Decking calculator: how many boards, joists and screws do I need?
Board rows across the deck, the joists under them and the fasteners at every crossing, plus waste. All sizes are typical values you can change.
Decking calculator
Enter the deck and your boards. Board size, gap, joist spacing and waste are typical values: enter the ones from your product and its installation instructions.
Typical size: check your product
Typically 400 mm centre to centre for softwood boards 28 to 32 mm thick, 300 mm for diagonal, thinner or composite boards; the board maker’s figure applies
You need
Enter the length and width of the deck.
Decking boards by deck size
Boards of 3.6 m × 14.4 cm along the length, 5 mm gap, joists 40 cm apart, centre to centre:
| Deck | Boards (length bought) | Joists | Screws or clips |
|---|---|---|---|
| 3 × 3 m (9 m²) | 21 (75.6 m) | 29.7 m | 416 / 208 |
| 4 × 3 m (12 m²) | 26 (93.6 m) | 36.3 m | 509 / 255 |
| 4 × 4 m (16 m²) | 33 (118.8 m) | 48.4 m | 654 / 327 |
| 5 × 4 m (20 m²) | 42 (151.2 m) | 61.6 m | 832 / 416 |
| 6 × 4 m (24 m²) | 54 (194.4 m) | 70.4 m | 951 / 476 |
| 6 × 5 m (30 m²) | 68 (244.8 m) | 88 m | 1197 / 599 |
| 8 × 4 m (32 m²) | 66 (237.6 m) | 92.4 m | 1248 / 624 |
Includes 10 % waste on boards and joists and 10 % spare fasteners. Boards: whole boards, in brackets the length bought.
For 3 × 3 m the rows decide: each 3 m row takes a board of its own, because the 0.6 m offcut cannot fill another row.
Worked example: a 5 × 4 m deck
- Area: 5 × 4 m = 20 m², boards 3.6 m × 14.4 cm, 5 mm gap, joists 40 cm apart, boards along the length.
- Rows across the 4 m width: (4 m + 5 mm) ÷ (14.4 cm + 5 mm) = 26.88, so 27 rows of 5 m, 135 m of boards.
- Plus 10 % waste: 148.5 m ÷ 3.6 m per board = 41.25, you order 42 boards; 16.2 m is left over.
- Check row by row: whole boards per row, offcuts used only where they are long enough for the whole piece, gives 41 boards. You order the larger figure, here 42.
- Joists across the boards: 5 m ÷ 40 cm + 1 = 14 joists of 4 m, 56 m, with waste 61.6 m.
- 27 rows × 14 joists = 378 crossings: with 10 % spare 832 screws or 416 hidden clips.
Slabs instead of boards? The patio slab and pedestal calculator gives the slabs and pedestals.
What to measure before you calculate
Measure the deck where the boards will go, not the whole garden. The length along the house and the depth into the garden make the rectangle for the calculator. If one side is angled or the deck wraps around a corner, split it into rectangles and calculate each one.
- Measure length and depth in at least two places: if the figures differ, the area is not square and the longer side counts.
- Check the height: board, joist and support together have to stay below the door threshold, with some room so that water does not run toward the door.
- Find out what is underneath: concrete, pavers, gravel or soil decides what the joists rest on.
- Measure the existing slope with a straightedge and a level across the whole depth.
- Note the cut-outs: steps, posts, planting beds and drains need cuts and often joists of their own.
Joist spacing, fixings and planning
There is no British standard that a garden deck must follow; builders work to common practice and the board maker’s instructions. Softwood boards 28 to 32 mm thick laid square to the joists take joists at 400 mm centres; diagonal boards, thinner boards and most composite boards take 300 mm. Every board is fixed to every joist with two screws, or with the clip system the board maker approves.
In England a deck or other raised platform more than 30 cm above the ground needs planning permission; lower decks are usually permitted development, within the limits on garden coverage. Wales, Scotland and Northern Ireland have their own rules, and a listed building or a conservation area always needs a check with the council.
- Screws: coated or stainless decking screws about two and a half times as long as the board is thick, two per joist, about 20 mm from the board edge.
- Butt joints sit on a doubled joist or a noggin, so that each board end gets its own screws; stagger the joints of neighbouring rows.
- Treated timber rated for ground contact for the frame near the soil, and end-grain preserver on every cut the supplier asks for.
Slope, water and the house wall
A deck needs a fall away from the house, usually about 1 to 2 %, that is 1 to 2 cm per metre of depth, 4 to 8 cm over 4 m. The fall comes from the frame, not from boards cut at an angle; boards running with the fall let water drain along the grooves.
