How many deck boards do I need? A simple calculator
To work out how many deck boards you need, divide the width the boards cover by one board plus one gap to get the number of rows. Then work out which stock length each row is cut from, and add a couple of spare boards. As a rough guide, 140 mm boards with 5 mm gaps take about 6.9 lineal metres per square metre, and 5½ in. boards with ⅛ in. gaps take about 2.13 lineal feet per square foot, before offcuts.
Square metres alone will mislead you, though. The length of boards you can buy, compared with the length of your deck, decides how much ends up as offcut.
How do I work out lineal metres of decking?
- Measure the deck across the boards, the direction the rows stack up.
- Divide by the board width plus one gap. That gives the number of rows; a part row is a board ripped down its length.
- Multiply the rows by the run length (the deck measured along the boards). That is your net lineal metres or feet.
- Work out which stock length covers each row with the least offcut, and count boards.
- Add a small spare allowance.
For a quick check, here is the net coverage of some common boards with their usual gaps:
| Board and gap | Net decking |
|---|---|
| 86 mm hardwood, 4 mm gap | 11.1 lineal m per m² |
| 90 × 22 treated pine, 4 mm gap | 10.6 lineal m per m² |
| 137 mm composite, 6 mm clip gap | 7.0 lineal m per m² |
| 140 mm hardwood, 5 mm gap | 6.9 lineal m per m² |
| 5½ in. wood, ⅛ in. gap | 2.13 lineal ft per sq ft |
| 5½ in. composite, 3/16 in. gap | 2.11 lineal ft per sq ft |
These are straight arithmetic from the board widths and gaps in Deckulator's catalogue. Your gap may differ: timber gaps depend on width and moisture, and composite gaps are set by the maker's clip.
Why does board length matter so much?
Every row is cut from whole boards, so the offcut depends on how your run length fits the lengths on sale. Typical stock lengths in Deckulator's catalogue are:
- Australian hardwood: mostly sold in random lengths, with packs made up of roughly 1.8 to 5.4 m boards.
- Australian treated pine decking: 3.6, 4.2, 4.8 and 5.4 m.
- Australian composites: usually a single length of about 5.4 to 5.5 m.
- US wood: 8 to 20 ft, depending on species. US composites: 12, 16 and 20 ft.
A board doesn't quite cover its own length. Deckulator holds back 10 mm of every board to square the factory ends, plus 4 mm per saw cut, so a 4.8 m board covers a run of up to about 4.79 m. A deck that is exactly 4.8 m long needs 5.4 m boards or butt joints.
Worked example: 4.5 m × 3.6 m deck
Merbau 140 × 19 boards with 5 mm gaps, running parallel to the house along the 4.5 m side:
- Rows: 3.6 m across gives 25 rows, 23 full boards plus an edge row ripped to about 125 mm on each side.
- Net decking: 25 rows × 4.5 m = 112.5 lineal metres.
- Boards: each row comes from one 4.8 m board, so 25 boards or 120 lineal metres, with 6.25% left as offcut.
- Spare: 5% of 25 boards, rounded up, is 2 more. Order 27 boards of 4.8 m, 129.6 lineal metres.
Make the same deck 4.8 m long and every row needs a 5.4 m board instead, which pushes the offcut to about 11%. A few centimetres of design can save a noticeable amount of timber.
The US version. A deck 15 ft 9 in. long and 12 ft deep in 5/4×6 treated pine with ⅛ in. gaps has 26 rows, each cut from one 16 ft board: 26 boards with under 2% offcut, plus 2 spare. Stretch it to exactly 16 ft and the rows no longer fit a 16 ft board. Deckulator then laid 37 boards in 10, 12 and 16 ft lengths, with butt joints on three joist lines and nearly 10% offcut.
How much waste allowance should I add for decking?
Many calculators add a flat percentage to the area and stop there. That hides the two different reasons you buy more decking than you lay:
- Offcut is set by your layout and the stock lengths. It can be near zero or well over 10%, as the examples show. Deckulator works it out from an actual cut list rather than guessing.
- Spare covers a split end, a board you don't like the look of, or a cut in the wrong place. Deckulator adds 5% by default as whole boards, and you can set it anywhere from 0 to 15%.
Picture frames, breaker boards, angled edges and diagonal boards all add cutting, so they change the offcut figure. It is worth comparing layouts before settling on one: see breaker boards and butt joints.
How many screws or clips do I need?
Face-fixed boards take two fixings at every joist. That is the standard in WoodSolutions TDG 21 in Australia, and the IRC (R507.7) calls for two nails or two No. 8 screws at each support for wood decking. Deckulator counts two screws per board per support, adds 5% and rounds up to the next 50. For hidden fixings it counts one clip per board at every joist, adds 5% and rounds up to the next 10. A 4.5 m row over joists at 450 mm crosses 11 joists, so it takes 22 screws.
Common questions
How many lineal metres of decking do I need per square metre?
About 6.9 lineal metres per square metre for 140 mm boards with 5 mm gaps, and about 11.1 for 86 mm boards with 4 mm gaps, before offcuts and spares.
How much extra decking should I buy for waste?
It depends on how your deck length fits the stock lengths, which can mean almost no offcut or more than 10%. Work out the offcut from a cut list, then add a small spare; Deckulator adds 5% as whole boards by default.
What length deck boards should I buy?
The length that covers each row with the least left over. A board covers slightly less than its nominal length once the ends are squared, so a deck exactly 4.8 m long will not come from 4.8 m boards without butt joints.
How many screws do I need per deck board?
Two at every joist the board crosses. A 4.5 m board over joists at 450 mm centres crosses 11 joists, so it takes 22 screws.
This guide explains common rules in plain words. It isn’t engineering advice: span tables, footings and barriers depend on your timber grade, soil, wind and local rules. Have a builder, certifier or building department check your design before you build.