the house.
- Enter the deck width in the direction the joists run, out from the house.
- Enter the actual board width and the gap you will leave between boards. Enter the stock board length you will buy.
- Enter the joist spacing, 450 mm for 19 mm hardwood or 16 inches in the US.
- Set the wastage allowance, then the number of screws per board at each joist, and finally the number of screws in the box you plan to buy.
What each result means
- Decking boards to order
- Stock-length boards to buy including your wastage allowance for offcuts, split ends and boards you reject.
- Deck area
- Length times width of the deck surface, the figure most quotes and composite price lists use.
- Rows of boards
- How many board widths plus gaps fit across the deck, rounded up so the last row is covered.
- Linear length of decking
- The total length of board laid before wastage, which is how many timber yards price decking.
- Joists
- Joists at your spacing along the deck, including both end joists. Their size comes from span tables.
- Joist timber
- Total length of joist timber at one deck width per joist. Bearers, rim boards and blocking are extra.
- Deck screws
- Fasteners at the number you chose per board at every joist crossing, the real count rather than a per area rule.
- Boxes of screws
- Screws divided by the box size, rounded up. Buy one extra box because screws get dropped and heads strip.
The formula
Rows = round up (deck width / (board width + gap))
Linear decking = rows x deck length
Boards = round up (linear decking x (1 + wastage % / 100) / stock board length), and never fewer than the number of rows
Joists = round up (deck length / joist spacing) + 1, counting both end joists
Joist timber = joists x deck width
Screws = rows x joists x screws per crossing
Boxes = round up (screws / screws per box)
Board width, gap and spacing are converted from mm or inches to metres or feet first.
How this number is used in practice
Joist spacing is set by the decking, not by preference. In Australia, 19 mm thick hardwood decking is normally fixed at 450 mm centres, while thinner boards and many composites need 400 mm or less, and diagonal laying needs closer spacing again because each board spans further between supports.
In the US, 16 inches on centre is standard for 5/4 wood decking and many composites require 12 inches, particularly on stairs and diagonals. Always use the composite manufacturer's own spacing, since the warranty depends on it.
Board gaps let water drain and allow timber to move. Hardwood swells in the wet and shrinks in the sun, so Australian installers typically leave 4 to 6 mm at installation depending on the moisture content, while kiln-dried boards may need more. Composite boards expand lengthwise with heat, so their instructions also set end gaps. Joins between boards must land on a joist, which is why longer stock lengths reduce waste and look better.
The frame under the deck is structural and is usually the part that needs approval. Bearer and joist sizes come from AS 1684 span tables in Australia and IRC section R507 or the American Wood Council's DCA 6 guide in the US. Decks more than about 1 m or 30 inches above the ground typically need a balustrade and often a building permit, so check with the local council or building department before ordering.
Worked examples
A 6 x 4 m hardwood deck, 90 mm boards, 5 mm gaps, 5.4 m lengths, 450 mm joists, 10% wastage
53 boards, 43 rows, 15 joists (60 m of joist timber) and 1,290 screws, 3 boxes of 500
A US 20 x 12 ft deck, 5.5 inch boards, 3/16 inch gaps, 16 ft lengths, joists 16 inches on centre
36 boards, 26 rows, 16 joists and 832 screws
A 4.8 x 3 m composite deck, 137 mm boards, 6 mm gaps, 5.4 m lengths, 400 mm joists, hidden clips
21 boards, 21 rows, 13 joists and 273 clips
When people use it
- Ordering decking and screws for a DIY deck in one delivery
- Comparing 90 mm and 140 mm boards on board count and screw count
- Checking whether composite decking at 400 mm joists changes the frame cost
- Pricing a deck replacement where the frame is kept but the boards are renewed
- Checking a builder's quoted material quantities
Tips
- Run boards along the house so rainwater drains away and joins are less obvious.
- Buy the longest boards you can handle. Fewer joins look better and waste less.
- Stagger joins across different joists and never let two adjacent rows join on the same joist.
- Pre-drill hardwood near board ends to stop splitting, and use stainless screws near the coast.
- Seal the end grain of cut hardwood boards to reduce end checking.
- Check local rules for balustrades and permits before building a raised deck.
What it assumes, and where it stops being accurate
- The deck is a single rectangle with boards running along its length and joists across it.
- Wastage is a single percentage. Picture framing, diagonal laying and breaker boards need more.
- Offcuts are reused in later rows where long enough, which is why the board count works from linear length.
- Joist timber counts joists only. Bearers, posts, footings, ledgers, rim joists and blocking are not included.
- Screws assume the chosen number per board at every joist crossing. Stairs, fascia and blocking need extra.
- No structural checking is done. Member sizes and spans must come from AS 1684, the IRC or an engineer.
Terms used on this page
- Joist
- The framing member the deck boards are fixed to, running at right angles to the boards.
- Bearer
- The larger beam supporting the joists, carried on posts or stumps. Called a beam in the US.
- On centre
- Spacing measured from the centre of one member to the centre of the next.
- Linear metre
- A length of board regardless of width, the unit many timber yards price decking in.
- Picture framing
- A border of boards laid around the edge of a deck, hiding the cut ends of the main boards.
Questions people ask
- How many deck boards do I need?
- Work out rows first: the deck width divided by board width plus gap, rounded up. Multiply rows by the deck length for the linear length, add wastage, then divide by the stock board length. A 6 by 4 m deck in 90 mm boards at 5 mm gaps needs 43 rows and about 53 boards of 5.4 m.
- What joist spacing should I use?
- 450 mm for standard 19 mm hardwood decking in Australia and 16 inches on centre for most US wood decking. Composite and thinner boards often need 400 mm or 12 inches, and diagonal decking needs closer spacing. Follow the decking manufacturer's instructions.
- How many screws per deck board?
- Two face screws per board at every joist is the standard fixing, so a board crossing ten joists takes twenty screws. Hidden clip systems usually use one clip per board per joist. Count the real crossings rather than using a per area rule.
- How big should the gap between deck boards be?
- Usually 4 to 6 mm for hardwood in Australia and about 3/16 inch in the US, enough for water to drain and for boards to swell in wet weather. Very dry kiln-dried boards may need more. Composite manufacturers state their own side and end gaps.
- Do I need a permit for a deck?
- Often, if the deck is attached to the house or raised above a set height, commonly about 1 m in Australia or 30 inches in the US. Rules vary by council and city, so check with the local building authority before you order materials.
Use this with
Deck Materials Calculator answers one part of the question. These pick up where it stops: Fence Post and Rail Calculator, Stair Rise and Run Calculator, Concrete Volume Calculator and Paint Calculator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- American Wood Council. DCA 6 prescriptive residential wood deck construction guide
- International Code Council. International Residential Code section R507 decks
- Standards Australia. AS 1684 residential timber-framed construction span tables
Deck Materials Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.