No water may stand under the deck. Keep the ground below sloped away from the house and the drains free. Where the deck is fixed to the house, flashing over the ledger keeps water out of the wall; the house wall and its weather barrier must not disappear behind the deck.
Wood or composite
Pressure-treated softwood, larch, cedar and tropical hardwoods move mostly in width: they swell when wet and shrink when dry, hence the gap. Hardwood is pre-drilled, or it splits at the screws. Timber turns grey over time unless it is oiled.
Composite boards grow mostly in length when the sun heats them. They usually need closer joists than thick wood boards, a gap at butt joints and ends, and often the maker's clip system. Enter its joist spacing and gap in the calculator: a closer spacing gives more joists and more clips.
Choose the board length to suit the deck
The board length decides the offcuts. On a 3 × 3 m deck with 3.6 m boards, every 3 m row takes a board of its own and the 60 cm offcut fills no other row: 21 boards, 75.6 m bought for 63 m laid. With 3 m boards it is 24 boards, 72 m with 10 % waste.
When a board length matches the deck, or the rows come out in whole boards, there are hardly any offcuts. Run the lengths on offer through the calculator and compare the length you buy, not only the number of boards.
Common mistakes
- Using the joist spacing of thick wood boards for thin or composite boards: the boards bounce and crack at the screws.
- Butt joints on a single joist: both ends share a narrow strip of wood and the screws split out.
- Joists on soil or in a hollow where water stands.
- No slope, or a slope toward the house.
- Plain steel screws in treated or tropical wood: they corrode and leave dark stains.
- Ordering only the length plus waste without checking the rows: with short rows the waste figure is not enough.
- Mixing boards from different deliveries: colour and size can differ.
Tools and materials
- Boards, joists, bearers, supports (concrete pads, posts or pedestals and pads) and weed membrane
- Coated or stainless deck screws or the board maker's clip system, plus spacers for the gap
- Cordless drill driver, for hardwood a countersink drill bit
- Mitre saw or circular saw, straightedge, spirit level, string line and tape measure
- End-grain sealer for the cut ends, if the board maker asks for it
Frequently asked questions
- How many decking boards do I need for 20 m²?
- For a 5 × 4 m deck with boards 144 mm wide and a 5 mm gap, running along the 5 m side: 27 rows of 5 m, 135 running metres, with 10 % waste 148.5 m. In 3.6 m boards that is 42 boards. With joists at 400 mm centres you need 14 joists of 4 m, 56 m, with waste 61.6 m, and 832 screws or 416 hidden clips.
- How far apart should decking joists be?
- Common British practice is 400 mm centre to centre for softwood boards 28 to 32 mm thick laid square to the joists, and 300 mm for diagonal boards, thinner boards and most composite boards. There is no national standard for a garden deck, so the board maker’s figure applies. A closer spacing means more joists and more screws.
- How many screws per decking board?
- Two at every joist the board crosses, or one hidden clip per crossing with a clip system. A 3.6 m board on joists at 400 mm centres crosses 10 joists and takes 20 screws or 10 clips. Use coated or stainless decking screws about two and a half times as long as the board is thick; your fixing system’s instructions apply.
- How big should the gap between deck boards be?
- About 5 to 6 mm for softwood boards, so that water drains and the wood can swell; the calculator starts from 5 mm. Composite boards take the gap their maker gives, often with the spacer of the clip system. The gap counts in the number of rows: the wider it is, the fewer rows.
- Which way should deck boards run?
- Usually away from the house, along the fall, so that water runs off along the boards and their grooves; a fall of about 1 to 2 % is usual. The joists run across the boards. The direction changes the quantities: rows across the shorter side are more but shorter, with fewer butt joints. Try both directions in the calculator and compare.
- What do the joists rest on?
- Never directly on soil. A deck at ground level usually has joists on bearers, and the bearers sit on concrete pads or on posts set in concrete; on a patio or a slab they rest on rubber pads or adjustable pedestals. How far apart the supports may be depends on the timber size and grade: the timber supplier’s span tables and the product instructions apply. Every butt joint gets a doubled joist or a noggin.
Draw the deck in the planner
Upload your floor plan and the free planner works out the exact packs and waste before you order.
create a layout planCalculation with typical values for boards, gap, joist spacing and fixings, not products. Joist spacing and fixings from common British practice; planning from the permitted development rules for householders in England. The manufacturer’s instructions apply.