When building a wood deck, knowing exactly how many boards, screws and joists to buy is essential. Buy too few and you’ll make extra trips to the store. Buy too many and you waste money. With MyPlanDIY’s free deck calculator you simply enter your deck dimensions and the calculator works out everything for you – including 10 % waste allowance for cuts.
The calculator covers the key materials: deck boards (typically pressure-treated pine or hardwood), stainless-steel screws and supporting joists. You can use the result as a shopping list directly, or create a project in MyPlanDIY and get a complete material list with prices and a budget.
Enter dimensions below – the result updates instantly
Measure the side the boards run along
Measure the side the boards run across
Typically 95, 120 or 145 mm
Recommended 5–8 mm for drainage
5 m × 4 m deck with 145 mm boards
Area
20.0
m²
Boards
30
pcs
Screws
486
pcs
Joists
9
pcs
Materials
Deck boards
135 running metres (incl. 10% waste)
30 pcs
Stainless steel screws (A2)
2 per board per joist
486 pcs
Joists / substructure
36 running metres (60 cm spacing)
9 pcs
Calculation breakdown
Creates a project with a material list based on your calculation
Say you want to build a deck that is 6 metres long and 4 metres wide – a popular size for a family deck. With standard 145 mm boards and a 5 mm gap, the calculation looks like this:
Calculation example
With these numbers you have a solid starting point for getting quotes from your local hardware store. Tip: Always order stainless-steel screws (A2 or A4) for outdoor use – they cost a little more but prevent rust stains on the wood. Read more in our complete guide to deck projects.
Calculating materials for a deck comes down to three things: the deck dimensions, the board sizes and the substructure underneath. Here are the principles behind the calculation:
Deck boards typically come in widths of 95, 120 and 145 mm. The most popular is 145 mm, which gives a classic look and requires fewer boards. Between each board you need a gap of 5–8 mm to allow rainwater to drain and the wood to expand. The “effective board width” is therefore the board width plus the gap – e.g. 145 + 5 = 150 mm.
To find the number of board rows, divide the deck width by the effective board width. A 4-metre deck with 150 mm boards needs 4000 ÷ 150 = about 27 rows. Each board is as long as the deck length (or pieced together from shorter lengths if the deck is longer than standard board lengths).
Joists are the load-bearing beams that the deck boards are screwed onto. They are placed perpendicular to the board direction, typically at 60 cm spacing (centre to centre). For thinner boards (19 mm) a 40 cm spacing is recommended. The number of joists is calculated as: deck length ÷ joist spacing + 1.
In practice, boards need to be cut at corners, posts and edges. There is also a risk that some boards are warped or have defects. That is why ordering at least 10 % extra is recommended. The calculator adds this automatically so you don’t run short a few boards in the middle of the project.
Every point where a board crosses a joist should be fastened with 2 screws. This gives a stable result and prevents boards from rocking. The count is calculated as: number of board rows × number of joists × 2. Always use stainless-steel screws (A2) outdoors – plain steel rusts and leaves ugly streaks on the wood.
Even though a deck seems like a straightforward project, there are several pitfalls that can cost time and money:
Not enough boards
Many people forget about cuts and waste. Boards get trimmed at ends and corners, and 10–15 % extra is normally needed. The calculator automatically adds 10 %.
Too much space between joists
Joists should typically be spaced at most 60 cm apart (centre to centre). Greater spacing results in bouncy boards and a risk of breakage. Use 40 cm for thinner boards.
Wrong screw type
Regular steel screws rust and leave ugly streaks. Use stainless A2 screws (A4 near saltwater). Hidden-head deck screws give the neatest finish.
No drainage
A 5–8 mm gap is necessary for water drainage. Without gaps, water pools on the surface and the wood rots faster.
Unstable foundation
Joists should rest on concrete pads, adjustable deck feet or compacted gravel. Placing them directly on soil leads to settling and uneven boards.
Proper planning helps you avoid most of these mistakes. See our guide to material lists to make sure you don’t miss anything.
The deck calculator gives you the material numbers. But a successful deck project requires more than a calculation. With MyPlanDIY you keep the whole project in one place:
Free account – no credit card required. Demo project included.
Related pages