Deck board calculator: boards, joists and screws
This deck board calculator works out how many decking boards you need for a deck of a given size, from the board width, the gap you leave between boards and the stock length you can buy. It also counts the joists under the boards from the joist spacing, estimates the screws (two per board at every joist) and the boxes to buy, and prices the lot from the figures you enter.
Decking is bought by the board, not the square metre, so the useful numbers are rows across the deck and boards along each row. Presets for treated softwood, hardwood and composite boards fill in typical widths, gaps and joist centres; change any of them to match your supplier.
Metric and imperial units are both supported, and the full working is shown so you can check it against a timber yard’s quote.
How to use the Deck Board Calculator
- Choose metric or imperial and enter the deck length (the direction the boards will run) and width.
- Pick a board type for typical sizes, or choose Custom and type the board width, the gap and the stock length of the boards you will buy.
- Set the joist spacing — 400 mm / 16 in for timber boards, 300 mm / 12 in for composite or boards laid diagonally.
- Enter a waste percentage and the number of screws per box; open “Prices” to add the price per board, per joist and per box of screws.
- Read the boards, rows, linear metres, joists and screws, then copy or share the materials list.
Deck Board Calculator features
Rows × boards per row
Counts whole boards the way they are laid, so short stock lengths and butt joints are allowed for.
Board presets
Softwood 140 × 28 mm, hardwood 90 × 19 mm and composite 140 × 25 mm with their usual gaps and joist centres.
Joists and screws
Joists from the spacing plus one, screws at two per board per joist with 10% extra, converted to boxes.
Linear length
Total running metres or feet of decking, which is how many suppliers price it.
Cost estimate
Boards, joists and screws priced separately with a total in ₹, $, £ or €.
Metric and imperial
Millimetres and metres or inches and feet, converted when you switch.
How the deck materials are calculated
- Rows = deck width ÷ (board width + gap), rounded up. 3 m ÷ (140 + 5 mm) = 20.7 → 21 rows.
- Boards per row = deck length ÷ stock length, rounded up: 4 m ÷ 3.6 m → 2 (the offcut from one row starts the next in practice, which the waste figure allows for).
- Boards = rows × boards per row × (1 + waste): 42 × 1.1 = 46.2 → 47 boards.
- Joists = deck length ÷ joist spacing + 1 = 4 ÷ 0.4 + 1 = 11, each as long as the deck is wide. Joists run at right angles to the boards.
- Screws = rows × joists × 2, plus 10%: 21 × 11 × 2 = 462 → 509, i.e. 3 boxes of 200.
Board sizes, gaps and joist spacing
Wet treated softwood shrinks as it dries, so a 3 mm gap becomes 5–6 mm; dry hardwood and composite keep the gap you set. Follow the manufacturer’s spacing for composite — it is usually tighter than timber.
| Board | Typical size | Gap | Joist centres |
|---|---|---|---|
| Treated softwood (pine) | 140 × 28 mm (5.5 × 1 in) | 5–6 mm | 400 mm (16 in) |
| Hardwood (ipe, teak, merbau) | 90 × 19 mm (3.5 × ¾ in) | 4–5 mm | 400 mm (16 in) |
| Composite / WPC | 140 × 25 mm (5.5 × 1 in) | 6 mm (per maker) | 300 mm (12 in) |
| Any board laid diagonally | — | same | 300 mm (12 in) |
Not included — plan these separately
- Posts, beams (bearers), ledger board, joist hangers and concrete footings depend on the deck height and span tables; the concrete calculator sizes the footings.
- Fascia boards around the edge: roughly the deck perimeter in extra board length.
- Steps down from the deck — see the stair calculator for risers and treads.
- Stain or oil: use the deck area (both sides of boards if you coat before fixing) in the paint calculator. A matching fence is covered by the fence calculator.
Deck Board Calculator FAQ
How many deck boards do I need for a 12 × 16 ft deck?
With 5.5 in boards and ⅛ in gaps across the 12 ft width you need 26 rows; with 16 ft boards that is 26 boards, 29 with 10% waste. The deck board calculator also gives 13 joists at 16 in centres and about 744 screws.
How many linear metres of decking per square metre?
Divide 1 m by the board width plus gap: for 140 mm boards with 5 mm gaps, 1 ÷ 0.145 = 6.9 linear metres per m². For 90 mm boards with 4 mm gaps it is 10.6 m per m².
What gap should I leave between deck boards?
5–6 mm for softwood, 4–5 mm for hardwood, and whatever the manufacturer specifies for composite (often 6 mm). The gap lets water drain and the boards move.
How many screws per deck board?
Two at every joist. A 4 m board over joists at 400 mm centres crosses 11 joists and needs 22 screws. The calculator adds 10% for mistakes.
Should I include waste?
Yes, 10% is normal and 15% if the deck is not rectangular or the boards are laid diagonally. Boards with defects or damaged ends are also covered by it.
Does this calculate the substructure?
Only the joists. Beams, posts and footings depend on height and span tables from your local code; use the joist count and deck size with those tables.